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The role of calcination temperature in iodine adsorption by different magnesium oxide powders
Powder
Technology,
49 (1987)
143
143 - 147
The Role of Calcination Temperature in Iodine Adsorption by Different Magnesium Oxide Powders S. F. ESTEFAN,
M. B. MORSI,
CMRDI, National
Research
G. A. EL-SHOBAKY
and I. F. HEWAIDY
Centre, Cairo (Egypt)
(Received January 1,1986)
SUMMARY
The variation in the physical properties of magnesium oxide powders prepared from dolomite ore via three different process technologies were examined, throughout the course of calcination at varying temperatures, using iodine adsorption and porosity determination.
INTRODUCTION
The different process technologies adapted for the recovery of magnesia from the two major sources, seawater and magnesiumbearing minerals, strongly influence the properties of the MgO product. The conditions during precipitation and subsequent ageing of the magnesium hydroxide crystals control their morphology. The degree of stacking may similarly be affected by mechanical treatment of the slurry before calcining. But the extent to which the physical properties of MgO are influenced is a complex function of calcining temperature, time, impurity level in the material and Mg(OH)2 morphology [ 11. For use as a pharmaceutical product for relieving indigestion or as a component in various cosmetics, magnesia should be light, chemically reactive, free of certain toxic impurities, and of very high specific surface area; on the contrary, magnesia used for the manufacture of heavy-duty basic refractories [2] should be dead-burnt (chemically inactive), of high density (not less than 3.3 g/cm3) and should be easily sintered to highdensity bodies. The demand for improved magnesia-based refractories to resist the severe working conditions experienced in the rotating basic oxygen steelmaking processes initiated a high level of research to characterize fully the properties of 0032-5910/871$3.60
magnesia, particularly with respect to the role of chemical impurities [ 21. Impurities are expected to have a marked effect on the kinetics of grain growth [ 31. Porosity may be considered one of these impurities of zero concentration and limited solubility. Major efforts will continue to be made by chemists and technologists to improve production methods and quality control to maximize product quality and minimize quality variations. The objective of this paper is to review the development and properties of synthetic magnesia as obtained by different process technologies and calcined at varying temperatures. The change in the physical properties of the MgO powders was examined throughout the course of calcination, using iodine adsorption and porosity determination.
EXPERIMENTAL
A selection of magnesia raw materials were produced from dolomite ore, calcined at 1100 “C, via three different process technologies, namely: (i) reaction with sea-bittern at controlled pH = 11, adapting the underliming technique 141; (ii) reaction with nitric acid-digested dolomite; (iii) reaction with ammonium chloride. The magnesium hydroxide products were calcined at different temperatures ranging between 550 and 1400 “C. The physical properties of the magnesium oxide products were investigated as follows: (1) Iodine adsorption measurements were carried out at room temperature (25 “C) using a standardized aqueous solution of iodine in potassium iodide. A fixed mass of each of the magnesium oxide powders (O.l1.0 g) was shaken vigorously in a known 0
Elsevier Sequoia/Printed
in The Netherlands
144
concentration of iodine solution for 72 h. The amounts of iodine taken up after equilibrium is established were determined by backtitration against a standard volumetric 0.2 N sodium thiosulphate solution. The amounts of iodine adsorbed per 100 g of magnesia were calculated. (2) Magnesium oxide samples calcined at 550 “C for 3 h were subjected to different doses of y-radiation, ranging between 5 - 50 Mrad, from a 6oCo source at a temperature of about 150 “C. The samples were stored for 1 week and then subjected to iodine adsorption from a standardized iodine solution. (3) The percentage of total porosity in the different magnesia samples was calculated using the equation Total porosity
% =(l-2)x100
where paPP and Ptrue are the apparent and true densities, respectively. The apparent density was determined by the pyknometric method, whereas the true density was determined from X-ray analysis by applying the equation
NXM Ptrue = -
VXL
where N is the number of moles per unit cell of crystal, M is the relative molecular mass of MgO, V is the volume of a unit cell (Q~)~, and L is the Avogadro constant.
RESULTS
AND DISCUSSIONS
The MgO content of magnesia raw material is not in itself the major characteristic controlling the properties and performance of the end-products. Actually, it is difficult to classify magnesia raw materials into categories purely on the basis of MgO content because other factors such as bulk density and the type, content, and relative proportions of the chemical impurities present in the magnesia have a fundamental effect on the slag resistance and high-temperature mechanical properties [ 21. However, chemical analyses, X-ray examinations and emission spectroscopic detection of trace elements, all together have confirmed that the three types of magnesia, prepared by the different process
technologies aforementioned, fall within the first category [ 51. The three calcined magnesium oxide products are of high purity, each containing not less than 98.5% MgO. The calcium and boron contents in the three samples are very low, averaging 0.65% CaO and 0.04% Bz03, respectively. All other impurities, Al, Fe, Mn, and Cu, are only detected in trace amounts [ 11. The properties of the MgO formed by the decomposition of Mg(OH)P are affected by the conditions under which the decomposition occurs, and depend on the heat treatment to which the MgO is subjected after decomposition, and on the morphology and physico-chemical properties of the precursor [ 11. In the present investigation, various physical changes taking place throughout the calcination process of magnesia powders, recovered from dolomite by different processes, have been explained. It has been noticed that very close to the decomposition temperature of the hydroxides, the magnesium oxide formed has a very small crystallite size [6]. At low calcination temperatures, there was small but significant crystallite growth without aggregation but, at approximately 925 “C, aggregation became evident and the deviation from theoretical increased once more. On raising the calcination temperature, there appeared to be a rapid increase in the particle size. There was also a simultaneous increase in the crystallite size, but the increase in the particle size was many times more than the increase in the crystallite size as indicated by X-ray line profile analysis 161. The results of iodine adsorption on to the three different magnesium oxide samples revealed that no significant change in the adsorption capacity of iodine was observed for all magnesium oxide samples heated at the temperature range 400 - 800 “C as shown, in Fig. 1. Eberle et al. [ 71 have studied the adsorption of iodine by magnesium oxide prepared by electrolysis and calcined at temperatures of 300 - 500 “C. Eventually, the values of iodine adsorption of 136 - 192 meq&/lOO g of MgO reported in their work are markedly low compared with the corresponding values presented in Fig. 1, and this may possibly result from wide divergence in the inter-facial active adsorption sites in the two types of
145
I
o
MgO
144-
recoveredby sea- bittemtechnique
(550 'C 3hwrs.j
F F
142-
J:
lLO-
o/0
./4
?! 9 '5 138 z ..! 2E
136j
/‘,,,, ,,, ,,,
134
26LxlXl
135 18-
0
lo-
2-
Lo
0i
\
"\ L.I.I.I.l.l.I.I.l.I.I 4m 5al6a 7ul &xl Sal 1Ku1103120011x)1~
lb)
Temp..!C
%I I
26-
7 8" D 2
22-
B0
lL-
.P .;;
lo-
& -_ .E (c)
la-
6_ 2r
10
20
30
40
50
60
Dose of )I -radiation (mega rad )
lL-
226-
0
/-"a
O-0
-1 \ ok 1.1.1.1.1.1.1.1.1.1 403 500 600 700 803 900 larlllml2al3al1~ Temp.:C
Fig. 1. Variation of iodine absorption with calcination temperature. (a), On MgO prepared by seabittern technique; (b), on MgO prepared by nitric acid technique; (c), on MgO prepared by NH&l technique.
magnesia. However, iodine adsorption progressively increased with increasing rirradiation dose for magnesium oxide samples calcined at 500 “C (Fig. 2). This is attributed to enhanced generation of new active adsorption sites on the magnesia surface caused by the highly energetic r-radiation. It is clear from Fig. 1 that iodine adsorption increased with increasing calcination temperature of magnesium oxides, attaining maxima for samples calcined at temperatures around 1000 “C. The increase in iodine adsorption by samples calcined at tempera-
Fig. 2. Effect of y-irradiation on iodine adsorption on MgO powder.
tures ranging between 850 and 1000 “C is attributed to the creation of active adsorption sites as a result of migration of surface OH groups. The subsequent decrease in iodine adsorption for samples calcined at temperatures higher than 1000 “C is a result of increased grain growth and sintering of the magnesium oxide samples as confirmed by electron micrographs [ 11. It has been shown that bulk impurities have a substantial effect on the properties of MgO powders. The presence of small amounts of impurity hinder the rate of grain growth and grain boundary migration [3]. The explanation for this is that it results from impurity drag at the boundary. This results in a greater temperature dependence because the boundaries must overcome the drag effect of the impurity. Activation energies for grain growth were calculated [8] as about 108 kJ mol-’ below about 1200 “C and 335 kJ mol-’ above about 1200 “c. Since the effect of impurities is to decrease grain growth, what is the effect of pores? Porosity is an impurity of a very gross kind because pores have only limited solubility [ 31. Although a pore or inclusion can reduce the driving force for grain boundary movement, it is difficult to imagine how a pore might alter the activation energy for grain boundary migration. Actually, thermal fluctuations may feed grain boundaries sufficient energy to overcome the restraining force of the pore and include the pore within the
146
19 - (A) MgO
i.l
z +
by sea - bittern
recovered
eye<
17-
. 1 . 0 * . ’ . ’ . ’ . 1 * ’ . 803 850 903 950 loco IO50 I100 1150
16
Temp..‘C G-4
t...‘.‘*‘.‘.‘...‘. &xl
850 903
950
loo0 1050 1100 1150
Temp.,‘C
@I (C )MgO
recovered
by
ammonium
chloride
technique.
0
27; O\ 26
0
25
24
be retarded and densification of aggregates could proceed. It is noteworthy that the maximum of iodine adsorption for the magnesium oxide recovered by the ammonium chloride technique was markedly lower than the corresponding maxima for the magnesium oxides recovered by the other two techniques. The explanation of this anomalous behaviour is that the presence of Cl- ion strongly enhances grain growth [8] of the product MgO. The crystal structures of Mg(OH)2 and MgClz are identical, and Clions can isomorphically replace OH- ions in the lattice. However, MgClz melts at 714 “C and boils at 1412 “C without dissociation, so that at normal calcining temperatures (800 - 1000 “C) it probably assists grain growth and sintering by forming a liquid phase in which ion transport is enhanced. This may also reflect diffusion problems resulting from variations in the pore volume and the pore-size distribution in the samples, or from recrystallization of MgO which decreases its reactivity. In the case of magnesia recovered from sea-bittern, the adaptation of the underliming technology [ 41 with the addition of a small amount of sodium hydroxide to control the pH of the pulp and to complete precipitation of any free magnesium ions, this sodium hydroxide washed out all Cl- ions entrained in the magnesium hydroxide precipitate and consequently Cl- ions could no longer affect grain growth.
0
23
2122
0\
I. I. I. I. I. 800 850 9ccl 950 loa,
I.
I.
8.
lo!3
llal
1150
Temp..‘C
Fig. 3. Change of total porosity with calcination temperature.
grain. Figure 3 delineates the dependence of porosity, for the three different magnesia samples, on the calcination temperature. Hey and Livey [9] have suggested that, if the surface diffusion coefficient in magnesia could be lowered, the growth process of crystallites and pores would
REFERENCES 1 M. B. Morsi, Ph.D. Thesis, Ain Shams Univ., Cairo (1985). 2 S. F. Estefan and L. G. Girgis, J. AufbereitungsTechnik, 9 (1978) 428. 3 R. C. Lowrie, Jr. and I. B. Cutler, Sintering and Related Phenomena, Gordon Aud Breach, New York, NY, 1967, pp. 527 - 541. 4 S. F. Estefan and F. T. Awadalla, J. Aufbereitunge-Technik, 3 (1983) 139. 5 M. Peatfield and D. R. F. Spencer, in Proc. Conf. Basic Oxygen Steelmaking: A New Technology Emerges, May. 1978, Whitstable Litho, Kent, 1979, pp. 107 - 122.
147 6 A. A. Ftamadan, M. B. Morsi, S. F. Estefan and I. F. Hewaidy, Proc. 8th Conf on Solid State Science, El-Minia University, El-Minia, Egypt, 24 - 27 Feb., 1985, p. 56.
7 H. Eberle, H. Wolfgang and H. Bartling, Ger. Pat. 2 060089, 1972. 8 J. Green, J. Mater. Sci., 637(1983) 18. 9 A. W. Hey and D. T. Livey, Trans. Brit. Ceram. Sot., 65 (1966) 627.
Technology,
49 (1987)
143
143 - 147
The Role of Calcination Temperature in Iodine Adsorption by Different Magnesium Oxide Powders S. F. ESTEFAN,
M. B. MORSI,
CMRDI, National
Research
G. A. EL-SHOBAKY
and I. F. HEWAIDY
Centre, Cairo (Egypt)
(Received January 1,1986)
SUMMARY
The variation in the physical properties of magnesium oxide powders prepared from dolomite ore via three different process technologies were examined, throughout the course of calcination at varying temperatures, using iodine adsorption and porosity determination.
INTRODUCTION
The different process technologies adapted for the recovery of magnesia from the two major sources, seawater and magnesiumbearing minerals, strongly influence the properties of the MgO product. The conditions during precipitation and subsequent ageing of the magnesium hydroxide crystals control their morphology. The degree of stacking may similarly be affected by mechanical treatment of the slurry before calcining. But the extent to which the physical properties of MgO are influenced is a complex function of calcining temperature, time, impurity level in the material and Mg(OH)2 morphology [ 11. For use as a pharmaceutical product for relieving indigestion or as a component in various cosmetics, magnesia should be light, chemically reactive, free of certain toxic impurities, and of very high specific surface area; on the contrary, magnesia used for the manufacture of heavy-duty basic refractories [2] should be dead-burnt (chemically inactive), of high density (not less than 3.3 g/cm3) and should be easily sintered to highdensity bodies. The demand for improved magnesia-based refractories to resist the severe working conditions experienced in the rotating basic oxygen steelmaking processes initiated a high level of research to characterize fully the properties of 0032-5910/871$3.60
magnesia, particularly with respect to the role of chemical impurities [ 21. Impurities are expected to have a marked effect on the kinetics of grain growth [ 31. Porosity may be considered one of these impurities of zero concentration and limited solubility. Major efforts will continue to be made by chemists and technologists to improve production methods and quality control to maximize product quality and minimize quality variations. The objective of this paper is to review the development and properties of synthetic magnesia as obtained by different process technologies and calcined at varying temperatures. The change in the physical properties of the MgO powders was examined throughout the course of calcination, using iodine adsorption and porosity determination.
EXPERIMENTAL
A selection of magnesia raw materials were produced from dolomite ore, calcined at 1100 “C, via three different process technologies, namely: (i) reaction with sea-bittern at controlled pH = 11, adapting the underliming technique 141; (ii) reaction with nitric acid-digested dolomite; (iii) reaction with ammonium chloride. The magnesium hydroxide products were calcined at different temperatures ranging between 550 and 1400 “C. The physical properties of the magnesium oxide products were investigated as follows: (1) Iodine adsorption measurements were carried out at room temperature (25 “C) using a standardized aqueous solution of iodine in potassium iodide. A fixed mass of each of the magnesium oxide powders (O.l1.0 g) was shaken vigorously in a known 0
Elsevier Sequoia/Printed
in The Netherlands
144
concentration of iodine solution for 72 h. The amounts of iodine taken up after equilibrium is established were determined by backtitration against a standard volumetric 0.2 N sodium thiosulphate solution. The amounts of iodine adsorbed per 100 g of magnesia were calculated. (2) Magnesium oxide samples calcined at 550 “C for 3 h were subjected to different doses of y-radiation, ranging between 5 - 50 Mrad, from a 6oCo source at a temperature of about 150 “C. The samples were stored for 1 week and then subjected to iodine adsorption from a standardized iodine solution. (3) The percentage of total porosity in the different magnesia samples was calculated using the equation Total porosity
% =(l-2)x100
where paPP and Ptrue are the apparent and true densities, respectively. The apparent density was determined by the pyknometric method, whereas the true density was determined from X-ray analysis by applying the equation
NXM Ptrue = -
VXL
where N is the number of moles per unit cell of crystal, M is the relative molecular mass of MgO, V is the volume of a unit cell (Q~)~, and L is the Avogadro constant.
RESULTS
AND DISCUSSIONS
The MgO content of magnesia raw material is not in itself the major characteristic controlling the properties and performance of the end-products. Actually, it is difficult to classify magnesia raw materials into categories purely on the basis of MgO content because other factors such as bulk density and the type, content, and relative proportions of the chemical impurities present in the magnesia have a fundamental effect on the slag resistance and high-temperature mechanical properties [ 21. However, chemical analyses, X-ray examinations and emission spectroscopic detection of trace elements, all together have confirmed that the three types of magnesia, prepared by the different process
technologies aforementioned, fall within the first category [ 51. The three calcined magnesium oxide products are of high purity, each containing not less than 98.5% MgO. The calcium and boron contents in the three samples are very low, averaging 0.65% CaO and 0.04% Bz03, respectively. All other impurities, Al, Fe, Mn, and Cu, are only detected in trace amounts [ 11. The properties of the MgO formed by the decomposition of Mg(OH)P are affected by the conditions under which the decomposition occurs, and depend on the heat treatment to which the MgO is subjected after decomposition, and on the morphology and physico-chemical properties of the precursor [ 11. In the present investigation, various physical changes taking place throughout the calcination process of magnesia powders, recovered from dolomite by different processes, have been explained. It has been noticed that very close to the decomposition temperature of the hydroxides, the magnesium oxide formed has a very small crystallite size [6]. At low calcination temperatures, there was small but significant crystallite growth without aggregation but, at approximately 925 “C, aggregation became evident and the deviation from theoretical increased once more. On raising the calcination temperature, there appeared to be a rapid increase in the particle size. There was also a simultaneous increase in the crystallite size, but the increase in the particle size was many times more than the increase in the crystallite size as indicated by X-ray line profile analysis 161. The results of iodine adsorption on to the three different magnesium oxide samples revealed that no significant change in the adsorption capacity of iodine was observed for all magnesium oxide samples heated at the temperature range 400 - 800 “C as shown, in Fig. 1. Eberle et al. [ 71 have studied the adsorption of iodine by magnesium oxide prepared by electrolysis and calcined at temperatures of 300 - 500 “C. Eventually, the values of iodine adsorption of 136 - 192 meq&/lOO g of MgO reported in their work are markedly low compared with the corresponding values presented in Fig. 1, and this may possibly result from wide divergence in the inter-facial active adsorption sites in the two types of
145
I
o
MgO
144-
recoveredby sea- bittemtechnique
(550 'C 3hwrs.j
F F
142-
J:
lLO-
o/0
./4
?! 9 '5 138 z ..! 2E
136j
/‘,,,, ,,, ,,,
134
26LxlXl
135 18-
0
lo-
2-
Lo
0i
\
"\ L.I.I.I.l.l.I.I.l.I.I 4m 5al6a 7ul &xl Sal 1Ku1103120011x)1~
lb)
Temp..!C
%I I
26-
7 8" D 2
22-
B0
lL-
.P .;;
lo-
& -_ .E (c)
la-
6_ 2r
10
20
30
40
50
60
Dose of )I -radiation (mega rad )
lL-
226-
0
/-"a
O-0
-1 \ ok 1.1.1.1.1.1.1.1.1.1 403 500 600 700 803 900 larlllml2al3al1~ Temp.:C
Fig. 1. Variation of iodine absorption with calcination temperature. (a), On MgO prepared by seabittern technique; (b), on MgO prepared by nitric acid technique; (c), on MgO prepared by NH&l technique.
magnesia. However, iodine adsorption progressively increased with increasing rirradiation dose for magnesium oxide samples calcined at 500 “C (Fig. 2). This is attributed to enhanced generation of new active adsorption sites on the magnesia surface caused by the highly energetic r-radiation. It is clear from Fig. 1 that iodine adsorption increased with increasing calcination temperature of magnesium oxides, attaining maxima for samples calcined at temperatures around 1000 “C. The increase in iodine adsorption by samples calcined at tempera-
Fig. 2. Effect of y-irradiation on iodine adsorption on MgO powder.
tures ranging between 850 and 1000 “C is attributed to the creation of active adsorption sites as a result of migration of surface OH groups. The subsequent decrease in iodine adsorption for samples calcined at temperatures higher than 1000 “C is a result of increased grain growth and sintering of the magnesium oxide samples as confirmed by electron micrographs [ 11. It has been shown that bulk impurities have a substantial effect on the properties of MgO powders. The presence of small amounts of impurity hinder the rate of grain growth and grain boundary migration [3]. The explanation for this is that it results from impurity drag at the boundary. This results in a greater temperature dependence because the boundaries must overcome the drag effect of the impurity. Activation energies for grain growth were calculated [8] as about 108 kJ mol-’ below about 1200 “C and 335 kJ mol-’ above about 1200 “c. Since the effect of impurities is to decrease grain growth, what is the effect of pores? Porosity is an impurity of a very gross kind because pores have only limited solubility [ 31. Although a pore or inclusion can reduce the driving force for grain boundary movement, it is difficult to imagine how a pore might alter the activation energy for grain boundary migration. Actually, thermal fluctuations may feed grain boundaries sufficient energy to overcome the restraining force of the pore and include the pore within the
146
19 - (A) MgO
i.l
z +
by sea - bittern
recovered
eye<
17-
. 1 . 0 * . ’ . ’ . ’ . 1 * ’ . 803 850 903 950 loco IO50 I100 1150
16
Temp..‘C G-4
t...‘.‘*‘.‘.‘...‘. &xl
850 903
950
loo0 1050 1100 1150
Temp.,‘C
@I (C )MgO
recovered
by
ammonium
chloride
technique.
0
27; O\ 26
0
25
24
be retarded and densification of aggregates could proceed. It is noteworthy that the maximum of iodine adsorption for the magnesium oxide recovered by the ammonium chloride technique was markedly lower than the corresponding maxima for the magnesium oxides recovered by the other two techniques. The explanation of this anomalous behaviour is that the presence of Cl- ion strongly enhances grain growth [8] of the product MgO. The crystal structures of Mg(OH)2 and MgClz are identical, and Clions can isomorphically replace OH- ions in the lattice. However, MgClz melts at 714 “C and boils at 1412 “C without dissociation, so that at normal calcining temperatures (800 - 1000 “C) it probably assists grain growth and sintering by forming a liquid phase in which ion transport is enhanced. This may also reflect diffusion problems resulting from variations in the pore volume and the pore-size distribution in the samples, or from recrystallization of MgO which decreases its reactivity. In the case of magnesia recovered from sea-bittern, the adaptation of the underliming technology [ 41 with the addition of a small amount of sodium hydroxide to control the pH of the pulp and to complete precipitation of any free magnesium ions, this sodium hydroxide washed out all Cl- ions entrained in the magnesium hydroxide precipitate and consequently Cl- ions could no longer affect grain growth.
0
23
2122
0\
I. I. I. I. I. 800 850 9ccl 950 loa,
I.
I.
8.
lo!3
llal
1150
Temp..‘C
Fig. 3. Change of total porosity with calcination temperature.
grain. Figure 3 delineates the dependence of porosity, for the three different magnesia samples, on the calcination temperature. Hey and Livey [9] have suggested that, if the surface diffusion coefficient in magnesia could be lowered, the growth process of crystallites and pores would
REFERENCES 1 M. B. Morsi, Ph.D. Thesis, Ain Shams Univ., Cairo (1985). 2 S. F. Estefan and L. G. Girgis, J. AufbereitungsTechnik, 9 (1978) 428. 3 R. C. Lowrie, Jr. and I. B. Cutler, Sintering and Related Phenomena, Gordon Aud Breach, New York, NY, 1967, pp. 527 - 541. 4 S. F. Estefan and F. T. Awadalla, J. Aufbereitunge-Technik, 3 (1983) 139. 5 M. Peatfield and D. R. F. Spencer, in Proc. Conf. Basic Oxygen Steelmaking: A New Technology Emerges, May. 1978, Whitstable Litho, Kent, 1979, pp. 107 - 122.
147 6 A. A. Ftamadan, M. B. Morsi, S. F. Estefan and I. F. Hewaidy, Proc. 8th Conf on Solid State Science, El-Minia University, El-Minia, Egypt, 24 - 27 Feb., 1985, p. 56.
7 H. Eberle, H. Wolfgang and H. Bartling, Ger. Pat. 2 060089, 1972. 8 J. Green, J. Mater. Sci., 637(1983) 18. 9 A. W. Hey and D. T. Livey, Trans. Brit. Ceram. Sot., 65 (1966) 627.