Kinetics of ion exchange in polyphase systems including crystallizing substances

ReactivePolymers, 17 (1992) 75-88

75

Elsevier Science Publishers B.V., Amsterdam

KINETICS OF ION EXCHANGE IN POLYPHASE SYSTEMS INCLUDING CRYSTALLIZING SUBSTANCES D.N. MURAVIEV, O. Yu. SVERCHKOVA *, N.M. VOSKRESENSKY and V.I. GORSHKOV

Departmentof Chemistry, MoscowState University,Moscow119899(Russia) (Received December 5, 1990; accepted in revised form October 3, 1991)

Kinetics of ion exchange has been investigated in triphase systems: (1) strong-base anion-exchanger AV-17× 8 in the CO2- form-CaCI 2 solution-CaCO 3 precipitate; and (2) sulfonate cation-exchanger KU-2 × 8 in the Ca 2+ f o r m - N a 2 C O 3 solutionCaCO 3 precipitate, and in quadriphase systems: (1) AV-17 × 8 in the CO 2- formCaSO4 • 2H20 precipitate-CaSO4 saturated solution-CaCO 3 precipitate; and (2) AV1 7 × 8 in the CO2- form-KU-2 ×8 in the Ca 2+ form-Na2CO 3 solution-CaCO 3 precipitate. It has been shown that in both tri- and quadri-phase systems, the rate of cation exchange is lower than that of anion exchange. The amount of the precipitate crystallizing on the surface of resin beads depends on the type of ion-exchange resin, its granulation and on the ratio of resin components in the case of quadriphase systems with a mixed bed. By varying this ratio and the granulation of the ion-exchangers it is possible to accomplish the precipitation mostly in the anion- or cation-exchanger. Under certain conditions the precipitation takes place mainly in the solution phase. The mathematical model of ion-exchange process accompanied by precipitation on the surface of the resin beads has been proposed. The rate of the ion-exchange process has been assumed to be controlled by diffusion within the porous layer of the precipitate fixed on the surface of the bead. As an example of practical application of the results obtained the method of simultaneous regeneration and separation of mixed bed resins exhausted in a water demineralization process has been proposed. Keywords: ion exchange; kinetics; polyphase systems; precipitation; model

INTRODUCTION

Biphase systems may be considered as the most traditional objects of investigations in

* To whom correspondence should be addressed.

the field of ion exchange. These systems involve, as a rule, a solid (sometimes liquid) ion-exchanger and a liquid solution. With addition into such systems some new phases, or their formation during ion-exchange interaction, may occur and this significantly complicates the problem of studying these systems. However, it may offer some a d v a n t a g e s

0923-1137/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

76

in comparison with the classical variant of an ion-exchange process. Polyphase systems (with more then two phases) most often applied in practice are mixed beds of ion-exchangers widely used for water deionization [1-4] and in ion-exchange synthesis [5] and also systems including slightly soluble substances besides solid resin and liquid solution. The ion-exchange interaction with the formation * of one or several precipitates can be either involved in the processes purposely or can be an undesirable phenomenon. Process designs related to the first case are usually used for shifting the ion-exchange equilibrium [9-12], in precipitation chromatography [13], and for other purposes. In the second case the precipitate formation may take place (e.g., under the regeneration of cation-exchanger in the Ca 2÷ form by concentrated H E S O 4 solution (precipitation of CaSO4) [14,15]) in water treatment processes (precipitation of iron hydroxides) [16] and in some other situations [16]. Precipitation as an impediment has been discussed in a number of publications dealing with the attempts to prevent formation of slightly soluble substances in ion-exchange columns and to decrease the danger of resin bed clogging. Addition of precipitation inhibitors [17], carrying out the process in sectional columns [18] or in counter-current units [19] have been used for this purpose. One of the main drawbacks of ion exchange and precipitation combination (except hydrodynamic problems) is increasing the resistance to mass transfer in the resin phase because of coverage of the exchanger beads with the precipitate layer. The same effect can be observed in fouling of the resin bed by organic material [20-22].

* Dissolution of precipitates using ion-exchangers (e.g., in ore pulp leaching process [5] and ref. 4, p. 295) is not considered.

D.N. Muraviev et al. / React. Polym. 17 (1992) 75-88

A number of theoretical and experimental studies on the kinetics of ion-exchange processes accompanied by ionic reactions have been carried out (see, e.g., refs. 23-25). In several publications envisaging ion-exchange systems with precipitate formation (see, e.g., refs. 26 and 27) no quantitative data on the kinetics of an ion-exchange process accompanied by precipitation have been found. For that reason the following problems are studied in this paper: (1) Kinetics of ion exchange accompanied by CaCO 3 precipitation in triphase and quadriphase systems involving strong-base anion-exchanger and sulfonate cation-exchanger in the CO 2- and Ca 2+ forms, respectively. (2) The mathematical model describing an ion-exchange process controlled by diffusion within the porous layer of the precipitate fixed on the surface of a resin bead. (3) The method of simultaneous regeneration and separation of mixed bed resins exhausted in a water demineralization process (as an example of a practical application of the results obtained).

EXPERIMENTAL Chemicals Solutions of HCL, NaOH, NaC1, Na2CO3, CaC12, MgSO4 and CaSO 4 • 2H20 were prepared from Merck standard solutions and from solid salts of p.a. quality used as received. The concentrations of H +, CO 2- and O H - ions were determined in all cases by potentiometric titration using a Radiometer pH-meter (Denmark) with a combined glass electrode. Calcium concentrations were measured by EDTA (sodium salt) titration [28]. Concentrations of C1- were determined by mercurometric titration using diphenyl carbasol as the indicator. Sodium concentrations were measured by flame photometry using a Flapho-4 (Germany) photometer. The absolute errors of titration methods were

D.N. Muraviev et al./React. Polym. 17 (1992) 75-88

about 5 × 10 -4 m e q / c m 3. The relative errors of Na ÷ analysis were no more than 5%. Strong-base anion-exchanger AV-17 × 8 and sulfonate cation-exchanger KU-2 x 8 were commercial materials (USSR). Resin fractions of known granulation were obtained by dry sieving of air-dry samples of the respective exchangers. The specific capacities of ion-exchangers were determined under dynamic conditions. Methods

Kinetic experiments were carried out using the limited volume m e t h o d [4] (batch conditions) according to the following techniques. A known amount of ion-exchanger (2.90 meq in the swollen state) and 100 cm 3 of water were placed into 250 cm 3 round-bottom flask and stirred by propeller at 1200 rpm *. T h e n 5 cm 3 of 1.164 m e q / c m 3 salt solution was pipetted into the flask. The m o m e n t of salt addition was counted as time zero and phase contact time varied from 10 to 2700 s. After a given period of time stirring was stopped and the resin was separated from the solution and CaCO 3 slurry by filtration. This procedure took no more than 5 - 7 s. After phase separation, the resin was carefully rinsed with distilled water that was collected together with the solution and CaCO 3 slurry into a graduated flask. Analysis of both resin and solution (with suspended CaCO 3) phases were carried out. In the case of quadriphase systems with mixed bed resin mixtures (composed of anion- and cation-exchangers of different granulation), the system was separated into the

* This value was determined to be optimum since it corresponded to the plateau range of F (resin conversion degree) vs. rotation speed curves obtained for some systems under study in an independent series of experiments.

77 resin components by drying and dry sieving. Separated resin samples were transfered to columns quantitatively and 0.2 M Na2SO 4 solution was passed through every column. During elution, resin beads (previously washed with water and being maximally swollen) shrank and CaCO 3 precipitate was remove from the surface of the resin bead and was eluted from the columns by the solution flow. Collected eluates were centrifuged and the analysis of both supernatant and CaCO 3 precipitate was carried out. Anion-exchanger samples were additionally eluted by a known volume of 0.1 M HC1 solution followed by analyzing the Ca/+ and H ÷ content in the eluates. The overall Ca 2+ content in the respective eluates corresponded to the CaCO 3 amount precipitated on the surface of the resin beads. In studying the quadriphase system with reprecipitation of C a S O 4 • 2HzO into CaCO 3 using an anion-exchanger in the CO32- form, initially 100 cm 3 of water and 5.12 meq of C a S O 4" 2 H / O (0.125-0.25 m m fraction of powdered dihydrate gypsum) were placed into the flask and were stirred at 1200 rpm during 15 min. In preliminary experiments it was shown that these values of stirring time and speed correspond to the condition of saturating the CaSO 4 solution upto constant concentration. After saturating, a known amount of anion-exchanger in the CO32- the form was transferred into the reaction vessel. This m o m e n t was zero time. After a given period of time, stirring was stopped and the resin was separated from the solution and the CaCO3-CaSO 4 slurry. Both solution and slurry phases were collected into a graduated flask containing a known amount of 0.1 M HCI. Then the analysis of H ÷ and Ca a÷ was carried out. Anion-exchange resin was transferred to a column and was eluted with 0.1 M HC1 solution followed by analyzing the Ca/+ concentration and the H + concentration change in the eluate. The overall Ca 2+ content in the eluate corresponded to the

D.N. Muraviev et al. / React. Polym. 17 (1992) 75-88

78

C a C O 3 amount precipitated on the surface of the resin beads. On the basis of analysis results the degree of resin conversion (F) and the C a C O 3 fraction precipitated on the surface of the exchanger beads (P) were calculated as follows: F = qi/Q and P = qp/Q, where qi is the amount of exchanged ion in the resin sample (meq), qp is the amount of C a C O 3 precipitated on the surface of the resin beads (meq); Q is the full ion-exchange capacity of the resin sample (meq). The relative errors of the F values were ~ 10% and for the P values they were no more than 15%.

RESULTS AND DISCUSSION The main parameters of the systems investigated are given in Table 1. Conditions of the kinetic experiments carried out with the bi-phase systems were similar to those for the tri- and quadri-phase systems. F values obtained for systems 1 and 2 have been calculated using the same expressions as in the case of systems 3-6 (see above). Experimental kinetic curves for systems 1-5 are presented in Fig. 1. As seen from Fig. 1 the conversion rates of cation-exchangers are much lower than that of anion-

TA B LE 1

Parameters of ion-exchange systems under investigation System No.

1

Initial composition

PrecipiSolution, conc.

tate 1

Precipitate 2, formed

Phase number

Overall reaction

R2Ca + 2NaCI = 2RNa + CaCI 2

resin 1,

resin 2,

ionic form, granulation

ionic form, granulation

(mm)

(mm)

KU-2 × 8

-

NaCI 0.065

2

AV-17 × 8

NaCI 0.065

2

C a 2+ f o r m

(meq/cm)

0.25-0.40 2

Ca 2+ form

R~CO 3 + 2NaCI = 2RNa + C a C O 3

0.25-0.40 3

KU-2 × 8 C a 2+ form 0.25-0.40

-

Na2CO 3 0.065

CaCO 3

3

R2Ca+Na2CO3 = 2RNa + C a C O 3

4

-

A V - 17 × 8 CO32- form 0.25-0.40

CaC1 0.065

CaCO 3

3

R~CO3+CaCI2= 2R'CI+CaCO 3

5

KU-2 x 8 Ca 2+ form 0.25-0.40 0.63-0.80

AV-17 x 8

NaC1 0.065

CaCO 3

4

R2Ca + R'2CO 3 + 2NaC1 -2RNA + 2R'CI + CaCO3

CaCO 3

4

R~2CO3 + C a S O 4 = R ' 2 S O 4 -t- C a C O 3

a b

a

b C

d

0.63-0.80 0.25-0.40 AV-17×8 CO32- form 0.125-0.25 0.25-0.40 0.63-1.00 0.80 a

CaSO 4 0.0256

CaSO4.2H20

(sat. at 20°C)

a Resin beads stuck in the holes of the 0.8 mm sieve.

79

D.N. Muraviev et aL / React. Polym. 17 (1992) 75-88

0.e~...o.._ 0.6 o.4 02

_~._._D

~ ' " 0 ~

----

__--

w.,o'o.oW

;

io

i"s t,min

Fig. I. Experimental kinetic curves of Cl- - C O ~ -

and

Ca 2+ - N a + exchange in biphase (1), triphase (2) and quadriphase (3) systems including strong-base anionexchanger (full lines) and sulfonate cation-exchanger (dotted lines).

exchangers both in the bi-, tri-, and quadriphase systems. In the case of biphase systems this fact can be explained by the different selectivities of anion- and cation-exchange resins towards exchanging ions. Actually the isotherm of C1--CO32- exchange is convex while that for CaE+-Na + exchange is concave, at least for the given experimental conditions. The increase in the number of phases in the systems under study leads to increasing F values reached in the same periods of time (cf. points on the respective curves corresponding to, e.g., 30 min). Another conclusion one can make from Fig. 1 concerns the more complex shape of kinetic curves 2 and 3 (corresponding to the tri- and quadri-phase systems, respectively) compared with that for

biphase system (curve 1). The last two results deal with on the one hand shifting the equilibrium in systems where ion exchange is accompanied by precipitation of CaCO 3 and on the other hand with the formation of the precipitate layer on the surface of the resin beads. This results in reducing the rate of mass transfer in the resin phase (cf. curves 1 and 2, full lines in Fig. 1). Kinetic curves of C a C O 3 precipitation on the surface of anion- and cation-exchanger beads in tri- and quadri-phase systems are shown in Fig. 2. As seen from Fig. 2 the amount of C a C O 3 formed on the surface of the resin beads in the triphase systems increases proportionally with the F values. Under the same periods of phases contact time the P values for the anion-exchanger are markedly greater than that for the cation-exchanger. In the case of the quadriphase system the C a C O 3 fraction precipitated on the surface of the anion-exchanger decreases considerably compared with the P values for the triphase system (cf. curves 1 and 2 in Fig. 2) while the P values for the cation-exchanger stay constant in both cases. Such a distribution of the precipitate has been observed for both 5a and 5b systems (see Table 1) composed with resins of different granulation and can be interpreted in terms of different rates of Ca 2÷ and CO ] release. The difference in cation and anion exchange in this case causes enrichment of the solution near the surface of the cationexchanger beads by CO ]- ions that leads to

0.4

0.2

~

s

lb

~

1~

3

4

2~

m

2s

O

.

"V t,min

O

is

Fig. 2. Kinetic curves of CaCO 3 precipitation on surface of anion- (1,2) and cation-exchanger (3,4) beads in triphase (open circles) and quadriphase (filled circles) systems.

80

D.N. Muraviev et aL / React. Polym. 17 (1992) 75-88

precipitation of CaCO 3 on the surface of the cation-exchanger. For elucidating the influence of the ratio of resins in the mixed beds on the F and P values for the anion- and cation-exchanger components the following series of experiments have been carried out. Kinetic experiments with several resin mixtures of different compositions (systems 5a and 5b in Table 1) have been performed using the method similar to that described above. The time of phase contact was in all cases 15 min. The results of this series of experiments are presented in Figs. 3 and 4 where the F and P values (respectively) are plotted vs. the equivalent fraction of the anion-exchanger in the resin mixture. It follows from Fig. 3 that curves 1 and 2 have a definite tendency to draw together in the range of low anion-exchanger content in particular. From this trend one can conclude that equal F values for both cation- and anion-exchangers may be achieved where there is significant cation-exchanger excess in the resin mixture. As seen

/

1.o

0.8

I

0

0

0

/

0.6

/ / / 0.4

0.2

0.o

0:3 anion

o:s exch.

o'.r

o.'9

resin fraction

Fig. 3. Conversion degree (F) of anion- (1) and cation-

exchanger (2) in mixed beds of different compositions vs. fraction of anion-exchangerin the resin mixture.

0.3 0.2 0.1

o.~

o'.~

o'.s

'

o.'7

0:9

0.3 0.2 0.I

0.2 anion

O.4 exch.

0.6 0.8 resin f r a c t i o n

Fig. 4. Fraction of CaCO3 precipitated on the surface of cation- (A) and anion-exchanger (B) in mixed beds of different compositions vs. fraction of anion-exchanger in the resin mixtures: 0.25-0.40 mm (filled circles); 0.63-0.80 mm (open circles).

from Fig. 4 more intensive precipitate formation is observed for the anion-exchanger of 0.63-0.8 m m granulation (low content) in the mixture in comparison with a mixed bed of equivalent composition (0.5:0.5). For the finer fraction of anion-exchanger the opposite trend is observed. It is interesting to note that the precipitate distribution in the cation-exchanger of the same granulations has an absolutely contrary character than the anion-exchange resin. One important conclusion can be made on the basis of the results considered above concerning minimizing the precipitation on the surface of mixed bed resins. It seems to be rather evident that to achieve this purpose (at least in this situation) one needs to even the fluxes of ionic species released from both the anion- and cation-exchanger components of the resin mixture (Ca 2+ and CO 2ion fluxes in this case). Assuming that the overall flux of each ionic species is proportional to the overall surface area of the respective resin component of the mixed bed it is obvious that to equalize the fluxes change of resin granulation or regulation of the mixed bed composition can be used.

81

D.N. Muraviev et aL / React. Polym. 17 (1992) 75-88

Before considerating the results obtained for system 6 (see Table 1) it seems to be useful to underline some of its features. The main peculiarity of this system deals with the combination of the ion-exchange process with reprecipitation of one slightly soluble substance (CaSO 4) into the other less soluble one (CaCO3). In this case two additional elementary steps of the process appear in comparison with ordinary ion exchange (e.g., in system 2, see Table 1). The former is dissolving the initial precipitate ( C a S O 4) and the latter is the formation of the new one (CaCO3). The dissolution of CaSO 4 has been determined to be a rather fast process ensuring a constant concentration in the solution phase. It makes the system such as this unusual in comparison with ordinary ion-exchange systems studied under batch conditions. For estimating rates of these two steps and comparing them with the rate of the overall process the following series of experiments was carried out. Since the system under study is isolated the conversion degree of

1.0

o. e

0.6

F

the resin should be equal to that for the precipitate phase (for transforming CaSO 4 into CaCO3). For this reason it was decided to study the rate of reprecipitation without the resin phase using N a 2 C O 3 solution injected into the reaction vessel trough a capillary. The N a 2 C O 3 flow rate was kept constant at such a level so that the concentration of the CO32- ions was equal to that of the SO 2- ions in the saturated C a S O 4 solution (0.92 c m a / m i n of 1.7 m e q / c m 3 N a E C O 3 solution). The m o m e n t of NaECO 3 solution injection was set at zero time. After a given time period the solution phase was separated from the precipitate of calcium salts. Then the analysis of calcium carbonate content in the precipitate phase was carried out. On the basis of analyses results the degree of CaSO 4 into C a C O 3 conversion (F) was calculated using the expression given above. The results of the kinetic experiments on reprecipitating CaSO 4 into CaCO 3 using NaECO 3 solution and anion-exchanger in the CO 3 form are presented in Fig. 5. As seen

I

@---I

2

~

~

3_

o

/~o

0.4

4

ro

0.2

o. o

zoo

4bo

6~o

so'o

~Ft,s

Fig. 5. Experimental kinetic curves of reprecipitation of CaSO 4 into CaCO 3 using Na2CO3 solution (1) and of SO42--CO32- exchange (2-4) accompanied by reprecipitation. Strong-base anion-exchange resin of granulations: 0.125-0.250 mm (2); 0.25-0.40 mm (3); 0.63-1.0 mm (4).

82

D.N. Muraviev et aL / React. Polym. 17 (1992) 75-88 0.2

P

1

2 0

0.1

0.0

~

~

5

I0

15

20

25

36

t, rain Fig. 6. Kinetic curves of CaCO 3 precipitation on surface of anion-exchanger of different granulation, 0.125-0.250 mm (1), 0.25-0.40 mm (2), 0.63-1.00 mm (3).

from Fig. 5, the kinetic curves obtained in the case of the quadriphase system are "delaying" (in achieving the same F values) in comparison to the absence of the resin (cf. curves 2-4 and curve 1 in Fig. 5). The value of the "delay time" is characteristic of the interbead ion diffusion process involving diffusion within the porous layer of the precipitate fixed on the surface of a bead. The kinetic curves of CaCO 3 precipitation on the surface of resin beads are shown in

Fig. 6. It follows from Fig. 6 that the maximum P values are observed for the finest resin fraction. Comparison of the P values with the respective F values (given in Fig. 5) corresponding to resin samples of the same granulation shows that the dependence between these two parameters is close to a directly proportional one. For making clear the influence of the precipitate layer on the rate of ion exchange the following experiment with "intermediate

0.6 I

At-

-"

t,min

36

2

0.4

0.2

0.0 5

10

15

20

Fig. 7. Kinetic curves of SO 2 - - C O 2- exchange accompanied by reprecipitation with intermediate rinsing of resin (1) and without (2).

D.N. Muraviev et al./React. Polym. 17 (1992) 75-88

rinsing" was carried out with 0.8 mm anionexchanger. The technique of the experiment was similar to that described above. After stopping phase stirring, the resin was quickly separated from the solution and the C a S O 4 CaCO 3 slurry and was rinsed by distilled water (no more than 1 min). In rinsing and mechanical mixing the resin sample some part of the precipitate layer was removed from the surface of resin beads. Then the resin was placed back into the vessel and was kept in contact with other phases for the same time period under the same conditions. Rinsing was carried out in the middle of the kinetic experiment and after that the phases were separated and then analyzed. The kinetic curves obtained in this series of experiments are presented in Fig. 7. As seen from Fig. 7 the the degree of resin conversion after rinsing in all cases is higher than that obtained for resins without this procedure. It confirms that the rate of ion exchange increases even after partial removal of the precipitate layer from the bead surface. The results obtained allows one to conclude that the limiting step of ion exchange in this system is diffusion within the layer of the precipitate fixed on the surface. On the basis of this conclusion the mathematical model of ion exchange process accompanied by precipitation on the surface of an exchanger bead has been proposed.

MODEL DESCRIPTION In the mathematical model it was assumed all resin beads with a radius R were invariable during the ion-exchange process. The precipitation layer has been assumed to consist of CaCO 3 crystals of density Pc~c03 with uniform packing, characterized by a packing coefficient of 0.64. The thickness of the layer l(t) has been supposed to depend on the time of phase contact. The interbead diffusion rate has been assumed to be much

83

greater than that within the precipitate layer and the SO 2- ion concentration inside the beads depends only on time. Precipitation of CaCO 3 is the result of the interaction between the CO32- ions released from the bead and the Ca 2÷ ions diffused to the surface. In precipitating on the surface the CaCO 3 layer moves from the bead boundary to the solution phase. The uptake of SO 2- ions by the resin can be described as follows: (4/3)rrR 3 dC(t)/dt=41rR2j(t)

(1)

where J(t) is the flux of SO42- ions through the bead boundary ( m e q / c m 2 s); C(t) is the average concentration of SO42- ions in the bead (meq/cm3). Since during the experiment l(t) << R the layer has been considered to be flat and the diffusion flux to be quasi stationary and equal to:

J( t) = D[C o - C ( t ) / K ] / l ( t )

(2)

where C o is the constant concentration of SO 2- ions in the bulk (meq/cm3); D is the diffusion coefficient of CaSO4 within the CaCO 3 layer (cm2/s); K is the equilibrium coefficient between the concentration of SO 2- ions in the bulk and in the resin phase. Since the formation of slightly soluble CaCO 3 shifts the equilibrium to the direction of SO 2- ion uptake the isotherm of ion exchange in this case is close to a rectangular one. Its range corresponding to the kinetic data obtained (F < 0.8) can be considered to be linear with tangent K. For describing the rate of CaCO 3 layer growth it has been assumed that only a certain part of CaCO 3 precipitate is fixed on the surface of the resin bead. The rest of the precipitate flakes off and accumulates in the solution phase. This assumption is in good agreement with the experimental data obtained (see Figs. 5 and 6) and allows one to conclude F(t)/P(t) = constant = k.

84

D.N. Muravievet al. / React. Polym. 17 (1992) 75-88

Taking into account the equivalence of SO42 - - C O 2- exchange one obtains:

From eqns. (5) and (2) it follows that under F = 1

(4/3)zrR3k d C ( t ) / d t = Ck4~rR 2 d l ( t ) / d t

K = Q/(4/3)TrRaNbCo

(3) where C k is the concentration of CO 2- ions in the precipitate layer (meq/cm3); and C k =pCaCO3/Ecaco 3 where Ecaco 3 is the equivalent of CaCO 3. Thus the overall process of ion exchange and the precipitation layer growth can be described by eqns. (1)-(3) with, the following initial conditions: t=0;

C(t)=0;

/(t)=0.

(4)

The connection between the C(t) and l(t) parameter with the experimentally measured F(t) and P(t) values can be described as follows:

F(t) = [(4/3)TrR3NbC(t)]/Q

(5)

P(t) = [4TcR2Nbl(t)Ck]/Q

where N b is the number of resin beads in the sample; Q is the full ion exchange capacity of resin sample used (2.56 meq).

1.0 F

(6)

Using eqns. (5) and (6) one can obtain the solution for systems 1-4: l o g [ l / 1 - F(t)]

-

F(t)

=CkD/k(16vrER4NECot)/Q

(7)

Equation (7) contains only one parameter D that cannot be measured in the experiment. R values were obtained from the experimental data as the average values for the given resin fraction taking into account the swelling coefficient being constant for resin samples of all granulations and being equal to 1.13 (see system 6 in Table 1). The parameter D and 4 parameters Ri(i = a, b, c, d see Table 1, system 6) were calculated by minimization of: n

E (/i,ex- ti,cal)2//(ti,ex- ti,cal)

(8)

i=1

fulfilled using the random search method. Here ti,ex and t~,ca~ are the experimentally

/ ~

0.8

0.6

0.2 0°0

t

zdo

450

40o

aoo

gt$ Fig. 8. Computed kinetic curves and experimental points for sulfate-carbonate exchange on resins of different granulations: 0.125-0.250 rata (1); 0.25-0.40 rata (2); 0.63-1.00 tara (3). Curve (4) computed for intraparticle diffusion (see text).

85

D.N. Muraviev et al. ~React. Polym. 17 (1992) 75-88

determined and calculated from eqn. (7) values of time corresponded to given the F value for all 4 fractions (systems 6 a - 6 d in Table 1), respectively; n is the number of experimental points. Calculated R i values were close to that determined from experimental data: Ra,ex = 0.0109 cm; Rb,ex = 0.0197 cm; Rc,ex = 0.0484 cm; Rd,ex = 0.0485 cm and Ra,caI = 0.0132 cm; Rb,ca I = 0.0214 cm; Rc,al = 0.0471 cm; R,cal = 0.0463 cm. The calculated coefficient of C a S O 4 diffusion within the CaCO 3 layer was D = 7.9 × 10 -9 cm2//s. The ratio Ck/k used in the D calculations (determined from the experimental F and P values) was found to be 10.6 + 0.3 for a 75% confidence interval. The results of computer treatment of the experimental data are presented in Fig. 8. Curves 1-3 correspond to results computed from eqn. 7 for resin fractions of different granulations. Curve 4 has been computed with the help of the interactive diffusion equation (see, e.g., ref. 25) for 0.25-0.40 m m resin fraction using the value of reciprocal diffusion coefficient of CO 2- and SO 2- ions (D) determined by a thin layer method [4]. Experimental D measurements have been carried out for exchange from 0.0256 m e q / c m 3 Na2SO 4 solution on anion-exchanger samples of 0.0125-0.250, 0.25-0.40 and 0.63-1.00 m m granulations. The value of D average for these three fractions has been found to be 4.3 x 10 -6 cm2/s. The experimental determined K value was equal to 36.

PRACTICAL APPLICATION On the basis of the results obtained the m e t h o d of simultaneous regeneration and separation of mixed bed resins has been proposed. The mixed bed under regeneration was composed of the strong-base anion-exchanger in the CO 2- form and a sulfonate cation-exchanger in the Ca 2÷ form and cor-

Ile,/ o o.

3 ~

5

D' • °i

e:~

t ~°

O

V7

[

9

Fig. 9. Laboratory-scale unit: (1) solution-resin valve; (2) upper column tank; (3,4) solution valves; (5,6) filters; (7) solution inlet; (8) solution outlet; (9) resin outlet; (10) regenerant tank; (11) pump.

responded to a mixed bed exhausted in a hard water deionization process (e.g., calcium bicarbonate removal). The scheme of the laboratory unit is presented in Fig. 9. Transforming cation- and anion-exchangers from the Ca 2÷ and CO 2forms into the Na ÷ and O H - forms, respectively, was carried out under dynamic conditions by subsequent treatment with NaOH solutions of different concentrations circulating in a reserved cycle. The idea of the method is based on coupling the ion-exchange processes both in cation- and anionexchangers with precipitation of CaCO 3. This

86 should lead to significant shifting of the equilibrium in ion-exchange reactions for both resins to the right, resulting in more complete transformation of them into the desirable ionic forms. Both Ca 2+ and CO 2- ions may be considered "to help" each other to release resin phases producing low soluble CaCO 3. Respective equilibrium constants corresponding to ion-exchange processes coupled with a precipitation reaction become directly proportional to the factor 1 / L [7], where L is solubility product of CaCO 3 (,~ 10-9). Changing the regenerant concentration sharply should improve the kinetics of the process because of better removal of the precipitate layer from the surface of resin beads after "osmotic shock". Another advantage of the method deals with more complete usage of regenerant in this process. The only admixture appearing in the NaOH solution during the regeneration process is the CaCO 3 precipitate that can be easily removed by filtration. This allows one to reuse the regenerant solution or use it in a reversed cycle. The efficiency of the regenerant in this method is higher than that in the case of an ordinary regeneration process. Actually, if in the conventional regeneration processes the regenerant acts only by the cationic or anionic part of its molecule (e.g., in treating exhausted cation-exchanger with n 2 s o 4 solution) in our method the regenerant reacts with ion-exchangers by both parts of the molecule. Anion-exchanger during the regeneration transforms into the desirable O H - form and cation-exchanger transforms into Na ÷ form. The last process may be called "preregeneration" of the cation-exchanger since it leads to replacing divalent (Ca 2÷) ions by univalent ones (Na +) that simplifies the final regeneration of this resin by acid solution and decreases the danger of CaSO 4 precipitate formation as in the case when using concentrated HESO 4 solution for this purpose.

D.N. Muraviev et al. / React. Polym. 17 (1992) 75-88

The experiments were carried out according to the following techniques. Samples of cation- and anion-exchangers of different granulation (15 meq of each resin) in the initial ionic forms were mixed and the resin mixture was placed in the column. Then 0.06 m e q / c m 3 NaOH solution was passed though the mixed bed under a flow rate of 130 cma/min. The rate of alkali upflow was chosen from the mixed bed fluidization condition. The overall solution volume was 500 cm 3. Under the given value of upflow rate the anion-exchanger was concentrated in the upper column section (because of its lower density) and the cation exchanger was accumulated in the bottom part of the column. CaCO 3 slurry formed was removed from the column with alkali flow and was precipitated in vessel 10 (see Fig. 9). The slurry removal was carried out through line 3 and additionally through line 4. The precipitate was separated from the regenerant solution by filtration. The first step of the regeneration process (treatment with 0.06 M N a O H solution) took about 60 min. The second step involved the mixed bed being treated with 1 M NaOH solution over 20 rain. After every regeneration step the resins were removed from the column through the resin outlet. Resins were collected in portions followed by drying. After drying every resin portion was sieved into the anion- and cation-exchanger components and were weighed, and the content of every component was determined for each resin portion. Then the analysis of both cation- and anionexchanger was carried out. After the first regeneration step the degree of conversion of the cation-exchanger was 50% and for the anion-exchanger it was 40%. The amount of CaCO 3 precipitated on the surface of resin beads was 11% for the cation- and 12% for the anion-exchanger. Further mixed bed treatment with 1 M N a O H solution led to osmotic shrinking of the resin beads that resulted in more com-

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plete removal of precipitate from the surface of the ion-exchangers. After the second regeneration step the degree of anion-exchange resin conversion into the O H - form was 88% and that for cation-exchanger (conversion into the Na ÷ form) was 72%. The amount of the precipitate on the surface of the resin beads decreased to 3% and 7%, respectively. Since the final cation-exchanger regeneration was carried out with acid solution it has to result in the complete removal of CaCO 3 precipitate from the resin phase. The results of resin separation carried out simultaneously with the regeneration process allow one to obtain the individual resin components almost without admixture of the other. The fraction of the resin mixture that contained more than 20% of resin admixtures was no more than 10% of the initial resin mixture amount. Thus the main advantage of the method proposed can be considered to be in utilization of a recyclable regenerant and in yielding compact solid wastes (CaCO3). The combination of a regeneration process with separation of the mixed bed into individual resin components allows one to carry out the final regeneration of each resin separately with alkali or acid solution.

CONCLUSIONS From the results of the present investigation, it appears that: (1) The results of studying kinetics of ionexchanger in triphase and in quadriphase systems involving strong-base anion-exchanger in the CO 2- form and sulfonate cation-exchanger in the Ca 2÷ form accompanied by CaCO 3 precipitation show that in both tri- and quadri-phase systems the rate of cation exchange is lower than that of anion exchange.

(2) The amount of the precipitate crystallizing on the surface of resin beads depends on the type of ion-exchange resin, its granulation and on the ratio of resin components in the case of quadriphase systems with a mixed bed. Varying this ratio and the granulation of ion-exchangers it is possible to accomplish the precipitation mostly in the anion-, or the cation-exchanger. Under certain conditions the precipitation takes place mainly in the s61ution phase. (3) The mathematical model of ion exchange accompanied by precipitation on the surface of resin bead has been proposed. The rate of the ion-exchange process has been assumed to be controlled by diffusion within the porous layer of the precipitate fixed on the surface of a bead. The results of computer treatment of the experimental data obtained with the help of the proposed model demonstrate satisfactory fit of computed curves and experimental points. (4) The method of simultaneous regeneration and separation of mixed bed resins exhausted in a hard water demineralization process has been proposed.

ACKNOWLEDGMENT The authors wish to express their gratitude to Dr. Vladimir Belnov for computing some results.

REFERENCES 1 C. Calmon, Ion exchange processes used in the production of ultra pure water required in fossil fuel power plants, Ion Exch. Solvent Extr., 9 (1985) 1. 2 T. Mottershead, Production and use of high purity water in the microelectronics industry,in D. Naden and M. Streat (Eds.), Ion Exchange Technology, Ellis Horwood, Chichester, 1984, p. 25. 3 J.T. McNulty, Anion exchange resin kinetics in mixed bed condensate polishing,/b/d., p. 50.

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4 F. Helfferich, Ion Exchange, McGraw-Hill, New York, 1962. 5 G. Dobos. Studies on the kinetics of ion-exchange leaching of the hungarian rhodocrosite ore of "urkut origin". React. Polym., 7 (1988) 277. 6 V.L. Bogatyrev, Ion Exchange Resins in a Mixed Bed, Chimia (Leningrad), (1968) in Russian. 7 A.I. Vulikh, Ion Exchange Synthesis, Chimia (Moscow), (1973) in Russian. 8 A.R. Glueck, Desalination by an ion-exchange precipitation complex process, Desalination, 4 (1968) 32. 9 E.V. Kazakov, A.V. Markodich, K.K. Kalninsh and I.F. Karpova. Mechanism of ferricyanide organic resins formation and the character of their exchange properties, Vestnik Leningr. Univ. Fiz. Khim., 10 (1970) 123, in Russian. 10 E.V. Kazakow, I.F. Karpova, Mechanism of ferricyanide organic resins formation and the nature of their ion exchange interaction. Vestnik Leningr. Univ. Fiz. Khim., 10 (1968) 108, in Russian 11 A.I. Ryabinin, U.A. Afansiev, E.I. Lazareva and V.I. Eremin, Study of boron sorption from brines under dynamic conditions by zirconium hydroxideanion exchange resin sorbents. Zh. Prikl. Khim., 48 (1975) 35. 12 A.I. Ryabinin, U.A. Afanasiev, V.I. Eremin, V.T. Panjushkina and N.N. Bukov, On the interaction of zirconium hydroxide with anion exchangers, Dokl. Akad. Nauk SSSR, 225 (1975) 1115. 13 K.M. Olshanova, Precipitation Chromatography. Izd. AN SSSR, Moscow, (1963) in Russian. 14 B.A. Bolto and L. Pawlowski, Reclamation of waste-water constituents by ion exchange, Part IV. Resin stability problems, Effluent Water Treat. J., 23 (1983) 371. 15 C. Calmon, Limitations and problems of the ion-exchange process, in C. Calmon and H. Gold (Eds.), Ion Exchange for Pollution Control, CRC Press, Boca Raton, FL, 1979, Vol. 1, p. 46. 16 C. Calmon, The ion-exchange process, ibid., p. 20. 17 F.E. Witmer, C.D. Beitelshees and L.A. Haugseth,

Use of ion exchange softening to enhance RO product water recoveries, AIChE Syrnp. Ser., 70 (1974) 170. A.P. Van der Meer, D.N.M.M. Weve and J.A. Wesseling, Hydrodynamics and mass transfer in a counter-current ion exchange pilot plant plate column, in D. Naden and M. Streat (Eds.), Ion Exchange Technology, Ellis-Horwood, Chichester 1984, p. 284. P. Vulliez-Sermet and E. Zaganiaris, Optimization of sulfuric acid condition of use in countercurrent regenerated cation exchange units, IWC Proc., (1984) 225. B.J. Hoffman, Problems observed with ion-exchange resins in field operation, IWC Proc., (1982) 545. L.R. Gess and L.M. May. Prevention of iron, organic and microbiological fouling via a new concept in ion-exchange maintenance, IWC Proc., (1983) 306. H. Small, The poisoning of ion-exchange resins. Inhibition of cation exchange by cathodic surface active agents, J. Am. Chem. Soc., 90 (1968) 2217. F. Helfferich, Ion exchange kinetics. V. Ion exchange accompanied by chemical reaction, J. Phys. Chem., 69 (1965) 1178. M. Gorala Rao and A.P. Gupta, Ion exchange process accompanied by ionic reactions, Chem. Eng. J., 24 (1982) 181. D. Petruzzelli, F.G. Helfferich, L. Liberti, J.R. Millar and R. Passino, Kinetics of ion exchange with intraparticle rate control: models accounting for interactions in the solid phase. React. Polym., 7 (1987) 1. A.L. Bunge and C.J. Radke, Divalent ion exchange with alkali, Soc. Pet. Eng. J., Aug. (1983) 657. G. Klein, Fixed-bed ion exchange with formation or dissolution of precipitate, NATO ASI, Ser. E, Ion Exch. Sci. Technol., 107 (1986) 199. H.A. Flaschka, EDTA titration, Pergamon Press, Elmsford, NY, 1964.

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