Technical note hydration of calcined magnesite at elevated temperatures under turbulent conditions

Mi.erals Engi.eering, Vol. 2, No. 2, pp. 263-270, 1989 Printed in Great Britain

0892-6875/89 $3.00 + 0.00 © 1989 Pergamon Press plc

TECHNICAL NOTE HYDRATION OF CALCINED MAGNESITE AT ELEVATED TEMPERATURES UNDER TURBULENT CONDITIONS A. MERCY RANJITHAM and P.R. KHANGAONKAR National Metallurgical Laboratory Madras Centre, (Council of Scientific and Industrial Research) Taramani, Madras-600113, India (Received 3 November 19881 revision 8 February 1989)

ABSTRACT The kinetics of the reaction between water and calcined magnesite have been investigated for the effect of different variables viz. temperature, particle size, calcination temperature, magnesium sulphate soaking effect and calcium chloride addition on the rate of hydration. The activation energy is 45±0.5 kJ/mol, indicating chemical reaction as the rate controlling process. The preliminary treatment of raw magnesite with MgS04 7H20 prior to calcination did not have any critical effect on the degree of hydration. Keywords Magnesite; calcination; hydration; particle size INTRODUCTION Hydration is an important step in the hydrometallurgical pressure carbonation of aqueous MgO slurries [I ].- A fairly extensive work on hydration of magnesium o x i d e has b e e n c a r r i e d out by earlier workers [ 2 - 5 ] . M o s t of t h e s e i n v e s t i g a t o r s h a v e f o c u s s e d t h e i r attention on the hydration of magnesium o x i d e by w a t e r v a p o u r or w a t e r in an u n s t i r r e d vessel. G l a s s o n [6] has determined the rate curves for two samples of magnesium oxide hydrated at 22 ° and 95oc in a stirred batch reactor, but he was unable to fit his experimental data to a single rate equation w h e r e a s B u d n i k o v [7] m e a s u r e d the rate of hydration of magnesium oxide calcined at high temperature from 800o-I 800oc. Layden and Brindley [8] studied the kinetics of v a p o u r p h a s e h y d r a t i o n of magnesium oxide. The hydration of magnesium oxide was also studied by Solove et al [9] whereas Gavirish and Galinker [10] studied the behaviour of fused m a g n e s i u m oxide in water at high temperature. The mechanism of hydration of m a g n e s i u m o x i d e w i t h w a t e r v a p o u r at room t e m p e r a t u r e was e x p l a i n e d by F e i k n e c h t and B r a u n [11] u s i n g g r a v i m e t r i c x-ray and electron microscopic methods. Smithson and Bakhshi [12] studied the kinetics of hydration of several samples of commercially prepared magnesium oxide in a batch reactor at low temperature and proposed a mechanism which agrees with that of the earlier workers [11]. Chauhan [13] carried out his hydration study with electrolytic magnesia and found the activation energy to be -13.3 Kcal/mol. Asifs et al [14] used the quartz spring method to study the uptake of water vapour on magnesia. Kithara and Furuta [15] investigated the hydration of calcined MgO powders and observed sigmoidal hydration curves with an initial accelerated rate. Fruhwrith and Herzog et al [16] investigated the wet hydration kinetics of MgO single crystal and powder samples with regard to H +, Mg 2+ concentration and temperature. The present study deals with the hydration of calcined magneslte temperatures under turbulent conditions.

263

at elevated

264

Technical Notes

EXPERIMENTAL Materials The m a g n e s i t e sample used in this present i n v e s t i g a t i o n was obtained from M/s Burn Standart Ltd., Salem, India. The chemical analysis of the sample is given in Table I. P e t r o l o g i c a l m i c r o s c o p i c study revealed that the sample contained mainly 95% of m a g n e s i t e and minor amounts of dunite and iron oxide. T h e sample was crushed to 1.68 mm, calcined at three d i f f e r e n t temperatures viz. 650 ° , 750 ° and 850°C in a muffle furnace for three hours and then ground in a p o r c e l a i n b a l l m i l l . S a m p l e s of t h r e e s e l e c t e d n a r r o w r a n g e s w e r e o b t a i n e d by dry sieving. All of the chemical used in this i n v e s t i g a t i o n were of reagent grade with d i s t i l l e d water for solutions. T A B L E I C h e m i c a l a n a l y s i s of the m a g n e s i t e sample

Radicals MgO CaO SiO 2 Fe203 AI203 Other oxides LOI

Percentage 44.43 1.16 3.24 2.02 0.07 0.23 48.84

A p p a r a t u s and p r o c e d u r e T h e h y d r a t i o n s t u d i e s w e r e c a r r i e d out in a 500 ml spherical three necked glass vessel placed in a t h e r m o s t a t i c a l l y controlled water bath ± 0.IOC. The reactor had provisions for thermometer, a sampling device, a condenser and a stirrer. To start the e x p e r i m e n t a l run the calcined m a g n e s i t e sample was first hydrated with cold water and d e c a n t e d so as to enable the removal of calcium oxide. The entire sample was then t r a n s f e r r e d to the reaction vessel containing the r e q u i r e d a m o u n t of f r e s h l y p r e p a r e d d i s t i l l e d w a t e r , w h i c h w a s a l r e a d y e q u i l i b r a t e d to the bath temperature. The stirrer speed was m a i n t a i n e d at 1120 rpm throughout the experimental work. 5 ml of the s l u r r y w a s r e m o v e d e v e r y 30 m i n u t e s and immediately filtered through a sintered crucible. In all experiments, at the end of each filtration the h y d r a t e d residue was washed twice with 10 ml portions of acetone to remove immediately most of the w a t e r in c o n t a c t w i t h t h e m t h e r e b y a r r e s t i n g any further h y d r a t i o n and ageing of the sample. The air dried samples were used for analysis. There is always some m a g n e s i u m h y d r o x i d e p r e s e n t in all oxide sample for w h i c h the c o r r e c t i o n was applied while c a l c u l a t i n g hydration. RESULTS AND D I S C U S S I O N Effect of temperature A p l o t of f r a c t i o n h y d r a t e d (e) vs time for six d i f f e r e n t t e m p e r a t u r e s is shown in Figures I and 2. From Figures I a n d 2 it is s e e n t h a t a m a x i m u m f r a c t i o n was hydrated at 80°C. The t e m p e r a t u r e for the r e a c t i o n was chosen to be 70°C as higher temperatures lead to quick loss of water due to evaporation. The rate h y d r a t i o n increased with rise in reaction temperature. Effect of c a l c i n a t i o n t e m p e r a t u r e The effect of three d i f f e r e n t c a l c i n a t i o n temperatures 650 ° , 750 ° and 850°C of

Technical Notes

265

magnesite on the rate of hydration is indicated in Figure 3. In the present study it was seen that magnesite was fairly active during hydration, when it w a s c a l c i n e d at 6 5 0 o - 8 5 0 o c and it can be seen that m a x i m u m c o n v e r s i o n e f f i c i e n c y was found with 650oc. The higher the calcination temperature of magnesite the lower was its degree of hydration. This trend is due to the fact that the activity of magnesia produced at low temperatures of calcination is influenced by lattice dilation and imperfect crystal structure. O'J

.

0.7

~ O.6 o,,* o.*

~

o.~

i" r

O.t o •f o*0

PARTtCgE SlZ~ :

O.J

REACTIO~ t E M P E r A t U R E f*C )

REACTION R fACTI rENPE~Aru,c~E ( ' c ) ~

r

, 30

p.

"~S+~

, so

, 90

, ~0

| ~0

i , rio ~0

, J , Z~O 2TO 300

~

So

~

30"5

~

~

70

~

~0

0.0

| 0

JO

SO

gO

~20 SO ~ 0

2~0 ia.O 270 ~00

T I N E , rain.

Figs.1

and 2

Effect of temperature on fraction of magnesia hydrated

0.9 o.B SLURRY CONCENTRATION : I,OQ/INoC

0.?

REACTION TEHPERATURE : 70"C

0.6

PARTICLE S I Z E : - 75+63JJm

0.5

CALCINATION TENPERATURE

0.4

=

=

( "C ) 650

Q

~. 0 . 3

0.2

=

=

750

l

A

850

0.1 0.0

0

I

i

I

I

30

60

90

120

I

i

I

I

I

I

150 tSO 210 240 270 300

TINE, rain.

Fig.3

Effect of calcination temperature on fraction of magnesia hydrated

Effect of particle size The effect of three different particle sizes viz. -75+63, -63+53 and -53+45 microns on the rate of hydration is shown in Figure 4. The degree of hydration i n c r e a s e d s u b s t a n t i a l l y with finer particles. It may however be noted that although the experiments were started with materials with a narrow size range as above, size degradation took place immediately to finer sizes. The details of the size degradation are reported in a separate communication. Effect of magnesium chloride addition concentration The effect of magnesium chloride addition concentration 2%, 4% and 6% on the rate of hydration was then examined. From the Figure 5, it can be seen that the rate of hydration is slightly influenced by the addition of MgCI 2. Effect of MgSO 4 7H20 soaking on hydration A c c o r d i n g to S t r e l e t s [17] a p r e l i m i n a r y t r e a t m e n t of m a g n e s i t e w i t h solution of MgSO 4 7H20 results in a considerable activation of the mineral.

a

To confirm this the magnesite sample was soaked in magnesium sulphate for four hours, f i l t e r e d and then c a l c i n e d u n d e r o p t i m u m c o n d i t i o n s f o l l o w e d by subsequent hydration.

266

Technical Notes

O.I 0.8

"~ 0.?

~

,~

N

A

T

I

O

l l " ~

c~ 0 . 8

/ 2 " /

0,5

0.4

Iif

~. 0.3

'/I"

N

TEMPERATURE : 750"C

s , , . . , ~:"OENT"A'ION, ,o.1, ..o RE AO, IOM , EMPERA,UR E : '0",

"'":'"

"'z" " " ....

I

~. 0,2

~

-63÷52

m....-A -:'5+63 i 30

0.0

I I I I I i I I I 60 90 120 150 180 210 21,0 270 300

TIME ~ rain.

Fig.4

Effect of particle size on fraction of magnesia hydrated

0.9 0"8

INATION TENPERATURE : 750"C 0"7

RY CONCENTRATION; 40g/INgO

0"6

ICLE SIZE : --75+63 /lm

¢J

ION TEMPERATURE : 70"C

O=

/

O.&

NgCll ADDITION [ % )

~" 0"3 0.2 b~-/*

0'I I

I

I

i

I

~

~

90

120

~0

0"0 0

l

I

l

I

MO 210 24,0 2 ~

6

i 3~

~NE pmln. Fig.5

Effect

of

magnesium

chloride

addition

on

fraction

of

magnesia

hydrated

In contrast to the above statement made by Strelets the results obtained Figure 6 indicates that magnesium sulphate soaking had no critical effect the rate of hydration.

in on

0"8 0.7

0.6

===================

O.S

c= O. I, k.. 0.3

o

CLE SIZE : -75+63/Jrn

p

N ~ C T I O N ~EMPERATU.E

/

:~C

AMOUNT OF . . S O , . ' M , O :

,sg.I'

0.2 0.1 0.0

|

|

30 60

Fig.6

!

90

i

i

|

l

a

|

i

f20 150 f#O 210 2&O 270 300 TIME prain.

Effect of magnesium sulphate soaking on fraction of magnesia hydrated

Technical Notes

267

Rate of hydration The c o m m o n l y o b s e r v e d rate c o n t r o l l i n g p r o c e s s in a h e t e r o g e n e o u s noncatalytic reaction is either diffusion control or chemical reaction control. The energy for solution diffusion is usually of the order of 5 Kcal/mol (21 KJ/mol.) or less and chemical reaction control usually occurs between 10 and 25 Kcal/mol. (42 to 105 KJ/mol.) [18]. Models involving diffusion process as the rate controlling were considered for the data in the present work but they did not give satisfactory agreement with the requirements of the above process. The integrated rate equation for chemical reaction, controlling the shrinking spherical particles is given by the following Eq. [19].

1-(1-c¢)1/3

= K,t

(1)

Attempts to plot the data by Eq. (I) a b o v e did not y i e l d a s t r a i g h t line relationship. According to S m i t h s o n and B a k h s h i the a b o v e Eq. may be applicable for hydration of similar sized particles whereas Mg(OH) 2 particles are composed of a ranged particle sizes. Smithson and Bakhshi indicated the alternate relationship -in (l-u) = kt

(2)

Since the product in the present investigation is also composed of a range of particle sizes, the present experimental data were analysed according to the Eq. (2) above The plots of -ln (I-~} vs time for six different reaction temperatures viz. 30.5 ° , 40 ° , 50 ° , 60 ° , 70 ° and 80oc are shown in Figure 7. From the slope of these linear plots the rate constant was calculated. A plot of log k vs I/T (Fig. 8) g a v e a s t r a i g h t l i n e , f r o m w h i c h the a c t i v a t i o n e n e r g y was calculated. In the present case activation energy was found to be 45 ± 0.5 KJ/mol which is h i g h e n o u g h to rule out the p o s s i b i l i t y of d i f f u s i o n being the rate determining step. The hydration of calcined magnesite at elevated temperature is apparently controlled by surface chemical reaction. The p l o t s of -in (I- u) a g a i n s t temperatures and particle sizes is energy obtained in the present case Smithson and Bakhshi for hydration lower temperatures.

r e a c t i o n time for d i f f e r e n t calcination shown in Figures 9 and 10. The activation is in close agreement with the findings of of commerclal MgO samples w i t h w a t e r at

CONCLUSIONS The rate of h y d r a t i o n i n c r e a s e s w i t h rise in r e a c t i o n t e m p e r a t u r e . The p r e l l m i n a r y t r e a t m e n t of raw m a g n e s l t e w i t h M g S O 4 7H20 s o a k i n g prior to calcination did not have any critical effect on degree of hydration. The rate of hydration is controlled by the rate of chemical reaction occurring at the M g O p a r t i c l e surface. T h e a c t i v a t i o n energy obtained in t h e p r e s e n t investigation was found to be 45 ± 0.5 KJ/mol. indicating the reaction to be chemically controlled. ACKNOWLEDGEMENT The authors are thankful to The Director, Natlonal Metallurgical Laboratory, Jamshedpur-831 007 for providing the necessary facilities to conduct this work successfully. LIST OF SYMBOLS k t T

= = = =

Reaction ReaCtion Reaction Fraction

rate constant time temperature reacted

268

Technical Notes

2.0 1"9

1.8 f.7 1.6

1.5 f.&

1.3 1.2

?

1.1

I

1.0 0.9

I

0"8

0.7

CALCINATION TEMPERATURE " 750"C

0"5

" SLURRY CONCENTRATION : 0"5

t, Og/ I MgO

O'&

PARTICLE SIZE : -75+631.1m

0"3

REACTION TEMPERATURE ('C ) c o 30.5 - 60

0"2

&O

o.~ t A

J~

50

70 x

x

80

0.0 0

30

60

90

120 150 180 210 2 ~

270 300 330 360 390 120 I,.50

T!ME p rain.

Fig.7

Plots of -in(l-a) 1.9

vs time at different

temperatures

2.0 2.1 2.2 2.3 2.4 2.5

.z

~

2"6

I 2"8

2.9 3.0 0 3.T 3.2

I 2.5

2,7

2.9 I/T

Fig.8

An arrhenius

3.f

!

I

3"3

3.5

x IO'~(K "~ )

plot for the hydration of MgO particles

Technical Notes

269

2.1 2"0 1"9 1.8 1.;' 1.6 1"5 1"&

1"2

',t .~ 1"0 0.9 #

0"8

SLURRY CONCENTRATION " LOg/INgO

-75+63~m

0.7

PARTICLE SIZE ."

0"6

REACTION TENPERATURE : 70"C CALCINATION TENPERATURE ( "C }

0"5

650

0"/, 0.3 0.2

o-.--a

?50

b~A

850

0.1 0.0

I

I

|

0

30

60

i

|

a

!

*

a

a

a

i

I

a

!

90 120 ;'50 10 210 2/,0 2?0 500 330 360 390 &20 TIRE t rain.

Fig.9

Plots of -in(1-~) vs time at d i f f e r e n t c a l c i n a t i o n t e m p e r a t u r e s

"1

/o

2"0

o

1.9

/

1.8

/

1.? 1.6 1.5 1.4 1.3 1.2

/

/

~ 1.0 e

CALCINATION TENPERATURE : 750"C

"{

0"9

SLURRY CONCENTRATION

O.B

REACTION TENPERATURE

0.?

pARTICLE SIZE

." &Og/tNgO :

70"C

( ~n) )

0.6

o~

-53.P45

0.5

~

-63+53

0.4

~

-?$÷63

0.3 0.2

I

0.I' 0.0

0

30 60

90 I20 150 180 2TO 240 2?'0 300 $30 360 TIME, rain.

Fig.10 ME 2~-H

Plots of -ln

(l-u) vs time at d i f f e r e n t p a r t i c l e sizes

270

Technical Notes REFERENCES

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Evans R.L. & St. Clair H.W., Ind. Eng. Chem., 41, 2814-2817,

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Bratton R.J. & Bindley G.W.,

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Hollerer

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Dai

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19. L e v e n s p i e l 0., Chemical Reaction New York, 350-375, (1964).

J. Chem.

I. & Rachett

17. Strelets Kh. L., Electrolytic Production Jerusalem Ltd., 23-28, (1977). 18. Sohn H.Y. & W a d s w o r t h Plenum Press, New York,

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