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Kinetics of magnesium hydroxide precipitation from seawater using slaked dolomite
Minerals Engineering, Vol. 7, No. 4, pp. 511-517, 1994 Copyright ~ 1 9 9 4 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892-6875194 $6.00 +0.00
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0892-6875(93)E0037-X
TECHNICAL NOTE KINETICS OF MAGNESIUM HYDROXIDE PRECIPITATION FROM SEAWATER USING SLAKED DOLOMITE
R.C. CARSON and J. SIMANDL* Dept. of Chemical Engineering, McGill University, 3480 University Street Montreal, Qu6bec, Canada H3A 2A7 * Author to whom correspondence should be addressed. (Received 17 August 1993; accepted 19 November 1993)
ABSTRACT
Precipitation of magnesimn hydroxide is the first step in the production of various magnesium compounds from seawater. Although the process is well known, kinetic data for the precipitation reaction using slaked lime and dolomite is not available in the literature, bz this study, the global kinetics of precipitation of magnesium hydroxide fi'om seawater were investigated by monitoring the total calcium and magnesium composition of species smaller than 2 I~m. The effects of temperature and dolomite particle size were also investigated. Keywords Magnesium hydroxide precipitation; dolomite
INTRODUCTION Production of magnesium compounds from seawater is a well known industrial process in which calcium hydroxide in the form of slaked lime or dolomite is used to precipitate magnesium hydroxide. The availability of inexpensive starting materials and the high purity of the product makes precipitating seawater magnesium an economically significant process. Magnesia from seawater is used in the refractory, pharmaceutical, pulp and paper, and waste water treatment industries. It also has flame retardant and refractory coating applications. The use of dolomite as a precipitant instead of lime increases product yield due to the presence of a mixture of both calcium and magnesium carbonate in the uncalcined ore. Despite the widespread use of this process, there is very little kinetic data available for the precipitation reaction. Previous research [1] focused on the kinetics of magnesium hydroxide precipitation by gel growth methods. Nucleation and crystal growth kinetics also have been predicted from a survey of available conductivity and induction period data [2]. In the present study, the mechanism of magnesium hydroxide precipitation from aqueous solution was investigated using slaked, calcined dolomite. The global reaction kinetics of magnesium hydroxide precipitation from seawater were determined using 50, 100, and 200 mesh (297, 149, and 74 micron respectively) dolomite particles. The effect of temperature on system kinetics was examined by performing experiments at temperatures between 0 and 50 Celsius using 200 mesh (74 micron) dolomite. 511
512
Technical Note
EXPERIMENTAL Materials Dolomite particles ground to 50, 100, and 200 mesh size (297, 149, and 74 micron) were used in a series of batch tests. The dolomite contained calcium and magnesium oxides at a ratio of 57:43. Impurities accounted for less than 1.68% of the total weight. Calcination occurred in stages at 400, 800, and 1000 degrees Celsius until a 46 % loss on ignition of the total mass was achieved from the release of carbon dioxide. A solution with an ionic composition similar to seawater was prepared by dissolving purified evaporated sea salt to a specific gravity of 1.025. The product used was "40 Fathoms Marine Mix" purchased from Marine Inc. A partial list of the species present in the solution is given in Table 1.
TABLE 1 Partial List of Species Present in Reconstituted Seawater Used in Experiments
Element
Concentration (ppm)
CI
18 600
Na
10 400
Mg
1 290
Ca
410
K
380
Br
62
B
4.9
F
1.9
Procedure A 100 ml slurry of slaked, calcined dolomite was placed in a 600 ml glass reactor, to which was then added 250 ml of seawater solution. This method of cation addition is termed a "reverse strike', and results in very high levels of supersaturation. The reactor contents were thoroughly agitated at 450 rpm using a magnetic stirrer, resulting in complete mixing within 3 seconds of seawater addition. An atomic absorption spectrophotometer was used to measure magnesium and calcium concentration at time intervals of approximately 30 seconds in the first five minutes of the experiments, and at 5 minute intervals thereafter. Results from these tests were used to determine the rate equations. Analysis After removal from the reactor, aliquots were analyzed immediately. In order to prevent the introduction of particulates into the analyzer, syringe filters with celhdose acetate membranes of 0.20 ~m pore size were used to withdraw 2 ml samples. The samples were atomized in an air-acetylene flame within 5 seconds of sampling. Although crystal nuclei and microcrystailites smaller than 0.20 ~m remained in the samples to be analyzed, they were sufficiently small to be registered in the absorbency reading. Therefore, the rate equations derived here may be considered to be global in their description of nucleation and crystal growth for magnesium species up to 0.20 I.tm in size.
Technical Note
513
RESULTS AND DISCUSSION Process Mechanism The magnesium hydroxide precipitation mechanism cart be divided into the following determine the magnesium and calcium ion concentrations in the reactor: 1)
Dissolution of the calcium hydroxide Ca(OH)2 --. Ca + +
2)
steps which
two
2OH"
+
Formation of magnesium hydroxide 2OI-I-
+
Mg ++ -. Mg(OH)2
1
Although both steps occur quickly, formation of magnesium hydroxide was identified as the rate-limiting step when using a 200 mesh (74 micron) calcium hydroxide source. Figures 1 and 2 show, as a function of time, the concentration of magnesium and calcium ions and microcrystallites smaller than 0.2 gtm when 200 and 50 mesh (74 and 297 micron) dolomite particles are used. For the 200 mesh (74 micron) dolomite particles, the calcium curve reaches an asymptote more quickly than the magnesium curve. When using 50 mesh (297 micron) particles, the two composition curves are very similar in shape. The rate of dolomite dissolution becomes dominant at a boundary particle size somewhere between 200 and 100 mesh (74 and 149 micron).
0.3
,
,
[]
[]
I
[]
I
l
[]
I
2500
0
2000
0.2
lSOO
, 8
[o c... r •
Mg z+
1000
o.1
8 u
500
0.0
i
0
I
100
i
I
I
i
I
I
200
300
400
500
600
0
700
"nme (s) Fig. ! Concentration of magnesium and calcium species greater than 0.2 ttm, for 200 mesh (74 micron) dolomite feed, 30°C
Technical Note
514
It should be noted that the small local maximum in the magnesium concentration curve was consistently observed in all replicates of the 200 mesh (74 micron) dolomite experiments.
0.30 3
'
'
'
'
'
'
2000
0.25 1500 I::
E e~ "
"~ 4-J
A•
0.20
tO
V
i 0.15
E,
I
l
0
(u u t-
8
o.lo
O0
looo * o
Ca2+ I M g 2","
8C 8 5oo
0
0
0.05 .
0.00
0
I
100
,
0
I
200
0
,
0
0
0
I
'
I
I
300
400
500
600
0
700
Time(s) Fig.2 Concentration of magnesium and calcium species greater than 0.2 pm, for 50 mesh (297 micron) dolomite feed, 30°C
Global Reaction Kinetics Precipitation can be divided into the following steps [2]: ions -. ionic clusters -. heterogeneous nuclei -. microcrystallites -. final primary crystals -. aggregates -. flocks The concentration data for species ranging from ions to microcrystallites smaller than 0.2 I.tm were modelled numerically. A global rate equation describing the combined rates of the first four steps listed above was thus obtained. For 200 mesh (74 micron) dolomite particles, d(Mg(OH)2) = d(Mg 2") = k(Mg2") 4.5 dt dt This is comparable to the rate order of 5 predicted by Packter [2]. The rate of reaction is independent of calcium concentration because hydroxide is present in excess due to the rapid calcium hydroxide
Technical Note
515
dissolution. The rate of change of magnesium concentration is therefore considered equal to the rate of disappearance of magnesium species smaller than 0.2 I.tm as given by the above equation. For 50 and 100 mesh (297 and 149 micron) dolomite, the dolomite dissolution rate is slower than the rate of magnesium hydroxide precipitation and there is no longer an excess of hydroxide present. The rate of magnesium hydroxide precipitation is limited by the rate of dissolution and the global rate equation has a different form: d(Mg(OH)2) _- d(Ca 2") __. k(Mg2+),(Ca2-)a dt dt where [3 = 2.5 and3 < ~ < 4 The global rate order with coarse dolomite particles is lower than that for fine particles. This is a result of the influence of the rate of mass transfer from the dolomite particle surface on the global rate of reaction. Effect of Temperature For the 200 mesh (74 micron) particles, the dependence of the rate constant k on temperature is shown in Figure 3, A decrease in temperature causes an increase in the rate constant, resulting in a faster reaction. This behaviour is uncommon for pure reaction kinetics but is prevalent in crystal growth kinetics. Thus, it is evident that nucleation and microcrystallite formation eclipse the ion reaction in the
5.0
!
I
4.5
4.0 3.5 v' O
o
3.0 2.5 2.0 1.5 1.0 3.2
i
!
3.4
I
I
i
3.6
(Temperature, K) "1 x 10 3 Fig.3 Relationship between temperature and the rate constant for 200 mesh (74 micron) dolomite
3.8
516
Technical Note
overall rate equation described previously. The experimental techniques used in this research study did not permit the isolation of ion reaction kinetics. These would be expected to exhibit an order and a temperature relationship different than those observed experimentally for the combined class of species smaller than 0.2 i.tm. A particle size analyzer was used to produce a distribution curve for the crystals present in the batch reactor after 30 minutes. The particle size distribution data, collected for crystals produced at 0° and 45° Celsius, are shown in Figure 4. No significant difference was observed.
100 90
~B
80
Ig
Ill
[]
wig ig
70 1,1'1
l.J
o~
ql
60
rl
C2.
50
uig
40
c:~
3O
r~ 4 5 " C "
O'C
~, 131
~o -~
0
0
25
50
75
1O0
125
Particle size (microns) Fig.4 Particle size distribution of solids found in the reactor after 30 minutes for two isothermal tests
CONCLUSIONS The form of the rate equation for the precipitation of magnesium hydroxide from seawater using dolomite depends upon the dolomite particle size. The rate of magnesium hydroxide precipitation was established by measuring the composition of species smaller than 0.2 ttm. At high supersaturations, combined nucleation and mierocrystallite growth was found to proceed according to the following equations: d(Mg(OH)9/dt = k (Mg2+) 4"5 for finely ground dolomite (200 mesh, 74 micron) d(Mg(OH)9/dt = k (Mg2+) a (Ca2+) # for coarsely ground dolomite (100 or 50 mesh, 149 or 297 microns) where 15 = 2.5 and 3 < cx < 4.
Technical Note
517
The rate constant k has a negative temperature dependence. No significant effect of temperature upon particle size distribution was observed.
ACKNOWLEDGEMENTS The assistance of Mr. Kin Hoe Chong, Ms. Serina Evers, and Ms. Kawai Tam made the rapid sampling and analysis possible. Dolomite used in the experiments was generously donated by Mr. J. Robitaille of Dolomine Inc. and Mr. Stuart Lee of Canspar. The financial support of the Brace Research Institute and the National Science and Engineering Research Council of Canada is gratefully acknowledged.
REFERENCES .
2.
Baird, T., Braterman, P.S., Cochrane, H.D. & Spoors, G., Journal of Oystal Growth, 91, no. 4, 610-615 (1988). Packter, A., Ct3,stal Research attd Technology, 20; no. 3, 329-336 (1985).
~rglllnon
0892-6875(93)E0037-X
TECHNICAL NOTE KINETICS OF MAGNESIUM HYDROXIDE PRECIPITATION FROM SEAWATER USING SLAKED DOLOMITE
R.C. CARSON and J. SIMANDL* Dept. of Chemical Engineering, McGill University, 3480 University Street Montreal, Qu6bec, Canada H3A 2A7 * Author to whom correspondence should be addressed. (Received 17 August 1993; accepted 19 November 1993)
ABSTRACT
Precipitation of magnesimn hydroxide is the first step in the production of various magnesium compounds from seawater. Although the process is well known, kinetic data for the precipitation reaction using slaked lime and dolomite is not available in the literature, bz this study, the global kinetics of precipitation of magnesium hydroxide fi'om seawater were investigated by monitoring the total calcium and magnesium composition of species smaller than 2 I~m. The effects of temperature and dolomite particle size were also investigated. Keywords Magnesium hydroxide precipitation; dolomite
INTRODUCTION Production of magnesium compounds from seawater is a well known industrial process in which calcium hydroxide in the form of slaked lime or dolomite is used to precipitate magnesium hydroxide. The availability of inexpensive starting materials and the high purity of the product makes precipitating seawater magnesium an economically significant process. Magnesia from seawater is used in the refractory, pharmaceutical, pulp and paper, and waste water treatment industries. It also has flame retardant and refractory coating applications. The use of dolomite as a precipitant instead of lime increases product yield due to the presence of a mixture of both calcium and magnesium carbonate in the uncalcined ore. Despite the widespread use of this process, there is very little kinetic data available for the precipitation reaction. Previous research [1] focused on the kinetics of magnesium hydroxide precipitation by gel growth methods. Nucleation and crystal growth kinetics also have been predicted from a survey of available conductivity and induction period data [2]. In the present study, the mechanism of magnesium hydroxide precipitation from aqueous solution was investigated using slaked, calcined dolomite. The global reaction kinetics of magnesium hydroxide precipitation from seawater were determined using 50, 100, and 200 mesh (297, 149, and 74 micron respectively) dolomite particles. The effect of temperature on system kinetics was examined by performing experiments at temperatures between 0 and 50 Celsius using 200 mesh (74 micron) dolomite. 511
512
Technical Note
EXPERIMENTAL Materials Dolomite particles ground to 50, 100, and 200 mesh size (297, 149, and 74 micron) were used in a series of batch tests. The dolomite contained calcium and magnesium oxides at a ratio of 57:43. Impurities accounted for less than 1.68% of the total weight. Calcination occurred in stages at 400, 800, and 1000 degrees Celsius until a 46 % loss on ignition of the total mass was achieved from the release of carbon dioxide. A solution with an ionic composition similar to seawater was prepared by dissolving purified evaporated sea salt to a specific gravity of 1.025. The product used was "40 Fathoms Marine Mix" purchased from Marine Inc. A partial list of the species present in the solution is given in Table 1.
TABLE 1 Partial List of Species Present in Reconstituted Seawater Used in Experiments
Element
Concentration (ppm)
CI
18 600
Na
10 400
Mg
1 290
Ca
410
K
380
Br
62
B
4.9
F
1.9
Procedure A 100 ml slurry of slaked, calcined dolomite was placed in a 600 ml glass reactor, to which was then added 250 ml of seawater solution. This method of cation addition is termed a "reverse strike', and results in very high levels of supersaturation. The reactor contents were thoroughly agitated at 450 rpm using a magnetic stirrer, resulting in complete mixing within 3 seconds of seawater addition. An atomic absorption spectrophotometer was used to measure magnesium and calcium concentration at time intervals of approximately 30 seconds in the first five minutes of the experiments, and at 5 minute intervals thereafter. Results from these tests were used to determine the rate equations. Analysis After removal from the reactor, aliquots were analyzed immediately. In order to prevent the introduction of particulates into the analyzer, syringe filters with celhdose acetate membranes of 0.20 ~m pore size were used to withdraw 2 ml samples. The samples were atomized in an air-acetylene flame within 5 seconds of sampling. Although crystal nuclei and microcrystailites smaller than 0.20 ~m remained in the samples to be analyzed, they were sufficiently small to be registered in the absorbency reading. Therefore, the rate equations derived here may be considered to be global in their description of nucleation and crystal growth for magnesium species up to 0.20 I.tm in size.
Technical Note
513
RESULTS AND DISCUSSION Process Mechanism The magnesium hydroxide precipitation mechanism cart be divided into the following determine the magnesium and calcium ion concentrations in the reactor: 1)
Dissolution of the calcium hydroxide Ca(OH)2 --. Ca + +
2)
steps which
two
2OH"
+
Formation of magnesium hydroxide 2OI-I-
+
Mg ++ -. Mg(OH)2
1
Although both steps occur quickly, formation of magnesium hydroxide was identified as the rate-limiting step when using a 200 mesh (74 micron) calcium hydroxide source. Figures 1 and 2 show, as a function of time, the concentration of magnesium and calcium ions and microcrystallites smaller than 0.2 gtm when 200 and 50 mesh (74 and 297 micron) dolomite particles are used. For the 200 mesh (74 micron) dolomite particles, the calcium curve reaches an asymptote more quickly than the magnesium curve. When using 50 mesh (297 micron) particles, the two composition curves are very similar in shape. The rate of dolomite dissolution becomes dominant at a boundary particle size somewhere between 200 and 100 mesh (74 and 149 micron).
0.3
,
,
[]
[]
I
[]
I
l
[]
I
2500
0
2000
0.2
lSOO
, 8
[o c... r •
Mg z+
1000
o.1
8 u
500
0.0
i
0
I
100
i
I
I
i
I
I
200
300
400
500
600
0
700
"nme (s) Fig. ! Concentration of magnesium and calcium species greater than 0.2 ttm, for 200 mesh (74 micron) dolomite feed, 30°C
Technical Note
514
It should be noted that the small local maximum in the magnesium concentration curve was consistently observed in all replicates of the 200 mesh (74 micron) dolomite experiments.
0.30 3
'
'
'
'
'
'
2000
0.25 1500 I::
E e~ "
"~ 4-J
A•
0.20
tO
V
i 0.15
E,
I
l
0
(u u t-
8
o.lo
O0
looo * o
Ca2+ I M g 2","
8C 8 5oo
0
0
0.05 .
0.00
0
I
100
,
0
I
200
0
,
0
0
0
I
'
I
I
300
400
500
600
0
700
Time(s) Fig.2 Concentration of magnesium and calcium species greater than 0.2 pm, for 50 mesh (297 micron) dolomite feed, 30°C
Global Reaction Kinetics Precipitation can be divided into the following steps [2]: ions -. ionic clusters -. heterogeneous nuclei -. microcrystallites -. final primary crystals -. aggregates -. flocks The concentration data for species ranging from ions to microcrystallites smaller than 0.2 I.tm were modelled numerically. A global rate equation describing the combined rates of the first four steps listed above was thus obtained. For 200 mesh (74 micron) dolomite particles, d(Mg(OH)2) = d(Mg 2") = k(Mg2") 4.5 dt dt This is comparable to the rate order of 5 predicted by Packter [2]. The rate of reaction is independent of calcium concentration because hydroxide is present in excess due to the rapid calcium hydroxide
Technical Note
515
dissolution. The rate of change of magnesium concentration is therefore considered equal to the rate of disappearance of magnesium species smaller than 0.2 I.tm as given by the above equation. For 50 and 100 mesh (297 and 149 micron) dolomite, the dolomite dissolution rate is slower than the rate of magnesium hydroxide precipitation and there is no longer an excess of hydroxide present. The rate of magnesium hydroxide precipitation is limited by the rate of dissolution and the global rate equation has a different form: d(Mg(OH)2) _- d(Ca 2") __. k(Mg2+),(Ca2-)a dt dt where [3 = 2.5 and3 < ~ < 4 The global rate order with coarse dolomite particles is lower than that for fine particles. This is a result of the influence of the rate of mass transfer from the dolomite particle surface on the global rate of reaction. Effect of Temperature For the 200 mesh (74 micron) particles, the dependence of the rate constant k on temperature is shown in Figure 3, A decrease in temperature causes an increase in the rate constant, resulting in a faster reaction. This behaviour is uncommon for pure reaction kinetics but is prevalent in crystal growth kinetics. Thus, it is evident that nucleation and microcrystallite formation eclipse the ion reaction in the
5.0
!
I
4.5
4.0 3.5 v' O
o
3.0 2.5 2.0 1.5 1.0 3.2
i
!
3.4
I
I
i
3.6
(Temperature, K) "1 x 10 3 Fig.3 Relationship between temperature and the rate constant for 200 mesh (74 micron) dolomite
3.8
516
Technical Note
overall rate equation described previously. The experimental techniques used in this research study did not permit the isolation of ion reaction kinetics. These would be expected to exhibit an order and a temperature relationship different than those observed experimentally for the combined class of species smaller than 0.2 i.tm. A particle size analyzer was used to produce a distribution curve for the crystals present in the batch reactor after 30 minutes. The particle size distribution data, collected for crystals produced at 0° and 45° Celsius, are shown in Figure 4. No significant difference was observed.
100 90
~B
80
Ig
Ill
[]
wig ig
70 1,1'1
l.J
o~
ql
60
rl
C2.
50
uig
40
c:~
3O
r~ 4 5 " C "
O'C
~, 131
~o -~
0
0
25
50
75
1O0
125
Particle size (microns) Fig.4 Particle size distribution of solids found in the reactor after 30 minutes for two isothermal tests
CONCLUSIONS The form of the rate equation for the precipitation of magnesium hydroxide from seawater using dolomite depends upon the dolomite particle size. The rate of magnesium hydroxide precipitation was established by measuring the composition of species smaller than 0.2 ttm. At high supersaturations, combined nucleation and mierocrystallite growth was found to proceed according to the following equations: d(Mg(OH)9/dt = k (Mg2+) 4"5 for finely ground dolomite (200 mesh, 74 micron) d(Mg(OH)9/dt = k (Mg2+) a (Ca2+) # for coarsely ground dolomite (100 or 50 mesh, 149 or 297 microns) where 15 = 2.5 and 3 < cx < 4.
Technical Note
517
The rate constant k has a negative temperature dependence. No significant effect of temperature upon particle size distribution was observed.
ACKNOWLEDGEMENTS The assistance of Mr. Kin Hoe Chong, Ms. Serina Evers, and Ms. Kawai Tam made the rapid sampling and analysis possible. Dolomite used in the experiments was generously donated by Mr. J. Robitaille of Dolomine Inc. and Mr. Stuart Lee of Canspar. The financial support of the Brace Research Institute and the National Science and Engineering Research Council of Canada is gratefully acknowledged.
REFERENCES .
2.
Baird, T., Braterman, P.S., Cochrane, H.D. & Spoors, G., Journal of Oystal Growth, 91, no. 4, 610-615 (1988). Packter, A., Ct3,stal Research attd Technology, 20; no. 3, 329-336 (1985).