Note: Descriptions are shown in the official language in which they were submitted.
2139967
.
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PATENT
Case 840P003
CLOSED SYSTEM MULTISTAGE ~u~;~u~IFICATION
RF~:~y:~ALLIzATIoN
Descril~tion
Bac~-lL~ n~i De8crition of ~h~ Invention
The present invention generally relates to
multistage recrystallization, and more particularly to a
multistage method and apparatus particularly well-suited
for running under closed conditions to m;n1mi7e the
5 inf luence of environmental variations on the method and
apparatus. Cryst-lli7~hle substances are purified to
e~L~ -ly high levelg, typically on the order of 99.999
percent purity. Materials are transrerred within and
between stages of the multistage method and apparatus in a
10 manner whereby no crystal 117~ material need b~c loved
between vessels or stages.
Purification of cry~ -111 7-hle aterlals by way
of L~_L~ l i 7-tion t 1qu~ has been known and
practiced for many years. Generally ~ lrin~, it is
15 cU~L~ -ly di~icult to move trace impurities rrom
crys~al l 1 ~ahl~ mat-~rial~ by ~t ~ l l l 7-tion fro
saturated lut~ n ther~for, or by other t~
including fra~ti~ crystallization and fr~ tion-
fre-zing ~LOct,lULe.. Tn~ e~ are zone refining
20 ~LUC~IUL~3 which typicrlly would increa~e product purity
by adding recrystal 1 i 7~tion stag~s, but this would be done
at the expense of the pcL ~ ge yield of product that is
formed at the last or purest recrystallization stage. For
example, it is well known to use fractional
25 crystallization in separating radium chloride from barium
chloride by a classic separation pLucc-luLæ that utilizes a
complicated grid pattern of many dozens of separate
e~ uL~tion dishes or crystallizers. Under this
. ~, _ . , . . . , , ,, _ .
2139g~
p~ lu~e, the crystal crop of each crystallizer is
tran~ferred to the adjacent crystallizer in one direction,
while the mother liquor i5 transferred in the opposite, or
count~ u~ direction. Although increasing
5 concentrations of radium chloride follow .. t of the
crystal crop from one crystallizer to the next, a very
large number of crystallizers are required, and oper~ting
costs for this type of system are very high.
Generally ~o:~k~nq~ currently available
10 applications for frA~t1~n-l crystallization have
recogni zed the desirability of operating at an optimum
reflux ratio while not ne~ ~ F~'t ily e~fectively providing
suitable specifics for implementing thi- desirable
approach . It can be i La.lt that ref lux ratio
15 conditions be controlled clo~ely, preferably in
association with the establishment and alaintenance of
steady _tate conditiona in each ntage of th~ ~yste~,
thereby facilltating generally automatic control of ref lux
ratio conditions by closely monitoring and ~etering
20 s~lected material transrers, inputs and/or outputs for
each stagQ of the syste~. Tl~ 'Cl as ~ in thiu
regard are Griffiths U.S. Patent~ No. ~,88S,061 and
No. 5,127,921, which are in~ c-ted by le~L~nCe
her~into. P. icAa of this type typically reguire the
25 transrer o~ ~olid, cryst~ ~at rial b t~een vesaels
and b tw~Qn ~tages of thes~ hultistag L~ tion
syst~ms .
The ~ulti~tage ~L_~c~ 7~tion art ~l~o
in~ ulti8tep ~ yDI allization syatems for
30 separating rluid Dlaterial having two or ~or~ di~rent
.~ s. For example, U.S. Patent No. R~. 32,2~1
describes having a ~ t crystallizQ on a cooled
aurface as material containing the t flows down
the cooled surface. With this approach, a lution of a
35 giv n ~tage is used to wash the crystal~ formed in that
_tage before the crystals are transferred to the next
pur--r _tag . This wash solution is the solution that is
21~9~67
.
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in thls stage at the time immediately preceding the
crystallization of any material therefrom. In an
~ :~i L of this development, the crystals are
transferred to the next purer stage as soon as appropriate
S valves cause the liquid of the next purer stage to contact
and dissolve the crystals. The wash solution, which is
used to wash the crystal crop of the given stage, is the
purest part of the solution present in that given stage
before any material is cryst~ fro~ it.
With the approach o~ Re. 32,2~1, the Ytages
preferably operate sequentially. During crystallization,
a self _, Ling solid layer of crystal cake is formed on
the chilled surface of the cry~tal 1~ r after the mother
liquor is drained. This system is not particularly well-
15 suited for refining crysfAlli7Ahle substances which cannot
form the self ~ Ling, ~dh~l~nL layer of solid crystal
cake on the chilled surface of the cryst~ 7--. This
system is not particularly use~ul for feed materials which
form a flowablc slurry or any other for~ that does not
20 adhere to a chilled surfac~, ~uch a~ 8-~h~t~n~ which are
~_L,~ ' fro~ a a~ tlon comprised of a ~olut- of
the Daterial being refined and a solvent for this
material. Additionally, crystal cakes formed by systems
of this type are not particularly well-suited for
25 ~ff~ctive washing of individual crystal~ without breaking
up th-- cak~ and th-n washing thQ pulveriz-d crystals with
a ~uitable wash Eollltit~n. Typically, these cakes are hard
and rigid, and only crystals on its exposed surface are
e~fectiv~ly washed, the interior crystals not being
30 readily reached by the ~agh 801utl~r
U.S. Patent No. 4,787,985 ~ ,, Ls multistage
purification through use of a sequential recrystallizer
wherein the size and purity of the crystals increases from
~tag~ to stage. Cry~tals are ~}- ~ by the use of
35 thi~ no-- device8 or wa8h column8, and this approach has
the disadvantage that crystals are not dissolved and then
.
~1399~7
recryst~lli7ed from a purer solution than that from which
they were originally formed.
U.S. Patent No. 4,588,414 suggests the use of a
~ystem including a centrifuge arr~r, --t. This
5 countGL~ uLL~.~L approach ~hO~i~c the use of temperature
differences between ~ 8, with crystals being
removed rrom one cry~Alli~-r and being f-d to another
crystallLzer at a higher temperature while the ~other
liquor moves to another crystallizer of lower t mperature.
It has been found that, by procee~1n~ in
accordance with the present invention, it i8 possible to
achieve ~u~G-~urification o~ crystAl 1; 7~hle substances by
removal of certain impurities therefrom through a
multistage recrystallization p~o~.Gdu~G that achieves
purity levels on the order of 99 . 999 percent with
extremely high yields that are exceptional for ~ultistage
~e~ 11i7~tion ~ ,61u e3. Tn~ d is an effective
manner of controlling reflux r~tio conditions while
achieving and r- i ntA i n i n~ steady state operating
20 conditions.
v of ~h- Tnvention
In summary, the present invention is a method
and n~ for .u~ urlfying crystal 1; ~ahle sub~.L.Ir- .=..
25 by a ~ulti-stage recry~tal 1 i 7-tion pLoc~l~ r- wherein a
plun~lity of ~tagGes ; nt~ i ng a lea3t pur~ stage and a
purest stag~ proceed through substantially duplicate
cycl-~ cAn~ ted substantially ~imultan~ol~ly within the
stages . The cryst~a~ 11 i 7ahle ~ubst~nce is dissolved in each
30 stage and transf~rred as a fully dissolved _ - ~ of a
~o-called new solution to the next purer tage at which
the crystalli~hle material is recrystalli7~d~ the
PL~JC61U~G ~ ntin~in7 until ~u~ified crystals are
formed in the purest o~ th~ stages. Cry~tals formed in
35 each stage are o ~ tQ~ fro~ their mother liquor to the
extent that ~other liquor is withdrawn fro31 between the
individual cry~tals of the cry~tall~7~1 mat~rial in a
, ~
~ 213996~
~annær which leaves the indivldual crystal~ particularly
well-oriented and exposable to washing ~ceduL~3
Preferably, these crystals are washed, and preferably the
wash f luid ls mother liguor from the next purer stage
5 All vessels and transfer eq~ t are closed to
environmental influences and do not require transfer of
solids, thereby making same particularly well-suited for
automatic and ~ 1 ly efricient op~ration and; v~_d
yieldO .
It is a general ob~ect of the present inventLon
to provide an i ~ method and apparatus rOr
Ou~_~yurifying crystA~ hle substances
Another ob~ect of the present lnvention is to
provide an i ~ d u~_L~uIification system which is
closed to the environment and utilizes multistage
Allization that i~ pec;~ly well-adapted to
automated, ~;ial scal- operation
Another object o~ the present invention iOs to
provide an; ~, 1 in multistage recrystall ~-ation
that effectively, efriciently and very clos¢ly controls
r~fluY ratio conditions and that can achi~vo and thon
maintain steady stat~ conditions
Another ob~ect of this invention is to provide
an i x)~l Du~_-yu.ification of cry~All;~hl~ ~ubOstances
which achiQv O ~r. ly high puri~ication t ~, with
particul~rly high yinld~
Another object of the present invention is to
provide an; ,~_d ~pa t~,s and method for
~u~ification of cryst~l 1 i7~hle Oubstances which
in~ the use of a~ ts whereby a wash refluY
from one stage of a multistage r~c ~a~llization ~ystem is
used to displac~ residual mother liquor from crystals
~ormed in a stage that operates at a lower purity level
than the stage from which ths wash originat-O
Another object o~ the present invention i~ to
provide an; ,~_d multiOtage 1 ~ lli2ation Oy-tem
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2139967
.
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wherein specially efficient and thorough separation of a
minor ingredient from a major ingredient is effected.
Another object of this invention is to provide
an i ,~ d apparatus and Dlethod that utilizes ~ultiple
stages and involves forming a new crystal crop in each
~tage from a new solut$on containing the dissolved
- t being purif ied together with its ~olvent .
Another ob~ect of this invention i8 to provide
an i, ~ d multistage fr~r~ cry~t~ tion syl;tem
having a controlled reflux ratio ~uch that the proper
ref lux ratio can be used to separate out a minor
ingredient having a given segregation coefficient.
Another object of the present invQntion is to
provide an i , ~ apparatus and method wher-in ref lux
leaving a given stage of a multistage system is as impure
as possible and such that the crystal material leaving
that stage i5 as free of its mother liquor as possible in
order to thereby ~-Yimi 7A the separation efficiency of
each stage.
Another ob~ect of th~ present inv~ntion is to
provide an 1 ~ ~ethod and , .-; rOr r~dily
pror.~s~;n~ crys~ hle ~aterial~ that ror~ a flowable
slurry in a crystallizer, thereby allowing the use of
conventional slurry producing cry8t~ r~s ~
Another ob~ect of the present invention is to
provide an i ~ method and ..~ r~ which can be
operated in a totally ~nrlosed manner and und-r super
a~ ,~ lc ~ 5 ~l~ thereby ~acilitating
s..~ ~ if ication ref ining of substances requiring
30 ,.-I~,_.I.e~t~d solvent to dissolve same in ~ ~Ylally
important quantitie~.
Another ob~ect of the present invention is to
provide a multistage recrystalli2ation process and
~a~ which is automated and ~ncl~ ~ and ~eets
35 sanitation requirements of industries ~uch as the
pharmaceutical industry.
. . _ , _ .
2~399~7
Th~se and other ob; ectD, ~eatures and advantages
of the invention will be clearly understood through
consideration of the following detail~d description.
5 Brief Descril~tion of the Draw; n~
In the course of this description, reference
will be made to the t-t'- ~' drawings, wherein:
Fig. 1 is a genorally schematic view or f low
diagram of a closed multistage Du~_.yu~irication system
10 according to thiD invention; and
Fig. 2 is a generally schematic view or flow
diagram of an alternate . ~'i~ for carrying out the
closed multi~tage recryst~ll;zAtion suy_.~uLification
according to the present invention.
Descri~tion o~ the Particll l Ar - ~ - ~ i - ~
In the arrA~_ ~ illustrated in Pig. 1, a
cloDed multiDtage ~e~.L~DL~llization system is shown to
include three different stages, each Or which, except for
20 portions of th~ purest stage, F~ -~~-~ 8"1- L~ ;A11Y
equal volume~ of _ t s, but with dirr~rQnt purity
level~ being ~aintained ~rom stag~ to stage. In th~
illustrated: '; , Stage L is the stage at which the
crystAll;~-hle ~aterial th-rewithin i8 at a purity level
25 that i- the lowest among th~ stagQs illustrated. Stage P
is th ~tage llt which th~ purity l~v~ th~ highe~t.
StAgQ I i~ _..Lative o~ intermediate stage~ that may
be provided and at which thQ purity level i8 of an
int~ -';AtQ degree, meaning that same is purer than Stage
30 L and 1--88 purQ than St~ge P. Th~ ;-'-- 'iAt- ~tage can
be omitted, or additional int ';Ate stag~s can be
added. In a typical situation, ~ suy_.yurity level on the
order of 9g . 99 percent purity iD attained when multiple
int~ ';AtQ stagQs are inc~,L~ ted. Ir Qvon greater
35 purity, or hyperpurity, is deE~ired, additional
intermediate ~tages could be utili2ed, although it ust be
appreciated that the greater the number Or ; nt~ Ate
. _ ~
2l~9g67
~tages, equipment cost, and processing cost and t$1~e are
in~ L cased . Flow rates between these stages are
substantially the same, and the ~low rates are developed
to ensure there i8 enough solvent to dissolve the crystals
5 at the designated ti~e Yithin each stage.
Each stage includes a liquid receiver 21a, 21b,
21c. Each liquld receiver can includ~ a r~flux ~ection.
In the illustrated s '-'i- ~, the reflux section talces
the form Or a separate reflux compartl ent 22a, 22b, 22c.
10 These devices serve to hold and/or metQr liguids during
various steps of each cycle. P~C9L~ILe control manifolds
23a, 23b, or similar assembly, is provided 80 that
c conditions within selected r~spective
compartments or vessels are substantially the same in each-
15 stage at a given processing time within the cycle.P,.- c control manifold 23a will, ~or example,
E~.- ize these co~partments or vessels in order to
remove liquid therefrom and trans~er it to another vessel.
Control manifold 23b may sub~ect the vessel or compartment
20 to reduced ~ conditions or vacuum conditions in
order to ~ssist in drawing liquid thereinto rrom another
vcssel or ~ i ~. The ~anifolds or other ~uitabl-
means such as the illu~trated vents can also achieve
venting of these vessels or . i - ~s in order to
25 ~cilitat~ Qntry of liguid ther~into.
mese liquid receiv~r v~ssels or compart~ ents
provid- ^nt lç~ ~ spaces of known volumes which can be used
in ~ets~ring volu~es passed thereinto or ~-~rpl i ed thereby
by either simply filling the vessel or compartment or by
30 the use o~ suitable capacity ~i~n~ ng devices. Volume
variations can also be ad~ust~d by ~n~ ng a known
number of glass or plastic beads 24 or the like, e~ch of
which displaces a known volume o~ liquid. A desired
nul ber of such beads can be added to these vessel~ in
35 order to f inely ad~ust internal volumes.
Each stage al~o includes a ~Lv- c..soL 25, 26, 27.
The y~vce3~v~ of the purest stage, Stage P, can take the
... _ . _ .. _ . . .. _ _ _ . _
21399~v7
form of a crystallizer 27 having a crystal bed volume
which is substantially larger than the crystal bed volume
29 provided by the other processors 25, 26. In this
~anner, the D~ L~ULified product can be collected and
5 stored within the crystallizer 27 as ~..~_,~uLifled
product. Pure solvent feed preferably ent~rs the system
through a suitable feed devic- 28. Cry~tals to be
purified are fed through an inlet 31 into a feed dissolver
32 which preferably has a liquid receiver 21d as~ociated
10 therewith. Each pL~ -q~ 25, 26 and t_e feed dissolver
32 typically include a mixing device 33a, 33b, 33d for
facilitating crystal dissolving during a particular phase
of the processing cycle. Each pLv-_e.ssv. 25, 26 ineludes a
screen or foraminous plate or filter 34a, 34b near the
15 bottom portion thereof. Each such screen or filter 34a,
34b is sized to prevent passage therethrough of
crystal l; ~hle material when it is in its cryst~
state, such as crystals 35a, 35b illustrated in Fig. 1.
Each screen or filter, however, per~its passage
20 tl~ v~l. Or any liquid material whieh must be
withdrawn from the p.. - at any given time during a
cycl~ .
The t-rm "new solution~ is ~nt^n~ to refer to
a solution o~ solute within solvent which is forQed within
2S p.vc~ v. 25, 26 or th~ likQ. In such a new solution, the
cry~ 7-hle ~atRrial i8 prQs~nt, but in it~ dissolved
~tate within th- solvent for the particular cryst~ll17~hle
material being ~.. -~e~ within the syste~, the new
~olution being at an elevated temperature that maintains
30 th- cry~ hle material in its dissolved state. A
~upply of new solution, which can be specifically referred
to as a feed solution, is formed ~ithin the feed dissolver
32 at substantially the same time that the new solution i8
for~ed in ~ach of th~ ~..oce~,iol~ 25, 26. Thi~ solution
35 formation is, for ~any crystall 1 ~-~hle ~aterials, assisted
by a heating assembly, such as the illustrat~d heat
oY~h-- 3 36a, 36b, 36d through which a fluid at an
~ 996~
--10--
elevated temperature pas6es. Durlng a run, a substantial
3upply of hot new feed solution is maintainQd in the feed
dissolver. This supply is sufficient to insure thAt there
would always be enough to provide the proper quantity of
S new ~eed solution to the Stage L ~ L vr at the proper
time during each cycle of a run.
At the beginning of a typical cycle for thQ
sy_tem illuD,trated in Fig. 1, cry_tal~ 35a, 35b, 35c and
35d are pre6ent in th~ir r. ,- ~ive vessel~ such as in the
10 quantities generally illustrated in Fig . 1. Recently
washed crystal6 35a, 35b are DU~JVl Led by ~creens 34a,
34b, re6pectively. Suu.:L~uLlfied product cry6tal~ 35c, to
the extent they had been p~vduved in a previou6 cycle, are
present in ~Lv~es~v~ cry6t~1 l lz~r 27. Cry6tAl feed 35d
15 can be present in the feed di6solver. Preferably, each
liquid receiver 21a, 21b, 21c i6 D8l11 LanLially ~ull of
~other liquor. In the pr~erred operation o~ thi~
o~i--rt~ the mother liguor is a so-called ~ixed mother
liguor, which is a mixture of mother liguor rG ~nin~
20 after crystallization in that same stage,: ~inod with
~other liguor u6ed to wash the same crystals in a
D~ step of the sa~e processlng
In the first gtQp of thig proc~
valve6 37a, 37b, 37c or si~ilar devices ar~D opened, and
25 y~ D~UL iz~d air or othQr ga~ uch a~ nitrogen ent r~
~r, 1~ ~ ivQ ligu$d receiver~ 21a, 21~, 21c whil-- valv~s
38a, 38b, 38c or the likQ are opened, with th~ rexult that
the mother liguor within the liguid receivers 21a, 21b,
21c, r~6pectively, i6 tranE~ported into the p.._- 25,
26 of thQ next 1Q88 purQ stage, or in thQ casQ of liquid
receiver 21a lnto the feed dissolver 32. A~ter completion
of this step, crystals 35a, 35b and 35d are covered with
or f looded by the thus transferred mixed ~other liguor.
Crystal dissolving i~ carried out in the second
35 step. ThiD dissolving takes place in the p_ ~ D. 25,
26. Typically, th~ dis601ving pro~ G1~1L~ i-- ~QoiDte~d by
activation of the mixing devices 33a, 33b and by applying
. ~
~ ~139967
--11--
heat to the mother liquor and crystals such as through
activation of the heat ~Y~hAr~ - 36a, 36b. By this
action, the slurry of crystals that had been in each of
the pLOC~85~ within the mother liquor transferred
S thereinto is dissolved and thereby transformed into the
new solution for each of the respectiv~ stages, which is a
normal or predetermined volume of new solution charge for
which the system is calibrated. This charge of new
solution is now ready for transfer as a liguld.
In the third step of this illustrated process,
all of the hot new solution within each of the pLoce~ D
25, 26 is transferred into the liquid receiver 21a, 21b of
that same stage. This transfer PL-_ -' æ in~ C opening
of valves 39a, 39b located between the exit from the
respective processor and the inlet to the respective
liquid receiver. This transfer could be assisted by
appropriate venting of the liquid receivers, ~uch as at
vents 41a, 41b and/or pressurization of each ~L~ce3~L by
suitable ~eans (not shown). By the time this step is
completed, a supply of hot dissolved reed s:luti~ is
transferred from th~ feed dissolv 32 into the liguid
receiver 21d by suitable transfer ar. _ , in~ n~
the opening of valve ~2. Prererably, equal volumes of the
thus transferred hot new ~lut~n and hot fQed 801uti~n
are present within the r--, e iVQ l$quid r ceivsrs 21a,
21b, 21d at thi~ time.
In the fourth step of this illustrated
yL~c~luL~, all of the hot new solution in each of the
liquid receivers 21d, 21a and 21b is transferred into the
pL~Ce~3~0L 2S, 26 and 27 of the next pur-r ~tag-. Thi~
transfer ~Loc~-lur~ includes the opening of valves 43d, 43a
~nd 43b located between the QXit of the respective liquid
receiver and the inlet to th~ respectivR ~LoceasvL. This
transfer could b~ assisted by _, vyL lata venting o~ the
35 yL-,c~oL(not shown) and/or pL~e~-.Llzation of each
liquid rec~iver by yL~9 izing manirold 23a and opening
valves 37d, 37a and 37b. By the time this ~tep is
, . ~
2~i3996~
.
--12--
complQted, the new feed sol~l~io~ in liquid receiver 21d
would hav- been trans~erred to pL0- -- 25, the new
solution in liquid receiver 21a would have been
transferred to p~ocesv~ 26 and the new solution in liquid
5 receiver 21b would have been tran6ferred to processor
crystallizer 27.
Each thus transferred E~olution is Dub~ected to
crystallizatiOn conditionD within a fifth ~tep o~ this
pL~_ - .. Cry~tallization is typically racilitated by
10 reducing the t~ u~ within the ~L. !r~ ~. 25, 26 and
the crystallizer 27, such as through the action of heat
Y~hA- J 36a, 36b, 36c, typically while stirring the
solution during formation of the crystals. A~ter
completion of this step, crystalD are formed within their
15 mother liguor, both of which are present within the
pL~..esDo~ 25, 26 and crystallizer ~L~ 27.
In the neYt, or siYth step this moth~r liquor is
trans~erred out o~ the ~.~,ce~sor 25, 26 and crystallizer
p~oc~ . 27 and into the liguid receiver vessel or
20 ve6sels of that same stage. In a preferred ~LL_, t in
this regard, a controlled quantity Or this ~other liquor
first iD transferred into th r~fluY . ~- ~ ~ 22a, 22b,
22c by opening valves 44a, 44b, 44c, typieally while vents
45a, 45b, 45c are open 60 as to facilitatQ ~ of the
25 mother liquor through the valved conduit. In a preferred
aLL -_ ~, thi~ transfer requir--~ only the rurther
assi-t~mce o~ gravity, although a pumping or ~L.~D.,Llzing
aLL , ~ can also be used as desired. Typically, the
refluY ~ 22a, 22b, 22c will not be large ~nough
30 to hold all Or thQ ~other liquor; aceordingly, mother
liquor transfer continues by opening valves 39a, 39b, 39c,
preferably while vents 41a, 41b, 41c are also open and
valves ~,~.a, 44b, 4~c are cloDed.
In a related, seventh ~tep, the crystal~ are
35 ~ub~ected to a dewatering ~Loc~d~Læ. A~s illuDtrated,
manifold 23b is activated 80 as to reduce the pL~ ~ ~, or
create a vacuum condition, within at lea~t liquid receiver
~1399~7
--13--
21a, 21b. This provides the function of a suction filter
or plate filter and achieves the beneficial result of
withdrawing the mother liquor which remains between the
crystals after the initial transfer of mother liquor out
5 of the yLoce6s-)lD 25, 26. During the course of the mother
liguor transfer of these sixth and seventh steps, the
newly formed cry~tals remain supported by the ~;creen or
filter 34a, 3~b. Advantageously, the crystals 35a, 35b
cover an area which i8 typically equal to or approximately
10 the same as the surface area of the screens 34a, 34b.
This area is preferably significantly larger than the
depth of crystals supported thereby in order that the
dewatering y~ æ may be morQ effectively accomplished
than if the mother liquor being subjected to the
15 dewatering conditions had to travel through a greater
depth of crystals. It is particularly important that the
crystals 35a, 35b be Dupported by the screen 34a, 34b or
the like, rather than, for example, caked along the
sidewalls of a crystallizer after crystallization has been
20 completed .
The eighth and ~inal step of this illustrated
~.oc d~. is optional, although it i~ pre~erred. Same is
a step of washing the ~ust dewatered crystals with mother
liguor separated from crystals formed in the next purer
25 ~tage. In th~ illustrated ~ , this is most
r~adily ~ ' by opening valves 46b, 46c and
.. lzing the reflux ~ t --~s 22b, 22c by
activating th~ manifold 23a while valves 47b, 47c are
open. As a result, the purer stage mother liguor passes
30 over and through the crystals within th~ ~,,. e 25, 26
in order to carry out a reflux wash proc~dlL-- to remove
most of the mother liquor from this stage that still wets
the dewatered crystals. The crystals are thus washed with
liquid from the next purer stage befor~ those crystals are
35 dissolved in the ~ix~d mother liguor of the next purer
~tage. The ffectiveness of thi3 waz~hing i~ facllitated
by the configuration of the cake containing crystals 35a,
.
213~967
--14--
35b which are aupported by the screen 34a, 34b, rather
than being caked along the sides of a crystallizer as
generally r~iar ~a~ed herein.
Byproduct in the form of mother liquor from the
5 least pure stage exits the aystem upon opening valve 46a,
which removal may be as~i~ted by pressurization of the
r~flux compartment 22a, ~uch aa by activation of the
manifold 23a and opening of the valve 47a. By the
~1L , t shown in Fig. 1, it i8 po~:ihle to have the
10 same volume of mother liquor exit the system during each
cycle as is transferred as reflux from each stage to the
next less pure stage. Also, enough pure solvent enters
the Stage P crystallizer to maintain the proper quantity
of mother liquor therein and to make up the reflux that
15 must be fed to the Stage I p,~ QaOsvr 26. A crystal
product 35c may be removed at any time in any ~uitable
manner, but typic~lly aft~r Oeveral cycle~ have been run
whereby a sizable quantity of ~Ou~L~uLified crystals are
available. A~ Ated cry-Otalg should be removed often
20 enough to prevent the cryO~ ?~r from b~ g
overloaded with cryatalO.
If d sired, only on~ p~v~ 26 can be used,
in which event p~. 25 and any other les-~ pure stage
~.vceOsvl~ ar~ ply v~s~el~ for receiving and/or forming
25 n~w solution. Each of ~uch new ~olutions of varying
purity i- then transferr d, on~ after th~ othor, to the
~ingl~ ~L.^ qq 26 which cry~All~ and otherwise
~L~ -~ th~ crystal~ a~ ~i e~ herein. For example,
vQss~l 25 in this arrAr~ ~ ~;till receives mother liquor
30 from th~ liguid receiver 21b and/or thQ r~rlux compartment
22b ~nd transfers th~ nQw ~olut~on formed therein to the
pLvceOsvr 26 through conduit containing valve 43~.
Fig. 2 shows an alternate, pre~erred ` ' i - t
of the closed multiE~tage l~_L~ allization systel~ of this
35 invention, three different stages again being ~Ohown. The
primary functional difference between the o"~hodi-- t of
Fig . 1 and the . ' i - ~ of Fig . 2 lies in the processor
2139967
.
--15--
a~semblies. Fig. 2 includes the combination o~ a
crystallizer, a centrifuge assembly, a mother liguor
circulation pump and a mother liquor heater, as the
pLocessor of Stagea L and I. With this arr~n~- t, the
5 ~L~ . as illustrated in the Fig. 1 ^ ' ~ ~ rt,
including their ~creens and the suction filter or plate
rilter arr~r, --t need not be included. I~other liquor
removal from and dewatering of the fre~hly prepared
crystals arQ accomplished by the centriruge assemblies.
In the illustrated Fig. 2 ~ t., Stage L is
the stage at which the crys~ 7~hle material therewithin
is at a purity level that is the lowest among the stages
illustrated. Stage P is the stage at which the purity
level is the highest. Stage I is r_~L. F~ ative Or
intermediate stages that may be provided ~nd at which the
purity level is of an intermediate degree, meaning that
same is purer than stage L and less pur~ than Stage P.
The int~ -'iAte stage can be omitted, or additional
int~ Ate stages can be added. In a typical situation,
a _ yurity lQv~l of on of the order of 99 . 999 percent
purity iB attained when ~ultipl~ i nt~ t~ stages ~r-
ir.cv~.oL~ted. If even greator purity, or l~ .u~lty, iD
desired, additional intermediate stages could be utilized,
although it must be appreciated that the greater the
numb_r Or int~ --iAt~ 8tages, ~, i t cost, and
i rg c08t ~nd ti~e are inc~ ? ~ - Flow rate~
betweQn these ~tages are DuL~,~a--Lially the same, and the
rlow r~t ~ arR dcveloped to ensure there is enough solvent
to di~olve the cry~tals at the designated time within
each stagQ.
Each stage in~ a liquid receiver 121a,
121b, 121c. 2ach liquid receiver can include a reflux
section. In the illustrated ' i , the ref lux
~ection takeD the for~ of a separate reflux _ L ~I t
122a, 122b, 122c. These devices serve to hold and/or
~eter liquids during various steps Or each cycle. A
~L~ c control ~anifold 123a, or similar assembly, iB
21399~7
--16--
provided 80 that pressure conditions within selected
respective compartments or vessels of each stage are
substantially the same during a given processing time
within the cycle. Pressure control manifold 123a will,
5 for example, pressurize these compartment~ or vessels in
order to remove liquid therefrom and transfer it to
another vessel. Preferably, thQs~ vessels or compartments
include vents in order to facilitate entry of liquid
thereinto. These liquid receivQr vessels or ~ ts
10 provide enclosed spaces o f known volumes which can be used
in metering volumes passed thereinto or supplied thereby
by either simply f illing the vessel or compartment or by
the use of suitable capacity signalling devices.
Each stage also 1n~ A~ a processor, generally
designated as 125, 126, 127. The processor of the purest
stage, stag2 P, can take the form of a crystallizer 127
having a crystal bed volume 129 which is substanti~lly
larger than the crystal bed volume provided by the other
processors 125, 126. In thi~ manner, the 25~ y~1L ~fied
product formed during each cycle can be collected and
~tored wlthin the cryst~ r-- 127 a-~ ~ y~ L lf ied
product. PurQ solvent ~eed preferably ~ntQrs the ~ystem
through a suitable feed device 128. Crystals to be
purif ied are fed through an inlet 131 into a feed
dissolver 132 which preferably has a liquid receiver 121d
n~sociated th~rewith. The feed dissolver 132 typically
~n~ a mixing device 133d for facilitating crystal
dissolving therewithin.
I~ach yLoc~ssoL 125, 126 is an assembly of a
crys~ 7-r lSla, 151b, a centrifugQ 152a, 152b, ~ liguid
circulation pump 156a, 156b and a liguid heater 136a and
136b. Each centrifuge includes a centri~uge basket 153a,
lS3b or similar holding device which is mounted in a
suitable fashion to a rotatable shaft lS~.a, 15~b having
suit~ble power means for rotating the shaft and the basket
or the like. Each basket 153a, 153b or the like has a
wall area ~ - ~e ~ of screen material which acts as a
... _ _ . .. , . . _ . . _ _ . .
21~9~67
--17--
foral~inous plate or filter. The 5creen or filter ~aterial
of each such basket 153a, 153b or the like i8 OiZed to
prevent passage therethrough of crystalliz~ble material
when it is in its crystallized state, such as crystals
135a, 135b illustrated in Fig. 2. The 8creen or filter
material of each basket, however, per~it~ passage
thel. thLo~ o~ any liquid asOociated with the crystals.
This includes mother liguor and new Osolution aO previously
def ined .
Enough hot feed solution ohOuld be present in
the feed dissolver to insure an adequate supply of it to
the Stage L crystallizer 151a during every cycle of a run.
Also, during each cycle, the feed dissolver receives a
charge of cold mixed mother liquor from Stage L, heats --
that charge to a predetermined temperature and provides
conditions to enable that heated solution to dissolve the
proper guantity of feed crystals. The heating of thi~
solution can be assisted by a heating assembly, ~;uch as
the illustrated heat ~Yr hJ~, 136d typically having a
jacketed p~ y through which a fluid at a controlled
elevated t~ ~LUL~ passes. AloO, heat ~~~~ -~_ O 136a
and 136b are used to heat ~other liguor in the~e stages
for the purpose of crystal dissolving therein.
At the beginning of a typical cycle for the
syste~ illuoL~rlt ~ ~ in Fig. 2, crystal-~ 135a, 135b and 135d
~r- pr~s-nt in their r. ~ ` ive vessels ouch as in th~
quantities generally illustrated in Fig. 2. Suye.l.urlfied
cryOtal~ 13Sc can also be pr~sent in the plO~ -L.LoL
cryst~ r 127 i~ the system had already been cycled to
form _ ~ ~L lrLed crystals during ~tart-up operation~.
Recently washed crystal~ 135a, 135b ar~ supported within
baskets 153a, 153b, ~, a_' ~vely. Crystal ~eed 135d can
be present in the feed dissolver 132. Preferably, liquid
receiverg 121a, 121b, 121c ar each substantially full of
mixed ~other llquor. In the pr~ferred operation of this
embodiment, the mixed mother liguor i~ a ~ixture ol' ~other
liquor of crys~ ?.tion from that sa~l~ stage, t "'
~ 2~39967
--18--
with mother liguor reflux from the neYt purer stage which
was used to wash the same crystals in a ~tep during the
previous cycle.
In the f irst step of this processing sequence,
valves 137a, 137b, 137c or other dQvices performing a
valving function are opened, and pres~urized air or other
gas such as nitrogen entQrs respectiv~ liquid rQceivers
121a, 121b, 121c while valves 138a, 138b, 138c or the like
are open, with the result that the mother llquor Yithin
the liquid receivers 121a, 121b, 121c, ., . e ~ively, is
transported into the centrifuge basket 153a, 153b of the
next less pure stage, or in the ca~e of liguid receiver
121a into the feed dissolver 132.
After completion of this step, crystals 135a,
135b and 135d preferably are covered with or flooded by
th~ thus transferred mixed mother liquor by having the
liquid level in each centrifuge catch basin reach hiqh
enough to f lood the centrifuge basket to a level high
enough to cover any charge o~ dewatered crystals therein
and to permit a complete mixlng Or the flooded crystals
with the heated mother liguor. The ~ nd of the
conduit rrOm the hot end Or the mother liquor heat
~Yrh~nq~r is positioned to agitate the mixture of crystals
and heated mother liquor in the basket. AltQrnatively,
2S the crystal~ can be dissolved in the c~ntrirug~ baslcet by
having th¢ centrifuge catch basin be eD-pty and by ~praying
th- h~ated mother liquor over the exposed surface of the
~a~s of crystals in the basket. In this case, the basket
~ay or may not be spinning at the time of crystal
di~solving.
Crystal dissolving i8 carried out in the second
step. This dissolving takes place in the centrifuges
152a, 152b. ~ypically, the dissolving ~
assisted by activation of a circulatlon circuit 155a, 155b
having heat ~Y~hAnq~rs 136a, 136b ~nd pumps lS6a, 156b in
ord~r to circulate and mix the mother liquor and crystals
whil~ heating them to facilitate crystal dissolving. By
2139967
.
--19--
this actLon, the crystals that had been in each of the
centrifuges within the ~other liquor transferred thereinto
is transformed into the required volume or charge of
heated new solution for each of the respective stages.
5 This charge ls ready to be transferred as a liquid.
In the third step of the ~~ c-lcs~
herein, all of the solution within each Or the centrifuges
152a, 152b is transferred into the p,~ceOsor crystalli2er
lSlb, 127 of thQ next purer ~tage. In thQ pref~rred
10 illustrated ~ t, the transfer is Rrfected by
continuing or Ie inq operation of the pump 156a, 156b,
while closing valve 157a, 157b and opening valve 158a,
158b .
The fourth step, which may be carried out for,
15 after, or simultaneously with the third step, achieves
transfer of hot feed solution from the feed dissolver 132
into the p.oCa3soY cry~t~l ~1 zer 151a Or the next purer
stage. In the case of the illustrated . 'c~ . t, this is
Stage L. This transfer is typically effected by first
20 filling liguid receiver 121d with a volume of hot feed
~olution th~t i8 the same as the normal or p,. ~ ~ n~C~
controlled volume of new solution charg~. This feed
solution charge is then transferred from liquid receiver
121d into thQ ~ 8~r cryst~ 7~ 151a. In the
25 illustrated ~ ` '~ t, this i8 accomplished by opening
valv 137d and applying a positive p,. by w~y of a
manifold 123a while the vent 141d is closed, valve 142 is
closed, ~nd the valve 143d is opened. At this time, equal
volu~e~ of thus trans~erred hot f~ed solution and hot new
30 ~olution ar~ pr~sent wi~hin the Le, e ~iv ~ y~vCe5~0L
crystallizers 151a, 151b. Additionally, a charge of this
same volume of the purest hot new solution i5 transferred
to ~LocessuL crystallizer 127.
~ach thus transrerred l;olution i8 sub~ected to
3S crystallization conditions within a fifth ~tep of this
}"~c~du~. Cry~tallization iB typically racilitated by
reducing the ~ LUL~5 within the p~ ss~,L crystallizer
21~9967
.
--20--
151a, 151b, 127, such as through the cooling action of
heat oyrh~)lg~-r 136a, 136b, 136c, typically while stirring
th- solut~on during formation of the crystals. After
completion of this step, crystals are formed within their
S l~other liguor to form a ~lurry of crystals in a mother
liguor within the E,L.ac~ssoL crystallizer 151a, 151b.
In the next, or sixth step this crystal and
mother liguor slurry i8 transferred out of the ~,~ceD~-r
crystallizer 151a, 151b and into th- c4ntrifuge basket
153a, 153b of that same tage and pL~ 6~ assembly.
Crystals remain in the basket, and mother liguor is
allowed to pass into the liquid receiver vessel or vessels
of that 2;ame stage. In a preferred arr~ I ~ in this
regard, this mother liquor transfer i~ ~irst made from
centrifuge catch basin 159a, 159b into the reflux
compartment 122a, 122b by opening valves 144a, 144b,
typically while vents 145a, 145b are open 80 as to
facilitate ~ L of the mother liguor through the valve
conduit th~ . A controlled volume of ~other
20 liquor is thus transferred in each ~tage.
Similarly, the purest mother liquor, whlch c~n
include ~olvent feed 128, is transf-rred to th~ liquid
receiver vessel or vessels of thAt s~me stage, which i~
Stage P. This transfer can f ir~t be made by pa~sing the
25 ~ame controlled volume into the rerlux CoDlpartnlent 122c by
op ning valve l~.~.c, typically whil- v~nt 1~5c i~ open.
These transfers can be initiated by gravity,
although pumping or ~ - izing can also be practiced.
In th~ pr~ferred ' ~ which is illustrated, the
30 separation of cryatals from their ~lother liguor within the
Stage L and Stage I ~oceR~,.a is facilitated by
activation of the centrifuge 152a, 152b.
Typically, the reflux ~ 122a, 122b,
122c wlll not be large enough to hold all of the ~lother
35 liquor; accordingly, mother liguor tran~fer continues by
opening valves 139a, 139b, 139c, preferably while vents
l~la, l~lb, 141c are also open. Valves 14~,a, 144b, 144c
-
~ 2139~67
--2 1--
may be closed by this time. In a related, ~ieventh step,
the crystals within the centrifuge basket 153a, 153b are
subjected to a dewatering prcce-luL~. Centrifugation
continues in order to provide the function of a suction
5 f ilter or plate f ilter and achieves the benef icial result
Or withdrawing the mother liquor which r-mains between the
crystals after the initial transfer of ~other liquor out
of the centrifuges. During the course of the nother
liguor transfer of these sixth and seventh steps, the
10 newly formed crystals re~ain supported by the centri~uge
basket 153a, 153b.
The last step of this illustrated ~Lvc~luLe i8
optional, although it is pr~ferred. It is a ~tep of
w~shing the ~ust dewatered crystals with mother liquor
15 separated from crystals formed in the next purer stage.
In the illustrated; 'i- ~, this is most readily
accomplished by opening valves 146b, 146c and ~61SDUL izing
the ref lux compartments 122b, 122c by activating the
manifold 123a while valves 147b, 147c are open. As a
20 re~ult, thQ purer stage Dlother liquor passes ovQr and
through the crystalD within th~ centrirug~- 125, 126,
particularly their centrl~uge baskets 153a, 153b in order
to carry out a reflux wash p~v._eV,- Lel to r~move lea~ pure
mother liquor that remains even after dewatering. The
25 washing is facilitated because ther- i8 no ne~d to break
up or pulveriz- a cake to e~fectively vash the crystals.
~ .LVdU`~ in the form of moth~r liquor from the
least pur- ~tage exits the ~ystem upon opening valve 146a,
which re~oval ~ay be assisted by pl ~ ization of the
30 r~flux . ~ t 122a, ~uch as by activation o~ the
manifold 123a and opening of the valve 147a while vent
145a is closed. By the arrJ t shown in Fig. 2, it is
possible to have the same volume of mother liguor xit the
~y~te~ during ach cycl- as is tram~ferred a~ r~rlux fro~
35 each stage to the next less pure stage. Also, enough pure
~olvent QnterD the Stage P crystallizer to maintain enough
~other liquor therein to supply vessels 122c and 121c with
2~ 399~7
.
--22--
an adequate quantity of mother liquor. Crystal product
135c may be removed at any time in any suitable manner,
but typically after several cycles have been run whereby a
sizable quantity of superpurif ied crystals are available
5 for final collection.
If desired, only one centrifuge pL~e~60~ 126
can be used, in which event centrifuge p-o<;..~ ,. 125 and
any other less pure stage pLoce~3~r~ are vessels for
receiving and/or forming new solution. Each of such new
solutions of varying purity is then transferred, one after
the other, to the single centrifuge pLOCegS~lL 126 which
crystallizes and otherwise ~.~,ce6G~6 the crystals as
dirc---~e~ herein. For example, vessel 125 in this
arr~ _ -t is a vessel having heating c~rAhi I ities to --
form new ~olution as needed, and it receives mother liquor
from the next purer staqe which is inccr~v.~ted into new
solution then transferred to the centrifuge p~oce!;s~. 126
through valve 162.
In some cases, a combination of a cry8t~ 7~ r
and a centri~uge, which is similar to that used for Stage
I, ~ay bQ employsd for 8tage P. In thi~ ca~, th~
centrifuge ~ay b~! o~ th~ type that Qffect~ the discharge
of its load of d _te.2~ crystals from its centrifuge
basket at the appropriats time during each cycl~. When
this approach i8 us~d, it i~ preferred to add ~olvent for
r~f lux makeup during Qach cycle ~or cry~t~l vashing .
Also, in this case, th~ amount of Stage P mother liquor
holdup can be sl~aller than if a large crystalli2er would
be used for that stage.
C-ntrifugQ utilization cuch a~ t~d in
Fig. 2 is believed to provide a greater degree of liquid-
solid separation than the suction filtor ~.. r, of
Fig. 1. The Fig. 2 centrifuge arrA, ~ ~ is typically
preferred in situations in which the mother liquor i8
35 viscous and difficult to drain from the crystal, the
solute iB ~xceptionally soluble in the solvent, or the
crystals are ~ l ly fine or difficult to drain.
21~9967
.
--23--
It will be appreciated th~t the use of liguid
recelvers and/or ref lux compartments of known volumes can
play an important part in the metering function which is
important to ~aintaining a controlled ref lux ratio and
5 steady state conditions Volume and/or weight
ction~ can b- ~ade by f illing th~ ves~ uch
as the r~fluY compartment, to a desir~d level which can be
s ignified, for example, by a liquid lQvel responsive probe
within or associated with th~ vessel~ in order to dætect
10 when a certain volume of mother liquor has been attained
Other approaches include the use of overf low conduits or
the like An adjustability function is achieved, for
~xample, by providing multiple liguid level probes, a
select~d one of which is activated in order to vary --
15 controlled quantities and/or mark adJu ,i ~8 for
processing di~ferent crys~A~ hle materials
With more particular reference to the various
controlling and/or metering functions and means, the
following controlling and/or metering aspectfi are
20 preferred During each cycle, substantially the ~ame
volume or controlled quantity of ~iYed ~other liquor
should be tran~ferred from each stage to the neYt less
pure stage During each cycle, ~ubstantially the same
volume or controlled quantity of feed ~olution or new
25 solution should be transferred fro~ ~ach stage to the neYt
pur r ~tage During ~ach cycle, a controlled quantity or
volu~e of mother liquor should be discharged from the
leant pur- ~tage as ~ from the refining system,
which controlled volume should be substantially the same
30 as that Or tho r~rluY add d to stag ~ L by ~ean~ Or rerluY
~ 122c During each cycle, the weight of
crystal feed aaterial which is ~ n~ A~cl in the new feed
solution transferred to the pl~ e3sol should be
~ubstant~ally the ~ ame a~ the cry~tal~ for ed within the
35 crystallizer of Stage P The ref 1UY ratio i~ thQ ratio of
the weight per cycle of refluY solute (which is discharged
from each stage of the system, ~onitored for eYample at
2 4--
the purest staqe) to the weight per cycle of crystal
product rormed in this same stage. Generally speakin7~
the higher the reflux ratio, the greater is the difference
in purity per stage from one stage to the next purer
stage, and the lower the reflux ratio, the greater the
amount of crystal product p L~,-luced in each cycle.
Controlling the reflux ratio may include controlling the
weight of reflux solute that is transferred per unit of
time from each stage to the next less pure stage, and by
controlling the weight of crystal product within the
solution which is transferred per unit time from each
stage to the next purer stage.
As generally stated herein, it is desirable and
n~C/--:sary to maintain the same amount of solvent in each
stage during each cycle of a run. The same volume of
mother liquor is to be maintained in each stage at the end
of each cycle. With reference to Fig. 2 and the Stage P
reflux receiver 122c and liquid receiver 121c, the volume
of reflux being transferred from Stage P to Stage L during
each cycle i8 controlled by the volume of the Stage P
reflux receiver. In each of Stages I and L, the liguid
receiver 121a, 121b is preferably provided with an
overflow condult 165a, 165b that has a valve 166a, 166b
and empties into the reflux recQiver 122a, 122b of the
aame st~ge. PrQferably, the overflow level of this
conduit 165a, 165b i~ variable (such as having its inlet
be ad~ustable in height) to permit any desired volume of
moth~r liguor to remain in the liquid receiver after the
overf low had been discharged through the overf low conduit
to the reflux receiver.
This ~ILLCOl, t. provides a variable metering
assembly operated in the following manner, for example.
Durlng the centrifuging of the slurry, the mother liquor
i~ collected in that stage ' s liquid receiver as usual .
However, this receiver 121a, 121b holds only a
predet~in~d volume of this liguor and permits the
overage to overf low into that stage ' 8 ref lux receiver .
During the period when mother liquor is f lowing into the
~139~fi7
.
--25--
liquid receiver, the valve in the overflow conduit is
open. At the end of this period when the f low of ~other
liquor to the r~flux receiver i8 complete, this valve iB
closed. During the reflux transfer and crystal washing
S period, the valve 166a, 166b is closed, and the ref lux
effluent of the crystal washing is collected in the liquid
receiver. Therefore, none of the reflux effluent enters
the reflux receiver and all of it is retained in the
liquid receiver.
When this variable metering ~.o~ e-lu,~ is used,
the volume of reflux transferred from each stage to the
next less pure stage during each cycle is equal to the
volume of reflux transferred from the Stage P refluY
receiver 121c to Stage I. Also, the volume of ~wther --
15 liguor effluent derived fro~ the centrifuging of the
crystals in a stage (such as Stage I or Stage L) is equal
to the holdup volume of that stage's liquid receiver plus
the ref lux volume received by that stage during a cycle.
The holdup volume is the volume of mother liquor retained
20 in a stage's liguid receiver after the excess other
liquor i~ through the overf low conduit . ~hen
this ~yste~ is op~rating under normal ~ ~ium
conditions, the weight of ~olvent in each Stage I and
Stage L is equal to that contained in th~ liquid receiver
25 and the ref lux receiver of that stage at the co pletion of
the ov~rf low of ~oth-r liquor to th~ r~f lux r~cciv .
For example, Stage P reflux receiver 122c could
hav~ ~ on- gallon volume, and the holdup volume of the
Stag L and the Stage I llquid receiver could each be 4
30 g~llon~, ~nd ach would retain ~. g~llons ~Ift~r th- ~ccess
of ~other liquor ha~ f lowed out through the overf lo~
conduit. Therefore, during e~ch cycle, one gallon of
Stage P reflux is transferred to Stage I. In this
exa~ple, ~s soon a~ the system reaches ~ h'-iUII~
35 operation, on~ gallon of mother liquor overf lows from the
Stage I and Stage L liquid receiver lnto that ~a~e ~ctage's
ref lux receiver during each cycle . The net result of this
_ ~
2139967
method of operating the refiner in this example is that
there would always be 5 gallons of mother liguor in each
stage (Stage I and Stage L) during the liguid-solid
separation period of each cycle. By suitable variation of
the metering oYerf low conduit 165a, 165b, thQ volume of
reflux for a run could easily be changed to, for example,
2 gallons. The volume of th~ Stage P re~lux receiver is
changed from one to two gallons, and the lovel of the
overflow conduit 165a, 165b is lowered to reduce the
holdup volume of each o~ these receivers to 3 gallons.
Mother liguor holdup in Stage I and Stage L remains at 5
gallons during the liguid-solid separation period of each
cycle .
An indirect metering approach can be used by --
providing sample ports or the like in particular vessels
as needed . The percentage of solute in the ref lux is a
function of its specific gravity, and monitoring sa~e ~ay
be useful in providing the proper volume of reflux needed
in order to result in having the correct weight of solute
transferred as reflux. It may also ~ desirabl-- and
preferred to Donitor and control the ~p~cific gravity of
the new solution made during each stage.
When a steady statQ condition is achieved with
the present invention, the ~ystem exhibit~ the following
charactQri~tic~. mQ ~ame w~ight of cry~tA~ hle
~atQrlal to b~ purified i~ present in th~ nQw ~olutisn of
each tagQ while crystallizing is occurring. Each stage
thus y~ ~a the same weight Or crystal product during
each cycl~ he same weight of L~y ~lu~ ~ ~olute is
pLud~ d during Qach cycle. me wQight of product per
cycle i~ substantially thQ same as the weight of feed
dissolved in the feed dissolver per cycle minus the weight
of L1YL~1U~ ~ solute ploduced per cycle. Each cycle has
ial ly the ~ame ref lux ratio . Steady ~tatQ
condition r-intonAn~ e typically in~ os ~lonitoring and
(when n~co~sAry) ad~usting the final t~ ~tuL~ of the
crystallization step of each stage. Under normal
21399~
operating conditions of the refiner, this temperature is
the same as the liguid-solid separation t~ _tuLe in the
processors. In addition, this temperature is also the
same as the crystallization temperature of the mother
liquor effluent from the liquid-solid separation step.
The weight of solute per liter of solution is a function
of the crystallization temperature of the solution and
usually increases with a rise in this temperature.
Therefore, the operator of th- refiner can best regulate
the weight of solute in each volume or liter of mother
liquor by adjusting the final t~ _Lur~ of the
crystallization step for each stage. It is preferred that
this temperature be the same for all stages and for all
cycles in order that the mother liguor of all stages
contain the same weight of solute per liter. All of the
new ~olution for recry~itallization which is formed in each
stage i8 transferred to the neYt pur~r stage during each
cycle. Product, L~rvduvL and feed are likewise
controlled
It will be appreciated th~t the multistage
r~ c~ { --tion system of this inv~ntion is one which is
carried out in a totally on~los^d ref iner which can
operate under 5u~ ric p~e _..IL~6. Such a totally
on~losed system can ~eet the sanitary standards required
25 by, for eYample, the rh~ ic -l industry.
S~.,.__~1 "' lc pl~ operation per it~ the refining
of subst~nces that must utilize -, I.eated solvent to
dissolve a L. - - - hle quantity of crystal material ln the
cryst~l dissolver of a given stage. Thi~ permits the
30 proc~o~-~in7 of ~aterials such as ter~rh~h-l ic ~cid, which
dissolves in water at 235- C.
~ ihen procee~n~ with the preferred method
incvL~.o~-ting centrifuge te~ hn~ , it i~ de~irable that
the slurry which f lows to the centrifuqe basket be
35 maintained in a very fluid state to thereby prevent any
possibility of clogging of the conduit leading fro~l the
Cry8tAl l~ ,70r to the centrifuge b~sket. Important in this
~ ~39967
--28--
regard is preventing the percentage of solid or crystal
material in the slurry from exceeding 10 to 20 percent by
weight ~pen~ling upon the sub6tance being refined. Found
to be of assistance in this regard is continuously pumping
5 the body of new solution in a circuit including the
centrifuge catch basin, th~ crystallizer and the
centrifuge basket. This cycle is generally illustrated in
Stage L of Fig. 2. The circuit inrl -- the crystallizer
l51a, the basket 153a, the catch basin 159, operation of
the pump 156a and opening of the valves 158a and 161 while
valve 162 is closed . The new solution erf luent leaving
the centrifuge catch basin is substantially free Or
crystals, with the crystals in the slurry having been
deposited into the centrifuge basket. The temperature
15 drop of the mother liquor passing through the heat
oYrhAn~or 136a should be limited such that the peL~_..t~lge
content by weight Or solid crystal material in the slurry
leaving the temperature dropping heat ~Y~hJ~ngor 136a would
be low enough so the slurry is sufriciently rluid to
20 pr~vent any po-~ihle clogging of the conduit 163 between
the crystA 11 i 7~t' and the centrifuge bask~t .
Possibl~ clogging can also bQ ~lini~i7~ by
conducting at least two distinct, ~L~33sively cooler
crystallization steps. The rirst such ~tRp would be at a
25 t~ to produce a vQry liquid slurry Or crystals
which ~ould easily pa~s through conduit 163. Aft~r the
mother liquor thereof is passed through thQ cyclo ~ust
d~ ~ in order to re-enter the crys~ Al 1 i z-r 151a, the
t~ ur~: provided by the heat oYrhr~ j 136a is lowered
30 ~o that additional crystal~ wlll rorm ror pa~age through
the conduit 163. Thus, each slurry passing through the
conduit 163 contains fewer crystals than if all of the
cryst~lli7Yhlo material within the new solution were
crystAl 1 ~ and p_ssed into the centrirug~ in a single
3 5 step .
During refining, Dlaterial balance and material
rlow ar~ controlled as follows. The same volume of war~
_ . .. , .. _ .. _ _ _ . _ . =
2139967
--29--
new feed solution from the feed dissolver is metered to
the stage L process vessel during each cycle. The same
volume of cold mixed mother liquor produced in the Stage P
crystallizer is metered to the process vessel of Stage I.
5 All of the mixed mother liquor which is collected in the
liguid receiver of each stage is transferred to the next
less pure stage during each cycle. All of the warm new
solution produced in the process vessel during each staqe
is trans~erred to the process ve~sel or crystallizer of
10 the next purer stage during that cycle. All of the
crystals which are produced in each stage ' 8 process vessel
during each cycle are separated from their mother liquor
and then dissolved in mixed mother liquor from the next
purer stage during each cycle. -
During any cycle, the same volume of reflux
liquid is transferred to the next less pure stage or
discharged as D~rv-lu~;~, and this same volume of pure
ref lux Ls generated in Stage P. During any cycle, the
same weight of reflux solute is transferred from each
20 stage to the next less pure stago; or, in the case of
Stage L, to a L~lvl~ L discharge conduit. It h~s been
found that the best method to do this i8 to transfer the
~ame volume Or re~lux per cycle from each ~tage and to
utilize the same liquid-solid separation t~ ~u~æ for
25 all llguid-solid separation ~vc~slu~G3 con~ tel during a
run. Enough teed crystal material and warm new feed
solution ~re ---intainod in the feed dissolver to provide
~nough crystal material and solution to the Stage L
process vess_l during each run. The Stage P cry8~ 1 i 70-'
30 has enough ~other liquor holdup to provlde the requlred
supply thereof ~or the next less pure stage during each
cycle .
Each body of warm new solution that is red to
the process vessel or crystallizer and each body of cold
35 mixed mother liquor that is transrerred to the process
vessel or feed dissolver contA i n~ substantially the same
weight of solvent during each cycle of a run. Each body
21~9967
--30--
o~ olution t~t ~lc~ tvo ~diac-nt proc~- ~ ls
' ~ I vith ~ con t nt ~^~ 3 ' wolght Or ~olv~t
~h1d $~ ~ a~ thQ ~lght in ~ach charg ~ of ~ n~r
r-ed
6 A-- rOr ~ t~ , dur~nq th~
w~ 3 ~rio~ o~ ~ch oycl, t~ ~ ulnl~
t ~ 1~ chl-v d ~n th ~l~rry
ach proco~ v~l or ~ hid lnl~
to b- abo~r th '- - at ~hlc~l
10 ~ol~nt cryut~ ~ld t~rt to rOr~ rro~ t~ ot~ r
13~uor 1~1 o, durlnq tho cry~l d~ol~r~ng p rlod Or
~ch cycl~, th ~iYtllr- Or cry-t-l u~ ~ ~oenor
ll~uor in ~ach proc--- ~-a-l ahould ~ h~atod
. ~1 3-- ~ ly to d~13~01v All Or e~ e~l~ ln tho
1~ ~xtur-, u~ llr ~t t, _ cnly ~~
, ~o~ tn- t ~ n th S~ ai--ol~
~ I' , t~ ~iYtur Or r~ cry~Cal- and noehor
liquor in t)~ ~ -olv r anoul~ ~~~ ae a
con tant c ~uch th~t ~ch charg~ or varlD n~v
20 r ~ld ~olutlo~ h~ th~ u~ crya"-~ t~ t, ~'
. am~ L ~ ~g thQ
pr nt lnwntlon r~ ~u~tabl rOr _ _ _ r~-t~on or
~r$ou~ "1- Inorg nlc ~1 orq n~ '~1~
Ylthin olv m U~roror, ~ucn ~a vat, ~coho~, th r-,
25 lc~t~, ~I tho lilC ~plary l~ ~ ~t 1t1, -1-
~t rhll~ in tnl~ r g~ inclu~ cl~lor~a~,
~_ chlorld~, C0~ cblorl~, borlun n3trat-,
coppar nitr~t~, olSiu~ loald-, b riu~ cmorla-, copp r
ulr t-, nicl~ ulr~t~, ~lu~ ~luninu~ ~ulrlt-,
30 a;, onlu~ olyb~la~ nyarocmorla-, ur-~,
~ o -, citr1c ac3d, acry~id, b-n~oic Ic~d, ~1
thQ llKo ~ns rn~ lc ~g ~a pl~ t l l ' ' ~alll E#Clr10
~,.. - ~uitabl- rc~ th- typ-- Or at-rl~l~
~L~
lc~ r ~yat-a or th~ typ- g~r~lly
311 ,~ ~1 ls~ F3g 1 i~ d rOr th I~ur~ on Or
2139967
--31--
potassium chloride, using water a5 the solvent. The
following steps are provided for readying the system.
Fifteen kilograms of distilled water and 10
kilograms of potassium chloride feed are added to the feed
S dissolver. This added water provides a permanent holdup
of 15 kilograms of water solvent which remains in the
~olution body during the entire run. This water and feed
are then heated to 40 C. and mixed until the feed
dissolver contains 4 kilograms of undissolved crystals and
10 21 kilograms of warm "new" ~eed solution. This solution
contains 15 kilograms of water solvent and 6 kilograms of
potassium chloride solute. It also has a crystallization
t~mperature of 40- C. Distilled water (S kilograms) i5
added to a vessel of the ~ v~_e6sv~ assembly of Stage L, as
15 well as to that of any Stage I. This addition provides a
permanent holdup of 5 kilograms of water solvent which
remains in the solution body of each such stage during the
~ntire run. Distilled water (10 kilograms) is added to
the crys~l 1 i 7~r p~vcesDvr of Stage P in order to provide
20 a permanent holdup of 10 kilograms o~ water ~olvent which
remains in th~ Stage P solution body th~v_~hv-.L th~ ~ntire
run. The sy-tem is run for a ~urficient nu~er of normal
cycles until potassium chloride passes through
pLv~L~sively purer stages and finally precipitates out of
25 the solution to form the Stage P crystal product. At this
stag~, the refiner i~ in a normal ~teady 3tat- operation.
Approximately 10 initial cycles achieves this initial
~tion in the Stage P cryst~
The ~Lvc~lu~c within the refiner iB described
30 ~inning with the cry8tallization step. The warm new
xolution or new feed solution that was just transferred to
th~ process vessels and to the crystallizer is chilled
from 45 C. to -10 C., causing approximately 800 grams
of potassium chloride crystals to form while stirring.
35 Stirring ceases, allowing the crystals to settle in a
~ubstantially ~ven depth on top of thQ scre~n of each
process vessel. Mother liguor i~ drained to th~ reflux
21~9967
\
--32--
compartment or vessel of that same stage until it ifi
completely full of 1240 grams of reflux. Mother liquor
drainage continues into each same stage liguid receiver,
and a vacuum is drawn to "pull" mother liquor between the
5 crystals into the liquid receiver of at least pure Stage L
and in Stage I. Enough Stage P mother liguor is drained
from the product crysta~ r to completely fill the Stage
P l~quid receiver with 6200 grams of cold mother liguor.
The transfer of mother liquor in each liguid receiver to
10 the process vessel of the next less purQ Stage L or Stage
I or to the feed dissolver is effected.
Next, the charge of Stage L mixed mother liquor
in the feed dissolver is heated to 40 C. and mixed with
the feed crystals until it dissolves 800 grams of these ~-
15 crystals. The charge of Stage I mixed mother liguor inthe Stage L ~ essol is heated to approximately 45- C.
and mixed with the Stage L crystals thQrein until ~111 of
the crystals have dissolved into it. me charge of the
Stage P mixed mother liguor in the Stage I ~Lo- CJ~ .L is
20 heated to approximately ~5- C. and is mixed with the Stage
I crystAls th~roin until all Or those cry~tal- have
dissolved into it. During thQ cycl- and ~ny time after
the Stage L reflux vessel is ~illed with Stage L lother
liquor, its contents ~re disch~rged from the refiner as
25 Ly~lu~,L solution which iB 1 kilogram water nnd 2~0 grams
o~ potA~i chloride solute.
During each cycle of this Example, 800 grams of
potassium chloride cry-,tals are transferred one stage
~e DLL~ ~ and 240 grams of reflux solute are transferred
30 one stage d~ LL~am. Also, during each cyclo,
approxi~ately 560 grams of stage P crystals rinal product
and 240 grams of reflux solute feed for the refiner are
pLv~ ced in the Stage P cry~tall~o-. In addition, 6200
gr~ma of Stage P mother liguor is ~JL ~ d~ICCI in this
35 crystal 1~7~--. A complete cycle i8 calculated to ke
completed every hour to provide ~ daily production of
L~uLified potas~ium chloride crystal product of 13.44
2139967
--33--
kilograms per day from 19 . 2 kilograms of feed, with 5 . 76
kilogra~s of L~y~Odu-,L solute being discharged each 24
hour day.
If desired, this b2~yludu-,L solution may be
S treated in an evaporator f or removing three-quarters of
the water solvent by evaporation. This permits three-
quarters of the potassium chloride solute to be ~._v~ed
as crystals of substantially the same purity as thQ feed
crystals, which would allow these crystals ~4 . 32
kilograms) to be recycled to the feed dissolver each day.
The mother liquor from the evaporator would be a very
impure byproduct.
The cold mother liquor is to contain 240 grams
of pota6sium chloride solute for each 1000 grams of water -
lS solvent and is to have a specif ic gravity of 1.14 gms/cm3 .
The warm new solution is to contain 400 gra~s of potassium
chloride solute for each 1000 grams of water ~olvent and
is to have a specific gravity of 1.2 gms/cm3. Each
~tage ' 8 ref lux receiver is to have an internal volume o~
1.09 liters. When each of these vessels is filled with
cold reflux mother liguor, there are 12~0 grams at the
given specific gravity. The wQight of ~iYQd ~other liguor
transferred during each cycl~ from out of the liquid
receiver of each stage to the crystals of the next less
pur- tage ~r-nta~-~ 5~000 gram~ of water and 1,200 grams
of p-?t~i chloride, for a total w~ight of 6,200 grams.
The volume calculated from its specific gravity is 5 . 438
litor-. The war~ new solution received in each stage ' s
liguid r~ceiver during each cycle weighs 7,000 grams,
containing 5,000 gra Or water and 2,000 gr~s of
potassium chloride and occ~ri~c a volume of 5.833 liters
calculated from it~ specific gravity. Neither the
liguid receivers nor the ~L-,- ess~ of Stage L and Stage I
should ever be complet~ly filled during any cycle Or a
run. Accordingly, each is to be larger than these
volumes, six liters being suitable. The volume of the
Stage P liguid receiver regulates the volu~e of ~ixed
~_ ._,_ _ _. __ . _ ,, . __ _ _ . , .. . .. _
9~67
--34--
mother liguor transferred to the next less pure process
vessQl during each cycle. In this Example, it is desired
that 5.438 liters of this mother liquor be transferred,
making the reguired internal volume of the Stage P liquid
5 receiver 5 . 438 liters . on the other hand, the volume of
the feed solution receiver should be 5 . 833 liters in order
to properly regulate the volume of new reed solution fed
to the Stage L process vessel during each cycle.
me solution body that is to transfer bQtween
10 the feed dissolver and the Stage L pLoces60~ during each
cycle is used to dissolve 800 grams of potassium chloride
fed into the feed dissolver and to deposit 800 grams of
dissolved potassium chloride into the process vessel
during each cycle in order to form the Stage L crystals. --
15 In this Example, this body of liguid may contain 15kilograms of wat~r solvent and a variable amount of
potassium chloride solute.
Another solution body which is to transfer
bQtween the Stage L and the stage I p~ O.~ L ~ during each
20 cycle is used to dissolve 800 grams of Stage L crystals in
the Stage L ~ and to deposit 800 gra3s of Stage I
crystals in the Stage I ~L ~ during ach cycl~. This
entire solution body may contain 5 kilograms of water
solvent and a variable ~mount of potassium chloride
25 ~olut~ .
Anoth~r ~olution body which i~ to transfer
between the Stage I y~oc~as~. vessel and the Stage P
crystallisQr during ~ch CyClQ i8 used to dissolve 800
gra~s of Stage I crystals in the Stage I process vessel
30 ~nd to provide 560 gr~ms of Stage P cry~tal-.
meorQtically, this i8 a deposit of 800 grams of Stage P
cry~it~lliz-hle material in the Stage P crystAlli.~er.
However, 240 grams of these crystals are c - ' in the
production of reflux feed out o~ the Stage P cry5t ~
35 making the net pro~ ti on of gtage 3 crystals 540 grams
per cycle. ~L~ u~ the entire run, this solution body
may contain a _ ~AntiAl ly constant 10 kilograms of water
. . . ~
99~7
--35--
solvent and a variable amount of potassium chloride
solut~, d~r~n~linq upon temperature conditions at any
particular moment in any cycle.
In this Example, reflux feed for the refiner is
5 to be generated in the Stage P crystalliz~r. During each
cycle, 1 kilogram of pure water solvent i8 to be
introduced into the Stage P cryst~ r to ~lake up this
required reflux feed. At a temperature of -10 C., this
kilogram of water will dissolve 240 grams of Stage P
10 potassium chloride crystals to produce 1240 grams of
reflux to account for the transfer of 1240 grams of Stage
P ref lux to Stage I during each cycle. This material
balance enables the Stage P solution body to contain
approximately the required 10 kilograms of water solvent
lS during the entire run.
The feed dissolver for this Example may have a
capacity to hold up to 10 kilograms of feed crystals and
up to 15 kilograms of warm new feed solution. The Stage P
crystAl 1; 7~r is large enough to hold up to 8 kilogra~s of
20 ~: lated crystals and up to 10 liters of l~other l$quor.
E le 2
A three stage ~ L l~ication sy~tem of the
type generally illustrated in Fig. 2 is used for the
25 purirication of g~-ni~linA hydrochloride, using water as
the olv nt.
Each centrifuge assembly for Stages L and I had
a perrorated basket lined with 100 mesh screen, an
int~rr-l diameter of 25 cm (~0 inches) and an inside depth
30 Or 25 c (10 inches). Each basket wa~ supported by ~
rotatable shaft driven by a motor mounted on its top end.
Each catch basin had an internal diameter of 3S cm (14
inches) and a depth of 3S cm, its interior bottom surface
being located 2 cm (0 . 8 inches) below th~ bottom of the
35 basket. The central axis of the catch basin is the same
as that of the basket. The catch basin had a bottom
opening at its lowest point leading to a lS liter (3
. ~
~g9fi7
--36--
gallon) per minute circulation pump in the position shown
in Fig. 2. The heat ~Yrh~n~r of each stage, located as
in Fig. 2, had a tube through which mother liquor was
pumped in an upward direction by the circulation pump.
5 This tube was ~urL~ullded by a tube of larger diameter
through which hot water was pumped in a downward
direction. The hot water was circulated to and from a
large tank equipped with thermostatically controlled
heating means to maintain th- hot water at a c ~ t
10 te~perature of 80 C. The crystallizer for each stage had
an internal dian~eter of 20 cm (8 inches), an internal
height of 55 cm (22 inches), and a helical coil in its
interior through which cold water can be circulated. The
crystallizer was equipped with a stirrer of the paddl~ :
15 type which agitated the slurry sufficiently to keep the
crystals in suspension th~u~ JuL each crystallization
period. Each crystallizer was equipped with a plug type
valve at its bottom which controlled the f low of slurry
from its lowest point to a conduit leadlng to the interior
20 of the centrifuge bafiket of the stage. me coil in each
cryst~ ^r wa~ chilled with cold wat^r.
The following I~OCadUL~8 aro c~nd~l~ted for
readylng the refiner for normal pr~l~ ti~
30 kilograms of distilled water and 120
25 kilogra~s of g--ni~lin^ hydrochloride f-ed crystal~ were
placed in th^ f-ed di~solver of the r~finer. Then, the
miYture of these materials was heated to 50 C. with
stirrlng to produce a body of warm (50- C. ) new feed
liolution having a crystallization t~ ~LULC of 50- C.
30 This heated body cont~in^d approximately 72 kilogra~s o~
solute and 28 kilograms of water ~olvent. Also, ~8
kilograms of undissolved crystals r. - i n^d in the feed
dissolver after the heating step had been completed.
Throughout the L~ in~^r of the run, the contents of the
35 feed dissolver was maintained at 50 C.
The Stage I llquld recelver was fllled wlth 20
kilograms of Stage I ~other liguor left over frol~ a
~1~9957
--37--
prQvious run. Also, the StagQ P liquid receiver was
filled with 20 kilograms of Stage P mother liquor which
was left over from a previous run. Each of these bodies
of left over mother liquor had a crystallization
5 temperature of 15 C and contained approximately 7
kilograms of water solvent and 13 kilograms of solute.
The Stage P crystallizer was filled with 40 kilograms of
Stage P mother liquor left over from a prQviously
con~ ed run. This had a crystallizatlon ~ tUL~ of
15- C. and contained 14 kilograms of water solvent and 26
kilograms of solute.
After these additions are made to the refiner,
three special or start-up cycles are conducted to yL_~L~88
crystal material from the feed dissolver through Stage L, ~-
15 I and then finally to the Stage P cry~tallizer. The firstof these cycles involves these steps:
1. The Stage L crystallizer i~ f illed with a
charge of 25 kilograms of warm feed solution rrom the
crystallizer. This charge contains approximately 7
20 kilograms of water and 18 kilograms of solute and occupies
A volume of 21. ~, liters.
2. The Stagc L cry~t~ r chills this charge
with stirring to 15- C. to produce a slurry containing
about 5 kilograms of crystals and 20 kilograms of mother
25 liguor.
3. Then, this slurry i8 allowed to flow
gravitationally into the Stage L centrifuge basket where
it i8 guickly c~ntrifuged to retain the dewatered crystals
in the c~ntri~uge basket and allow the mother liquor to
30 f low into the Stage L liquid r~ceiver. (ThQ StagQ L
ref lux receiver need not be used at this ti~e . ) The
~ctual t~ tUL e and the crystalliz~tion temper~ture of
the ~other liquor effluent from the centrifuge is 10- C.
4. The Stage I mother liquor i~ transferred
35 fro~ the Stage I liquid rQceiver to thQ stage L ~other
liquor heating circuit where it is circulated through the
Stage L crystals in the Stage L centriruge basket while
21399fi7
--38--
being heated to a final temperature of 60- C. As a result
of this heating and being mixed with th~ Stage L cry~tals,
it rapidly dissolves all of these crystals. This results
in a body of Stage L new solution having an actual
5 temperature of 60 C., a cry6tallization t~ tu.e of
50 C. and a composition of 7 kilograms Or water solvent
and 18 kilograms of solute.
The second of these cycles involves these steps:
1. A 25 kilogra~ charge of new feed solution is
10 transferred from the reed dissolver to the Stage L
crystallizer. At this same time, the 25 kilograms of
Stage L new solution is transferred from the Stage L
heated mother liguor circuit to the Stage I crystallizer.
2. The Stage L and the Stage I crys~ rs
15 chill their charges of new solution to 15 C. with
stirring .
3. The Stage L slurry is transferred from the
Stage L crystallizer to the Stage L centrifuge basket and
the Stage I slurry is transferred from the St~ge I
20 crystallizer to the Stage I centrifuge basket.
4. ThQ StagQ L ~nd Stage I centrifuges are
operated. m Stage L crystals are retained in th~ Stage
L centrifuge basket, and the Stage I crystals are retained
in thQ Stage I basket. The Stage L mother liquor is
25 collect-d ln th~ Stage L liquid r~ceiv r, and thQ Stage I
~oth~r liguor i~ ~ol 1~1 L~ ~ in the Stag~ I liguid receiver .
5. The Stage L mother liquor is transferred
from the stage L liquid receiver to the feed dissolver
wher~ it i~ heated to 50 C. ~nd dissolves an additional 5
30 kilograms of foed crystals. Just befor~ thil~, a 20
kilogram charge of Stage I mother liquor, left over from a
previously c~n~ t~d run, i8 placed in the Stage L liquid
receiver. Then, this Stage L mother liquor is transferred
to the Stage L mother liquor heating circuit where it is
35 heated to 60 C. and dissolves all of the Stage L crystals
in the Stage L centrifuge basket. Thi~ results in a
normal sized body of new Stage L ~olution in said circuit.
,, . . ~
~ ~39967
--39--
At the same time, the 2 0 ki lograms o~ Stage P mother
liquor i8 tran6ferred from the Stage P liguid receiver to
the Stage I mother liquor heating circuit. This circuit
i5 now operated in the usual manner to dissolve the 5
5 kilograms of Stage I crystals out of the Stage I
centrifuge basket and produce a body of 25 kilograms of
heated Stage I new solution.
The third of these cycles involves repeating the
rive steps c^n~ t~d in the second cyclQ plu~ these
10 additional operAtions in each of them:
1. The body of Stage I new solution i8
transferred from the Stage I mother liquor heating circuit
to the Stage P crystallizer .
2. The Stage P crystallizer chills its 255 kilogram charge of Stage I new solution to 15 C. and
.~,du~es 5 kilograms of Stage P crystals.
3. The newly formed Stage P crystals (from step
2) may be retained in the stage P cryst~l 1 i 7~ until it is
convenient to harvest them.
~.. Twenty kilogr~ms of the Stage P ~other
liquor, having ~ t~ ..L~ of 15- C., i~ tr~nsferred
from the Stage P cryst~llizA~- to the Stag~ P liquid
receiver.
~ t the conclusion of this third cycle, the
25 refiner i- r~ady for the rir~t of ~ ~eri~s of nomlal
proA~ ti . cycl~ ach of these is ~ith th .. of
two k~lograms Or reflux solute ~ Lr~am for every five
kilograms of crystal motion upstream. In this c~se,
d ~ . .,~ ~eanr~ rrom a stage to the next ~Itage
30 of lovur purity. In this case, the voluDle of e~ch stage's
r~flux receiver is ad~usted to hold 3.077 kilograms of the
~olution having a speci~ic gravity of 1.12 g~s/cm3.
Therefore, the volume of each reflux receiver is 2.75
liters. ~S~ of material during each norDI2l1 cycle5 include:
Net crystal production of Stage P 3 kgs
Reflux ~olute generated in Stage P 2 kgs
~13~7
--40--
Water addition to stage P to make ref lux 1077 gms
Crystal production in each staqe 5 kgs
Solute leaving as byproduct from Stage L 2 kgs
Feed crystal6 dissolved in feed dis601ver 5 kgs
Each prod~ ti~n cycle incu-~v c.tes these steps:
1. LoAdin~l each stage's crYstallizer with new
solution: Stage L crystallizer receivers 25 kilograms of
new feed solution from the feed dissolver. Stage I
cryst~l 1 i 7~r receives 25 kilograms of Stage L new solution
from the Stage L mother lio,uor heating circuit. Stage P
crystallizer receives 25 kilograms of Stage I new solution
from the Stage I mother lio,uor heating circuit.
2. Crvstallization ste~7: The new solution
introduced into the crystallizer of each stage is chilled ~-
to 15 C. to produce 5 kilograms of crystals and 20
kilograms of mother liouor of the respective stage.
3. J,iouid-solid ~ ration stem: In stages L
and I, the slurry ~ust pl~-lu-,~d in the crystallizer of the
same st~ge is centrifuged with the dewatered crystals
being retained in the centrifuge basket and the ~other
lio,uor Qf f lu~nt being collscted in the ref lux receiver and
the liquid r-ceiver of the ~ame ~t~ge. The r~flux
receiver of both of the6e st~ges i5 ~d~usted to retain
2.75 liters of mother lio,uor as reflux. The balance of
16.923 KGS (lS.l liters) of the mother liquor of eAch
~tage i~ ~ol 1 ~ct^~l in that same st~g~ ' 8 lio,,uid receiver .
4. Refl~ transfer ~n~l crystal w~,~;hin~7 step:
This step is conducted in Stages L and I and involves the
transfer of the reflux mother lio,uor from the reflux
receiver of the next purer stage to the crystals in its
centriruge basket. These centriruge baskets may be
~pinning ~t the time of cry6tal washing. The effluent
from this washing operation is collected in the lio~uid
receiver of the same stage as the centrifuge in which the
washing is conducted. In both of these stages, the rerlux
wash erfluent, originating from the next purer stage,
~ixes with the mother liouor ~lready ~ol lectecl in the
2139g67
liquid receiYer of that stage. This mixture is called
mixed mother liquor. Thus, at the end o~ this step, there
are 20 kilograms of Stage L mixed mother liquor in the
Stage L liquid receiver and the same weight o~ Stage I
5 mixed mother liquor in the stage I liquid receiver.
5. Crystal ~Cnlvin~ steD: The mixed mother
liquor of each Btage i8 transferred to a heated mother
liquor circulation system of the next lower purity stage.
This circuit of each of these stages provides means to
10 cause the heated mother liquor to mix with and dissolve
the crystals in the centrifuge basket of said stage. This
results in a body of 25 kilograms of new Stage L solution
in the Stage L mother liquor heated circuit and a like
amount o~ Stage I new solution in the Stage I heated --
15 mother liquor circuit.
At this time, the ref iner is ready ~or the start
of the next cycle. As additional cycles are conducted, a
sufficient supply of feed crystals are inLrvdu- ~ into the
feed dissolver to prevent it from running out of crystal
20 material. Su~ urified crystal product is harvested from
the St~ge P crystallizer often enough to prevent that
cry6tallizer from be~ 1n~ overlo~ded with crystals.
It will be understood that the ~-mh~11 Ls of
the present invantion which have been described are
25 illustrative of some o~ the ~pplication~ o~ th~ principles
of the pr~sent invention . Nu vù8 modif ications may be
mad~ by those skilled in the art without departing from
the true spirit and scope of the invention.
._ . _, . .. .
