Crystallization as a Separations Process - American Chemical Society

Chapter 25

Reactive Crystallization of Magnesium Hydroxide

Downloaded by PENNSYLVANIA STATE UNIV on August 11, 2014 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch025

Hideki Tsuge and Hitoshi Matsuo Department of Applied Chemistry, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama 223, Japan

Crystallization of magnesium hydroxide by a continuous mixed suspension mixed product removal crystallizer was conducted to make clear the characteristics of reactive crystallization kinetics of magnesium hydroxide, which was produced by the precipitation from magnesium chloride with calcium hydroxide. The following operating factors were investigated affecting the crystallization kinetics; the initial concentration of feeds, residence time of reactants, feed ratio of reactants, and concentrations of hydroxide and chloride ions. It was clarified that the nucleation rate and the growth rate are correlated by the power law model and that the kinetic order in the power law model is correlated with concentrations of OH and Cl . -

-

The production of sparingly soluble materials by simultaneous reaction and c r y s t a l l i z a t i o n has been used widely in the chemical i n d u s t r i e s . To make clear the c h a r a c t e r i s t i c s of c r y s t a l l i z a t i o n of sparingly soluble materials by chemical reactions is important for better design and more e f f i c i e n t operation of reactive crystallizers. Many works on c r y s t a l l i z a t i o n k i n e t i c s have been made in continuous mixed suspension mixed product removal (CMSMPR) c r y s t a l l i z e r s . The nucleation rate B° and the c r y s t a l growth rate G have been correlated by the following power law model: B°=kG (1) The present work deals with the reactive c r y s t a l l i z a t i o n of magnesium hydroxide, a well-known sparing soluble m a t e r i a l , from magnesium chloride with calcium hydroxide. Magnesium hydroxide is produced i n d u s t r i a l l y by the p r e c i p i t a t i o n from brine with 3 )

l

0097-6156/90/0438-0344$06.00/0 © 1990 American Chemical Society

In Crystallization as a Separations Process; Myerson, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

25,

Reactive Crystallization ofMagnesium Hydroxide

TSUGE&MATSUO

calcium hydroxide and is used i n d u s t r i a l l y as d e s u l f u r i z a t i o n agents and materials of steel plant r e f r a c t o r y , while few studies have been made on the c r y s t a l l i z a t i o n k i n e t i c s of magnesium hydroxide. Dabir, Peters and Stevens studied the k i n e t i c s of magnesium hydroxide by a CMSMPR c r y s t a l l i z e r mainly for the lime-soda ash water softening process, but their experimental ranges were rather narrow. Packter discussed the crystallization kinetics of magnesium hydroxide in batch crystallizer. Therefore, it is important to c l a r i f y the crystallization kinetics of magnesium hydroxide for better understanding of i t s c r y s t a l l i z a t i o n process. The objectives of the present paper are as follows; 1) to discuss the effect of the operating f a c t o r s , that i s , the i n i t i a l concentration of feeds, residence time of reactants, feed r a t i o of reactants, and concentrations of hydroxide and chloride ions on the c r y s t a l l i z a t i o n k i n e t i c s of magnesium hydroxide, 2) to correlate the k i n e t i c s with the power law model, 3) to make clear the effect of anion concentrations on the k i n e t i c order in the power law model. 2)


4)

Experimental Experimental apparatus and procedure. Figure 1 is a schematic diagram of the experimental apparatus. The c r y s t a l l i z e r was a 1 l i t e r s t i r r e d tank reactor made of a c r y l i c r e s i n and is considered to be a continuous MSMPR reactor. The reactor was 0.1m in diameter and the l i q u i d height 0.14m. The impeller used was of the 6-blade turbine type and operated at 450 rpm to ensure complete mixing. Feed solutions were pumped into the c r y s t a l l i z e r continuously to produce magnesium hydroxide. The product was continuously withdrawn from the c r y s t a l l i z e r . The reaction temperature was maintained at 25 °C by constant temperature bath. Sampling was begun after 10 residence times, when the steady-state had been reached. Crystals obtained were photographed by the scanning electron microscope (SEM) and their sizes were analyzed by a d i g i t i z e r . Irrespective of c r y s t a l form, the maximum length of c r y s t a l was used to describe the s i z e of individual c r y s t a l . Aqueous solutions of magnesium chloride (reagent grade) and calcium hydroxide (reagent grade) react as follows: MgCl + Ca(0H) = Mg(0H) + C a C l 2

2

2

2

Exper imenta1 condi t ions. Table 1 lists the experimental conditions. Co shows the apparent i n i t i a l concentration of reactants in the c r y s t a l l i z e r . Series 1,11 and I were conducted with stoichiometric feed r a t i o , with changing the feed r a t i o of reactants and with sodium chloride addition under the constant C of magnesium chloride and calcium hydroxide, respectively. 0


345

CRYSTALLIZATION AS A SEPARATIONS PROCESS

346


Table 1 Experimental conditions

oeries i

I

II

in in

Run No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Co MgCl2 2.02 4.90 10.2 14.7 20.7 10.2 10.2 10.2 10.2 10.2 9.88 9.90 9.79 9.79 9.80

3

Imol/m ) Ca(0H) NaCl 0 2.01 4.90 0 10.2 0 14.7 0 0 9.92 4.89 0 0 10.2 12.2 0 14.5 0 0 10.2 9.88 19.7 9.90 48 9.79 78.8 9.79 98.5 9.80 186 2


25.


TSUGE&MATSUO

The residence times of 10, 20 and 30 minutes.

reactants

in the c r y s t a l l i z e r were

5,


Analysis of the CSD Data From the population balance for a MSMPR c r y s t a l l i z e r operated under the steady-state condition, the population density n for size-independent c r y s t a l growth is given as n = n°exp(-l/G0 ) (2) where G and 0 are growth rate and residence time. Figure 2 shows the CSD plotted on semi logarithmic coordinates, whose ordinate shows the number of c r y s t a l s N. In the present CSD a n a l y s i s , population density n could not be obtained d i r e c t l y , but N was obtained by counting individual c r y s t a l s in a fixed s i z e range from SEM microphotographs. As the volume and size range, in which N c r y s t a l s are involved, are constant for each run, N is considered to be proportional to n and the following equation can be assumed: N oc e x p ( - l / G 0 ) (3) While the CSD in Figure 2 seems to show the maximum, the data points of larger p a r t i c l e s were measured more accurately in this CSD analysis so that the linear r e l a t i o n of larger p a r t i c l e s in Figure 2 was adopted to analyse the CSD. The increase of accuracy of measurements of CSD or the discussion of the CSD analysis by Bransom m o d e l w i l l be made f u r t h e r . The linear c o r r e l a t i o n indicates that Equation 3 is s a t i s f i e d and c r y s t a l growth obeys the AL law. From the slope of CSD, the growth rate G is obtained. The dominant p a r t i c l e s i z e In, and the nucleation rate B° are related as f o l l o w s ; 1» = 3G0 (4) B° = 9 P / ( 2 p f v l , » V ) (5) where p , fv and V are, respectively, c r y s t a l density, volume shape factor and the volume of c r y s t a l l i z e r . The production rate P in Equation 5 is calculated from Equation 6 by the mass balance of magnesium ion; P = M(C -C)F (6) where M, Co, C and F are molecular weight, i n i t i a l concentration of Mg , concentration of M g in the c r y s t a l l i z e r and feed rate, respectively. n

5)

3

r

c

c

r

r

0

2+

2+

Results and Discussion Volume shape f a c t o r . Figure 3 shows SEM microphotograph of the t y p i c a l c r y s t a l of Mg(0H) obtained for series I. As the c r y s t a l form is composed of d i s k l i k e units and the c r y s t a l structure of Mg(0H) is C d l type, the standard unit of Mg(0H) c r y s t a l is considered to be a d i s k . The r a t i o of the length L to thickness D of the disk of c r y s t a l unit was measured for each experimental condition, so that i t was found that L/D was nearly constant at 6.4. The c r y s t a l volumes were calculated for 2

2

2

American Chemical Society Library 1155 ISth St., N.W, In Crystallization as a Separations Process; Myerson, A., et al.; Washington, 20036 ACS Symposium Series; American OX. Chemical Society: Washington, DC, 1990.

2

347


348


irEMl

1 2 3 4 5

6 7 8 9 10

Feed tank Pump Liquid flow meter Crystallizer Constant head

Liquid exit Impeller Thermometer Motor pH meter

11 12 13 14

Recorder Const, temp, bath N2-Cylinder Gas exit

Figure 1: Schematic diagram of experimental apparatus

0

0.5

1.0

1.5

2.0

6

I x 1 0 (ml

Figure 2: Crystal

size distribution

Figure 3: SEM photograph of Mg(0H)

2



TSUGE & MATSUO

the unit numbers of d i s k s , n „ , so that the volume shape factors f were obtained as a function of n„ as shown in Figure 4. The broken l i n e in the figure shows the volume shape factor of sphere, k / 6 . Average unit number of disk fi„ increases with increases of the concentrations of OH" and C I " . As 2 is a function of both concentrations of OH" and C I " , fi„ is correlated by two parameter least squares method within an accuracy of + 15Z as follows; fi„=7.41[0H-]° [C1"]° (7) where [OH"] and [CI"] are expressed in mol/1. Equation 7 shows that the c r y s t a l surface grows with the increases of OH" and CI" concentrations. v

U


1 3 4

1

6

8

Crystallization kinetics. Figure 5 shows the r e l a t i o n between the dominant p a r t i c l e size In, and the residence time 9 for each run. Im decreases l i n e a r l y with increase of 9 in log-log p l o t . The slope of lines decreases with the addition of NaCl. Figure 6 shows the r e l a t i o n between the growth rate G and the residence time 9 in log-log plot for each run. The p a r a l l e l straight lines are written as G oc 0 - i i (8) The effect of the experimental series on the slope is minute. Figure 7 shows the r e l a t i o n between the c r y s t a l nucleation rate B° and 9 . B° decreases l i n e a r l y with increase of 9 in log-log p l o t . The slopes of lines increase with the increases of OH" and CI" concentrations, which are caused by the dependency of supersaturation of Mg(0H>2 on the OH" and CI" concentrations. The phenomena shown in Figures 5~7 are considered as follows: with the increase of the residence time 9 , the feed rates of reactants decrease so that both G and B° decrease. The decrease of l with the increase of 9 is caused by the decrease of G rather than the increase of 9 . Figure 8 shows the r e l a t i o n between B° and G with 9 as a parameter. From the material balance, the r e l a t i o n between B° and G can be written by the following equation: B° = ( 1 / 6 f P c ) M 0 - G (9) The broken and dotted chain l i n e s , respectively, show the results of Series I and Series tt for constant 9 . The s o l i d lines show the r e l a t i o n between B° and G with concentrations of OH" and CI" as a parameter and are expressed by Equation 1, so that the k i n e t i c order i is obtained from these slopes. K i n e t i c order i increases with increases of OH" and CI" concentrations . As i is a function of concentrations of OH" and C I " , i is correlated by Equation 10 by two parameter least squares method within an accuracy of + 10Z as shown in Figure 9, where the ordinate and abscissa show k i n e t i c order i calculated by Equation 10, i a i , and by e x p e r i m e n t , i , r e s p e c t i v e l y . i=2.63[OH"]° [Cl"]° (10) where [OH"] and [Cl"]are expressed in mol/1. m

4

v

3

T

C

eZP

1 2 4

2

0

9


349



350



25.


TSUGE&MATSUO

5

10

2

eis) Figure 6: Relation between G and 0

n

Dabir et a l showed that the k i n e t i c order i is written by Equation 11 for the reactive c r y s t a l l i z a t i o n of Mg(OH) produced by magnesium chloride and sodium hydroxide. i=8100[OH"] - 0.43 (11) where [OH"] is expressed in mol/1. Figure 10 shows the r e l a t i o n between i and [OH"] of our experimental and Dabir et al's results. The difference of both data is caused by the difference of the reaction system, the i n i t i a l concentrations of feeds and the measuring method of c r y s t a l s i z e , that i s , Dabir et a l . used a Coulter counter. 2

Conclusion Reactive c r y s t a l l i z a t i o n experiments of magnesium hydroxide were conducted to clarify the characteristics of reactive c r y s t a l l i z a t i o n k i n e t i c s by a continuous MSMPR c r y s t a l l i z e r .


351


352


10

r—i—i

i i i ii|

10

5

r

2

eisi Figure 7: Relation between B° and 9


Reactive Crystallization of Magnesium Hydroxide


TSUGE&MATSUO

10~

4

10"

3

10"

2

I0H"1 Imol/ll Figure 10: Relation between i and

[OH']


353

354


K i n e t i c order i of Mg(OH) made by p r e c i p i t a t i o n from raagnesi chloride with calcium hydroxide are correlated wi concentrations of hydroxide and chloride ions as follows; 124 2 0 9 i = 2 . 6 3 [ O H - ] ° [Cr]° where [OH ] and [CI"] are expressed in mol/1. 2

-

Legend of symbols B° nucleation rate C concentration C apparent i n i t i a l concentration D thickness of disk c r y s t a l F feed rate f volume shape factor G growth rate i k i n e t i c order ICAI k i n e t i c order calculated by Equation 10 iexp k i n e t i c order obtained by experiment L diameter of disk c r y s t a l 1 p a r t i c l e size lm dominant p a r t i c l e s i z e M molecular weight MT suspension density Ν number of p a r t i c l e η population density n° nuclei density n number of unit 3 mean number of unit P production rate V volume of c r y s t a l l i z e r Θ residence time ρ C c r y s t a l density


0

3

_1

m" s mol/m mol/m m m /s

3

3

3

v

m/s

m m m kg/mol kg/m 3

3

1

3

1

m" m" m" m"

u

U

r

kg/s m s kg/m 3

3

Literature Cited 1. Bransom, S.H., D.E. Brown, and G. P. Heeley: Inst. Chem. Eng. Symposium on Ind. Crystallization, 1969, p26. 2. Dabir,B., R.W.Peters and J.D.Stevens: Ind. Eng. Chem. Fundam., 1982, 21, 298-305. 3. Garside, J . and M.B.Shah: Ind. Eng. Chem. Process Des. Dev., 1980, 19, 509-514. 4. Packter, Α.: Crystal Res. and Technol., 1985, 20, 329-336. 5. Shirotuka,T. and K.Toyokura: Kagaku Kogaku, 1966, 30, 833839. RECEIVED May 17, 1990


Crystallization as a Separations Process - American Chemical Society

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