CA2139967A1 - Closed system multistage superpurification recrystallization - Google Patents

Abstract

A method and apparatus are provided for superpurifying crystallizable substances to a purity level of 99.999%. Included is a closed multistage system which maintains controlled transfers of material between a plurality of stages. The method and apparatus avoid the need to transfer solid crystals from one stage to another and effect efficient and very effective separation of purified crystals from their mother liquor.

Description

.

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 . ~, _ . , . . . , , ,, _ .

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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 .

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 - , .

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.
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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~

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 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~
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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 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 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 .

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 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 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 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 .

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 .

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 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 .

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 .

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 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 .

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 _ ~

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 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~
_ . .. , .. _ .. _ _ _ . _ . =

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 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 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 \

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 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 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 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 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 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 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 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 25 5 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 cycle 5 include:
Net crystal production of Stage P 3 kgs Reflux ~olute generated in Stage P 2 kgs ~13~7 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.

._ . _, . .. .

Claims (29)

1. A method for superpurifying crystallizable substances by a multistage recrystallization procedure, comprising:
providing a plurality of stages having vessels with crystallizable substances therein and operating said stages by proceeding with a sequence of steps in a cyclic manner through a plurality of substantially duplicate cycles conducted substantially simultaneously within each stage such that the crystallizable substance is dissolved in each stage beginning with a least pure stage, transferred as a fully dissolved component of a new solution to the next purer stage, and recrystallized in the next purer stage until superpurified through recrystallization in the purest stage, the sequence of steps including:
(a) transferring a quantity of mother liquor from a liquid receiver of each stage into a processor having crystallizing capabilities of each next less pure stage containing crystals of the crystallizable material;
(b) dissolving the crystals of step (a) within the mother liquor of step (a) to form a new solution in each processor of the next less pure stage;
(c) transferring each new solution formed in step (b) to the processor of the next purer stage;
(d) crystallizing crystallizable material within each new solution of each processor to form crystals, said crystals of the most pure stage being a recrystallized superpurified substance;
(e) separating the crystals formed in step (d) from its mother liquor in at least the processors of stages other than the most pure stage to provide separated crystals, and collecting the thus separated mother liquor for subsequent use in step (a) of a next cycle; and (f) discharging mother liquor as a byproduct from the least pure stage.

2. The method of claim 1, further including the step of washing the separated crystals of step (e) with mother liquor from the next purer stage.

3. The method of claim 1, wherein said step (e) separating includes further separating mother liquor from the separated crystals of step (e) to thereby withdraw mother liquor from between the crystals and provide dewatered crystals.

4. The method of claim 3, further including the step of washing the dewatered crystals with mother liquor from the next purer stage.

5. The method of claim 2, wherein the mother liquor transferred in step (a) is a mixed mother liquor formed by combining mother liquor from that same stage with mother liquor passed into that stage during said washing step.

6. The method of claim 4, wherein the mother liquor transferred in step (a) is a mixed mother liquor formed by combining mother liquor from that same stage with mother liquor passed into that stage during said washing step.

7. The method of claim 1, wherein step (a) is carried out within a centrifuge processor having a basket containing the crystals whereby the new solution formed by dissolving step (b) is contained within the centrifuge; the step (c) transferring of each new solution is to a processor crystallizer of the next purer stage; and said step (d) crystallizing step is carried out within each said crystallizer.

8. The method of claim 7, wherein said step (d) crystallizing is carried out in a plurality of crystallization steps each crystallizing less than all of the crystallizable material within each new solution.

9. The method of claim 3, wherein step (a) is carried out within a centrifuge processor having a basket containing the crystals whereby the new solution formed by dissolving step (b) is contained within the centrifuge;
the step (c) transferring of each new solution is to a processor crystallizer of the next purer stage;
said step (d) crystallizing step is carried out within each said crystallizer; and said separating and further separating steps are carried out by the centrifuge of at least the stages other than the most pure stage.

10. The method of claim 7, wherein said dissolving step (b) includes recycling the crystals and mother liquor between the centrifuge and a heat exchanger which raises the temperature of the crystals and mother liquor.

11. The method of claim 1, wherein the plurality of stages are closed to undesired environmental influences external of the stages.

12. The method of claim 1, wherein the quantity of mother liquor transferred is substantially the same in each of step (a) and is substantially the same as the mother liquor discharged in step (f).

13. The method of claim 1, wherein equal quantities of crystallizable material are transferred during each stage.

14. A method for superpurifying crystallizable substances by a multistage recrystallization procedure, comprising:
providing a plurality of stages having vessels with crystallizable substances therein and operating said stages by proceeding with a sequence of steps in a cyclic manner through a plurality of cycles conducted within each stage such that the crystallizable substance is dissolved in each stage beginning with a least pure stage, transferred as a fully dissolved component of a new solution to the next purer stage, and recrystallized in the next purer stage until superpurified through recrystallization in the purest stage, the sequence of steps including:
(a) transferring a quantity of mother liquor from a liquid receiver of a stage of a multistage recrystallization system into a vessel, the quantity of mother liquor being from the next less pure stage and containing crystals of the crystallizable material;
(b) dissolving the crystals of step (a) within the mother liquor of step (a) to form a new solution;
(c) transferring each new solution formed in step (b) to the processor;
(d) crystallizing crystallizable material within each new solution in the processor to form crystals, said crystals from the most pure stage being a recrystallized superpurified substance;
(e) separating the crystals formed in step (d) from its mother liquor in at least the stages other than the most pure stage to provide separated crystals and collecting the thus separated mother liquor for subsequent use in step (a) of a next cycle; and (f) discharging mother liquor as a byproduct from the least pure stage.

15. The method of claim 14, further including the step of washing the separated crystals of step (e) with mother liquor from the next purer stage.

16. The method of claim 14, wherein said step (e) separating includes further separating mother liquor from separated crystals of step (e) to thereby withdraw mother liquor from between the crystals and provide dewatered crystals.

17. The method of claim 16, further including the step of washing the dewatered crystals with mother liquor from the next purer stage.

18. The method of claim 14, wherein the mother liquor transferred in step (a) is a mixed mother liquor formed by combining mother liquor from that same stage with mother liquor passed into that stage during said washing step.

19. The method of claim 17, wherein the mother liquor transferred in step (a) is a mixed mother liquor formed by combining mother liquor from that same stage with mother liquor passed into that stage during said washing step.

20. The method of claim 14, wherein step (a) is carried out within a centrifuge processor having a basket containing the crystals whereby the new solution formed by dissolving step (b) is contained within the centrifuge; the step (c) transferring of each new solution is to a crystallizer of the processor; and said step (d) crystallizing step is carried out within said crystallizer.

21. The method of claim 17, wherein step (a) is carried out within a processor including a centrifuge having a basket containing the crystals whereby the new solution formed by dissolving step (b) is contained within the centrifuge;
the step (c) transferring of each new solution is to a crystallizer of the processor;
said step (d) crystallizing step is carried out within the crystallizer; and said separating and further separating steps are carried out by the centrifuge.

22. The method of claim 20, wherein said dissolving step (b) includes recycling the crystals and mother liquor between the centrifuge and a heat exchanger which raises the temperature of the crystals and mother liquor.

23. An apparatus for superpurifying crystallizable substances by a cyclic multistage recrystallization procedure, comprising:
a plurality of stages including a least pure stage and purest stage, each said stage having a liquid receiver and a processor, said stages proceeding through substantially duplicate cycles conducted substantially simultaneously within stages;
means for adding a controlled quantity of crystallizable substance within mother liquor including a solvent for the crystallizable substance to a processor vessel of each stage except for the purest stage, each said controlled quantity of crystallizable substance being transferred from the liquid receiver of the stage next purer than the stage to which the crystallizable substance is added;
means associated with each processor vessel for dissolving crystallizable substance within its mother liquor to form a new solution from the crystallizable substance and mother liquor within each processor vessel;
a feed dissolver having means for dissolving within the solvent feed crystallizable material to be purified to form a new feed solution within the feed dissolver;
means for transferring the new feed solution from the feed dissolver and for transferring each new solution from each processor vessel into the processor vessel of the next purer stage, said processor of the purest stage being a crystallizer;
at least one of said processor vessels has means for forming crystals from the new feed solution or new solution transferred thereinto, said means for forming crystals including a foraminous member for supporting a collection of crystals, the crystals being within the mother liquor from which the new solution was formed, the crystals from the purest stage mother liquor being superpurified product crystals;
each liquid receiver having a reflux portion and means for transferring a controlled quantity of mother liquor from the processor vessel with means for forming crystals to the reflux portion of the same stage;
means for transferring residual mother liquor remaining in the processor vessel having means for forming crystals into the liquid receiver of the same stage, said transferring means including means for withdrawing mother liquor from between said crystals resting on the foraminous member of said processor vessel to form dewatered crystals;

means for transferring the controlled quantity of mother liquor within each reflux portion, except for the reflux portion of the least pure stage, to the processor vessel having crystals from the next less pure stage therein to thereby wash the dewatered crystals in the processor vessel with the mother liquor from the reflux vessel; and means for transferring the controlled quantity of mother liquor within the reflux vessel of the least pure stage to a collector for byproduct.

24. The apparatus of claim 23, wherein the processor vessel having means for forming crystals comprises the combination of a processor crystallizer upstream of and in slurry passing communication with a centrifuge, said means for transferring new solution opens into said processor crystallizer, and said means for withdrawing mother liquor from between crystals includes a centrifuge assembly of the centrifuge device.

25. The apparatus of claim 23, wherein each component of the apparatus is closed to the environment during the cyclic multistage recrystallization procedure.

26. The apparatus claim 23, further including a manifold system which imparts pressurized fluid simultaneously to each liquid receiver to transfer mother liquor out of each liquid receiver.

27. The apparatus of claim 23, further including a manifold system which connects to each liquid receiver to impart reduced pressure thereto for assisting the means for transferring residual mother liquor from between crystals resting on the foraminous member to thereby form the dewatered crystals.

28. The apparatus of claim 23, wherein each of said processor vessels of each stage except for the purest stage has means for forming crystals from the new feed solution or new solution transferred thereinto.

29. The apparatus of claim 28, wherein each of said processor vessels comprises the combination of a processor crystallizer upstream of and in slurry passing communication with a centrifuge, said means for transferring new solution opens into said processor crystallizer, and said means for withdrawing mother liquor from between crystals includes a centrifuge assembly of the centrifuge device.