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Influence of the method of alumina modification on formation of low-temperature solid solutions in magnesia—alumina systems
Applied Catalysis, 72 11991 ) 63-69
63
Elsevier Science Publishers B.V., Amsterdam
Influence of the method of alumina modification on formation of low-temperature solid solutions in magnesia-alumina systems N.A. Koryabkina, Z.R. Ismagilov*, R.A. Shkrabina, E.M. Moroz and V.A. Ushakov Institute of Catalysis, Prosp. Akad. Lavrentieva, 5, Novosibirsk, 6,30090 (I_~¢qSR) cReceived 20 July 1990. revised manuscript received 21 November 1990}
Abstract
Formation of solid solutions in magnesia-alumina systems has been studied. It has been shown that depending on the method of introduction of magnesium and the temperature of heat treatment, formation of three types of solid solutions with the structure of ~'-alumina is possible; the anionic, the anionic-cationic and the cationic type. The mechanical strength of the obtained samples of spherical alumina depends on the method of modification; the strongest samples are produced if magnesium is added to aluminium hydroxide with subsequent calcination at 820 K.
Keywords: solid solutions, magnesia-alumina, calcination, heat treatment.
INTRODUCTION
It is known that oxidation catalysts which operate under fluidized bed conditions may change considerably during use, which is connected with the sintering of the support. Phase transformations and mechanical decomposition of the catalyst granules also occur under these conditions. For this reason, one of the most important requirement imposed on the combustion catalyst supports is a high mechanical strength and thermal stability. One of the most widely used methods to increase the strength of supports and catalysts is chemical modification [1-4]. However, there is no complete description of the mechanism by which modifiers achieve their strengthening activity in the literature. The majority of the explanations are connected with the sintering activity of introduced additions at temperatures > 1273 K, which is supposed by the authors of refs. 3-5 to lead to strengthening of supports, i.e. to strengthening of the alumina, the latter being modified by addition of alkaline earth and rare earth elements (Ca, Mg, La, Zr, etc). It is noteworthy
0166-9834/91/$03.50
(~ 1991 Elsevier Science Publishers B.V.
64 that formation of, for example, magnesia-alumina compounds depends on the method of their preparation [6-8]. After coprecipitation of magnesium and aluminium hydroxides with subsequent calcination it has been discovered that formation of the spinel MgAlvO4 takes place at 673 K, while, in samples obtained by mixing aluminium and magnesium hydroxides, the spinel phase cannot be detected even at 1073 K [6]. Thermal decomposition of the mixture of nitrate salts of aluminium and magnesium leads to the formation of spinel within the interval 573-773 K, as has been shown in ref 7. Crystallization of magnesium-aluminium spinels, produced from the gels of nit rates of Mg ~+ and Al ~+, and mixing of the oxides of these metals [8 ], leads to tbrmation of 'disordered' spinels at 923-1273 K. It is noteworthy that the structures of magnesia-alumina compounds, containing> 10 wt.-% of magnesia are commonly studied [9]. However, as is mentioned in ref. 10, even if the magnesia content is 2-4 wt.~c at 823 K a magnesium solid solution ~in y-A120:~) is detected, as well as a decrease in the density of defects and an increase in the strength of spherical granules. In ref. 11 the formation of low-temperature solid solutions in alumina, containing < 10 wt.-% magnesia, was studied. On the basis of the oxidehydroxide formula given by authors, some variants of the arrangement of Mg ~÷ cations in the structure of l'-Al._)O:3 were discussed and structural formulas of compounds were proposed with respect to the amount of addition. It has been shown that the structure of low temperature solid solutions is characterized by a considerable defect density of cationic ~due to the filling of non-spinel positionsl and anionic (due to the presence of hydroxyl groups} lattice frameworks. However, data on the relationships between mechanical strength of granules and the amount of modifying additive, conditions of preparation, etc., were not reported. The present work represents a continuation of work initially published in refs. 10 and 11, and is concerned with studying the influence of the method of introduction of magnesium nitrate on the formation of low temperature magnesia-alumina compounds and on the mechanical strength of spherical granules prepared from them. EXPERIMENTAL Spherical alumina supports (diameter of granules = 2-3 mm ) were prepared from an industrial hydroxide having a pseudoboehmite structure. Formation of the granules was carried out by the hydrocarbon-ammonia method. Magnesium modification was conducted by two methods: introduction of magnesium nitrate solution to spherical granules of aluminium hydroxide (series 1 and to spherical granules of )'-alumina (series 3). The magnesia content of all samples was 2-8 wt.-%. The samples were calcined in the temperature range
65
383-823 K. The magnesium-modified samples were studied in comparison with the nonmodified 7-alumina (series 2 ). X-ray characteristics of samples were obtained from diffractograms, recorded by a DRON-1.5 apparatus, using Cu K a radiation monochromated by a graphite monochromator. Phase composition was determined by diffractograms taken at the rate of counter operation of 1 deg/min.; parameters of the unit cell Ca) for ?-AI._,O3 were determined using the (440) line at a rate of counter operation of 1/4 deg/min. The accuracy of determination of the parameters of the unit cell 3a= _+0.003 A. The size of the regions of coherent dissipation (RCD) was determined from widening the diffraction lines using the Selyakov-Sherrer formula; the accuracy of determination is % 5 A. The coefficient b, connected with the position of cations in spinel-like structures [ 11 ], was determined from a correlation of the heights of the (311 ) and (222) lines since, for the samples studied, the width of diffraction lines does not change on introduction of magnesium (dispersivity is unchanged); thus the use of the ratio of heights is still correct for obtaining b. The interplane distance (d/n) for the pseudoboehmite was determined in two directions [0.2.0] (20~ 14= ) and [2.0.0] (20--49 ~ ) to an accuracy of_+0.0005 ,i. (at a registration rate of 1/4 deg/min ). The mechanical crushing strength of the granules was determined by the standard method by means of an MP-9C apparatus. An average value of strength (P~,.) was calculated from the volume of aggregate data for 30 granules, the accuracy of determination being + 10%. RESULTS AND DISCUSSION
Table 1 illustrates the basic characteristics of the samples studied. It is obvious that introduction of magnesium into aluminium hydroxide (series 1) confers a sharp increase on the strength of the granules, which is most evident at 723-823 K. Impregnation of)'-A120:3 with magnesium nitrate (series 3) does not lead to any increase in the strength of the granules. In order to explain the observed effects, the interaction between the addition and aluminium hydroxide (alumina) was studied, The X-ray data presented in Table 1 illustrate that the process of the formation of structurally modified supports may differ according to its dependence on the method by which the magnesium is introduced. For example upon introduction of magnesium to aluminium hydroxide with subsequent drying at 383 K, aluminium hydroxide of the pseudoboehmite structure is formed, in which the Al :~+ cations are statistically substituted for Mg 2 *, and OH - anions for NO~-, i.e. a solid solution of the pseudoboehmite structure is formed, which is illustrated by the changed interplane distance (20-~49 ~ ). Besides, a decrease of 10-15% in the integrated intensity of [0.2.0] diffraction peak (20~ 14 ° ) is observed, which is connected with a decrease in the quantity of
66
.o<
2
,~
..L,.L~2.
•
"2.
~%u2
t
f
J ~.~
A
,r.
d,d 2~
.~
e',
'=_
,~
E
£ ,.2,.2,..~
E
.=_ ¢-
.o~
"~ ~ t
a
J ~
©G C,
<
.
67
the crystalline phase. This may be due to a disordering of the smallest ( <50 h,) particles. At 623 K the interplane distance d/n in the modified samples (series 1) ranges from 6.14 to 6.16 and the RCD from 83 to 76 ~,. With a subsequent increase of temperature, protospinel )'-A1203, promoted with magnesium cations, is formed from the solution of pseudoboehmite structure. This protospinel contains Mgz + in the tetrahedral position and is characterized by an increase of the cell parameter and coefficient b as compared to )'-A1203 [ 11 ]. As is shown in ref. 11, if the MgO content is 5-7 wt.-%, the number of O H groups in the protospinel structure increases. And the structural formula of this solid solution, e.g. at 7 wt.-% of MgO, is as follows: A1~'.75A16 '25 M~.5 Mgl.25 [A1 ]9.o 022.5 (OH)9.5 This compound may be related to a solid solution of the cation-anion type, meaning that the OH groups present represent the total quantity of anions (NO~ and O H - ). In the case in which the magnesium nitrate is introduced to 7-A1203 (series 3), the process of interaction probably takes place in some other way. Upon impregnation the salt solution is distributed within the porous space of the already formed }'-A1203. Drying leads to crystallisation of Mg(N03)2 salt in the matrix pores, although its diffraction lines may be registered only in the dried samples containing> 10 wt.-% of MgO. When the temperature is increased (523-723 K), decomposition of magnesium nitrate takes place with the formation of highly dispersed magnesium oxide (this phase is also recorded with samples containing > 10 wt.-% of MgO). In that case nitrate groups in the anionic frame of alumina (the cell's parameter increases somewhat ), but formation of a solid solution of the cationic type with a simultaneous restructuring of protospinel does not occur, which is confirmed by the lack of any observed change in the coefficient b. This compound may be conventionally related to the solid solution of the anionic type. At such temperatures ( ~<723 K) the granules' strength does not change. At 823 K an interaction between MgO and }'-A1203 takes place, leading to the formation of a solid solution of the cationic type (the cell parameter grows). Some increase of the coefficient b points to the initiation of restructuring. It is obvious that the intensity of the interaction between Mg 2+ and A1203 should depend on the amount of magnesium and the calcination temperature. As it is seen from Table 2, which illustrates the series 3 samples, upon increasing the content of MgO ( >/6 wt.-% ) or when increasing the temperature up to 973 K, a cationic solid solution of magnesium in )'-A1203 is formed as a result of a solid-phase reaction between magnesium oxide and alumina. These data are in good agreement with the concept of the mechanism and kinetics of the solid-phase reaction of spinel formation, where t he basic diffusing component is magnesium oxide. The following confirms the fact that in this case a solid solution of the cationic type is formed. As is shown in ref. 11,
68 TABLE 2 Properties of alumina modified by magnesium Content of MgO ( wt.c~.~
723 K
97:] K
P,, ( MPa ~
Phase composition
a. I,~ p
b
P~, I MPa )
Phase composition
a. I.~ b
0 4.5 6.1 7.2
26 "2"2 26 31
)'-AI_,O~ i'-AI.O~~' )'-AI,O, e )'-AL,O,"
7.919 7.926 7.9:31 7.938
1.22 1.2,5 1.34 1.52
29 :32 31 32
;,'-AI_,O, i'-Al_)O,' i'-A],O~~ '-AI:O., ~
7.917 7.943 7.962 7.967
1.35 1.71 1.83 2.21
"Solid solution of the anionic t.x~leof the structure of ",'-aum na. ~Solid solution of the cationic t.xlaeof the structure of )'-alumina. if the c o n t e n t of MgO i s > 7 wt.-% in samples of series 1 a decrease in the a m o u n t of a n i o n s ( O H - a n d N O ; groups ) is observed, d e t e r m i n e d as the total loss in weight ( T L W ) at 1073 K, a n d as this takes place the coefficient b grows and the solid solution s t r u c t u r e is described by the formula: AI~.:~_, (Mg, Al)s.,-, [AI],, .... 4, O.,.2.~+, (OH)9..~_, i.e. the s t r u c t u r e of this c o m p o u n d is close to the spinel one. Analogous c h a n g e s take place in case of the series 3 samples. Indeed, a s h a r p increase of b a n d the lattice a p a r a m e t e r occurs upon increasing the a m o u n t of m a g n e s i u m , and especially upon increasing the t e m p e r a t u r e . T h e anion cont e n t , e s t i m a t e d by T L W , for the samples of series 3, is less t h a n that of the sample o f series 1 (at equal Mg c o n t e n t and at the same calcination t e m p e r a ture ). If the c o n t e n t of MgO is 4.5 wt.-% at 823 K a sample of series 1 has T L W 3.5%, while t h a t of series 3 is 2.5%. As m a y be seen from T a b l e s 1 and 2, t h e r e is no evidence of a n y considerable s t r e n g t h e n i n g of spherical granules produced by i m p r e g n a t i o n of )'-Al~O ~with a solution of Mg( N O , )2. F r o m the results discussed above one may conclude that, upon i n t r o d u c i n g m a g n e s i u m into a l u m i n a , solid p r o t o s p i n e l solutions of t h r e e types are formed: the anionic, cationic and c a t i o n i c - a n i o n i c . T h e s t r u c t u r e of these c o m p o u n d s d e p e n d s on the m e t h o d of i n t r o d u c t i o n of the modi~,ing additive a n d on the c o n d i t i o n s of the s u b s e q u e n t heat t r e a t m e n t . Any increase in the s t r e n g t h of the spherical granules is p r o b a b l y c o n n e c t e d with the f o r m a t i o n of" solid solutions of the m i x e d type (the a n i o n - c a t i o n i c ones ) at certain p a r t i c u l a r ratios b e t w e e n the m a g n e s i u m / a l u m i n i u m a n d the anions.
69 REFERENCES 1 2 3 4 5 6 7 8 9 10 11
V.S. Kanalina, A.F. Starovos~tova, V,N, Anokhin, Catalysis and Catalysts. Leningrad. 1983. p. 127. E.Z. Colosman. A.L. Klyaehko-Gurviteh, V..J. Yakerson. Kinet. Katal., 13 ~1972 t 1036. P. Pena. P. Mizanzo..J.S. M o y a a n d S . DeAza. J. Mater. Sci.,20 11985)2011. F. Lange, B, Davis, D, Ralligh, J. Am. Ceram. Soc., 66 ( 1983 ~ 50. V.V. Veselov. T.A. Levanyuk, C.A. Chyornaya, Catalysts and Processes of Hydrocarbons Conversion. Naukova Dumica, Kiev, 1982. p. 14. M.D. Effros, G.S. Lemeshonok, N.F. Ermolenko, Izv, AN BSSR, set. khim. nauk. 4 11971 14. T.F, Limar. Yu. A. Kogan, L.A. Shepolenko and N.G. Kisel, [zv. AN SSSR. set. neorg, mater., 8 119721 500. R.K. Datta, Rustum. Roy. Am. M: eralogist., 5',] (1968t 1456. A.S. Ivanova, V.A. Dzisko, E.M. Moroz, Kinet. Katal. 26 ( 19851 429. I.A. Ovsyannikova. G.I. Gol'denberg. N.A. Kor3'abkina et al.. Appl. Catal., 55 11989 ~ 75. E.M. Moroz, V.N. Kuklina, V.A. Ushakov. Kinet. Katal., 28 ~1987 ) 699.
63
Elsevier Science Publishers B.V., Amsterdam
Influence of the method of alumina modification on formation of low-temperature solid solutions in magnesia-alumina systems N.A. Koryabkina, Z.R. Ismagilov*, R.A. Shkrabina, E.M. Moroz and V.A. Ushakov Institute of Catalysis, Prosp. Akad. Lavrentieva, 5, Novosibirsk, 6,30090 (I_~¢qSR) cReceived 20 July 1990. revised manuscript received 21 November 1990}
Abstract
Formation of solid solutions in magnesia-alumina systems has been studied. It has been shown that depending on the method of introduction of magnesium and the temperature of heat treatment, formation of three types of solid solutions with the structure of ~'-alumina is possible; the anionic, the anionic-cationic and the cationic type. The mechanical strength of the obtained samples of spherical alumina depends on the method of modification; the strongest samples are produced if magnesium is added to aluminium hydroxide with subsequent calcination at 820 K.
Keywords: solid solutions, magnesia-alumina, calcination, heat treatment.
INTRODUCTION
It is known that oxidation catalysts which operate under fluidized bed conditions may change considerably during use, which is connected with the sintering of the support. Phase transformations and mechanical decomposition of the catalyst granules also occur under these conditions. For this reason, one of the most important requirement imposed on the combustion catalyst supports is a high mechanical strength and thermal stability. One of the most widely used methods to increase the strength of supports and catalysts is chemical modification [1-4]. However, there is no complete description of the mechanism by which modifiers achieve their strengthening activity in the literature. The majority of the explanations are connected with the sintering activity of introduced additions at temperatures > 1273 K, which is supposed by the authors of refs. 3-5 to lead to strengthening of supports, i.e. to strengthening of the alumina, the latter being modified by addition of alkaline earth and rare earth elements (Ca, Mg, La, Zr, etc). It is noteworthy
0166-9834/91/$03.50
(~ 1991 Elsevier Science Publishers B.V.
64 that formation of, for example, magnesia-alumina compounds depends on the method of their preparation [6-8]. After coprecipitation of magnesium and aluminium hydroxides with subsequent calcination it has been discovered that formation of the spinel MgAlvO4 takes place at 673 K, while, in samples obtained by mixing aluminium and magnesium hydroxides, the spinel phase cannot be detected even at 1073 K [6]. Thermal decomposition of the mixture of nitrate salts of aluminium and magnesium leads to the formation of spinel within the interval 573-773 K, as has been shown in ref 7. Crystallization of magnesium-aluminium spinels, produced from the gels of nit rates of Mg ~+ and Al ~+, and mixing of the oxides of these metals [8 ], leads to tbrmation of 'disordered' spinels at 923-1273 K. It is noteworthy that the structures of magnesia-alumina compounds, containing> 10 wt.-% of magnesia are commonly studied [9]. However, as is mentioned in ref. 10, even if the magnesia content is 2-4 wt.~c at 823 K a magnesium solid solution ~in y-A120:~) is detected, as well as a decrease in the density of defects and an increase in the strength of spherical granules. In ref. 11 the formation of low-temperature solid solutions in alumina, containing < 10 wt.-% magnesia, was studied. On the basis of the oxidehydroxide formula given by authors, some variants of the arrangement of Mg ~÷ cations in the structure of l'-Al._)O:3 were discussed and structural formulas of compounds were proposed with respect to the amount of addition. It has been shown that the structure of low temperature solid solutions is characterized by a considerable defect density of cationic ~due to the filling of non-spinel positionsl and anionic (due to the presence of hydroxyl groups} lattice frameworks. However, data on the relationships between mechanical strength of granules and the amount of modifying additive, conditions of preparation, etc., were not reported. The present work represents a continuation of work initially published in refs. 10 and 11, and is concerned with studying the influence of the method of introduction of magnesium nitrate on the formation of low temperature magnesia-alumina compounds and on the mechanical strength of spherical granules prepared from them. EXPERIMENTAL Spherical alumina supports (diameter of granules = 2-3 mm ) were prepared from an industrial hydroxide having a pseudoboehmite structure. Formation of the granules was carried out by the hydrocarbon-ammonia method. Magnesium modification was conducted by two methods: introduction of magnesium nitrate solution to spherical granules of aluminium hydroxide (series 1 and to spherical granules of )'-alumina (series 3). The magnesia content of all samples was 2-8 wt.-%. The samples were calcined in the temperature range
65
383-823 K. The magnesium-modified samples were studied in comparison with the nonmodified 7-alumina (series 2 ). X-ray characteristics of samples were obtained from diffractograms, recorded by a DRON-1.5 apparatus, using Cu K a radiation monochromated by a graphite monochromator. Phase composition was determined by diffractograms taken at the rate of counter operation of 1 deg/min.; parameters of the unit cell Ca) for ?-AI._,O3 were determined using the (440) line at a rate of counter operation of 1/4 deg/min. The accuracy of determination of the parameters of the unit cell 3a= _+0.003 A. The size of the regions of coherent dissipation (RCD) was determined from widening the diffraction lines using the Selyakov-Sherrer formula; the accuracy of determination is % 5 A. The coefficient b, connected with the position of cations in spinel-like structures [ 11 ], was determined from a correlation of the heights of the (311 ) and (222) lines since, for the samples studied, the width of diffraction lines does not change on introduction of magnesium (dispersivity is unchanged); thus the use of the ratio of heights is still correct for obtaining b. The interplane distance (d/n) for the pseudoboehmite was determined in two directions [0.2.0] (20~ 14= ) and [2.0.0] (20--49 ~ ) to an accuracy of_+0.0005 ,i. (at a registration rate of 1/4 deg/min ). The mechanical crushing strength of the granules was determined by the standard method by means of an MP-9C apparatus. An average value of strength (P~,.) was calculated from the volume of aggregate data for 30 granules, the accuracy of determination being + 10%. RESULTS AND DISCUSSION
Table 1 illustrates the basic characteristics of the samples studied. It is obvious that introduction of magnesium into aluminium hydroxide (series 1) confers a sharp increase on the strength of the granules, which is most evident at 723-823 K. Impregnation of)'-A120:3 with magnesium nitrate (series 3) does not lead to any increase in the strength of the granules. In order to explain the observed effects, the interaction between the addition and aluminium hydroxide (alumina) was studied, The X-ray data presented in Table 1 illustrate that the process of the formation of structurally modified supports may differ according to its dependence on the method by which the magnesium is introduced. For example upon introduction of magnesium to aluminium hydroxide with subsequent drying at 383 K, aluminium hydroxide of the pseudoboehmite structure is formed, in which the Al :~+ cations are statistically substituted for Mg 2 *, and OH - anions for NO~-, i.e. a solid solution of the pseudoboehmite structure is formed, which is illustrated by the changed interplane distance (20-~49 ~ ). Besides, a decrease of 10-15% in the integrated intensity of [0.2.0] diffraction peak (20~ 14 ° ) is observed, which is connected with a decrease in the quantity of
66
.o<
2
,~
..L,.L~2.
•
"2.
~%u2
t
f
J ~.~
A
,r.
d,d 2~
.~
e',
'=_
,~
E
£ ,.2,.2,..~
E
.=_ ¢-
.o~
"~ ~ t
a
J ~
©G C,
<
.
67
the crystalline phase. This may be due to a disordering of the smallest ( <50 h,) particles. At 623 K the interplane distance d/n in the modified samples (series 1) ranges from 6.14 to 6.16 and the RCD from 83 to 76 ~,. With a subsequent increase of temperature, protospinel )'-A1203, promoted with magnesium cations, is formed from the solution of pseudoboehmite structure. This protospinel contains Mgz + in the tetrahedral position and is characterized by an increase of the cell parameter and coefficient b as compared to )'-A1203 [ 11 ]. As is shown in ref. 11, if the MgO content is 5-7 wt.-%, the number of O H groups in the protospinel structure increases. And the structural formula of this solid solution, e.g. at 7 wt.-% of MgO, is as follows: A1~'.75A16 '25 M~.5 Mgl.25 [A1 ]9.o 022.5 (OH)9.5 This compound may be related to a solid solution of the cation-anion type, meaning that the OH groups present represent the total quantity of anions (NO~ and O H - ). In the case in which the magnesium nitrate is introduced to 7-A1203 (series 3), the process of interaction probably takes place in some other way. Upon impregnation the salt solution is distributed within the porous space of the already formed }'-A1203. Drying leads to crystallisation of Mg(N03)2 salt in the matrix pores, although its diffraction lines may be registered only in the dried samples containing> 10 wt.-% of MgO. When the temperature is increased (523-723 K), decomposition of magnesium nitrate takes place with the formation of highly dispersed magnesium oxide (this phase is also recorded with samples containing > 10 wt.-% of MgO). In that case nitrate groups in the anionic frame of alumina (the cell's parameter increases somewhat ), but formation of a solid solution of the cationic type with a simultaneous restructuring of protospinel does not occur, which is confirmed by the lack of any observed change in the coefficient b. This compound may be conventionally related to the solid solution of the anionic type. At such temperatures ( ~<723 K) the granules' strength does not change. At 823 K an interaction between MgO and }'-A1203 takes place, leading to the formation of a solid solution of the cationic type (the cell parameter grows). Some increase of the coefficient b points to the initiation of restructuring. It is obvious that the intensity of the interaction between Mg 2+ and A1203 should depend on the amount of magnesium and the calcination temperature. As it is seen from Table 2, which illustrates the series 3 samples, upon increasing the content of MgO ( >/6 wt.-% ) or when increasing the temperature up to 973 K, a cationic solid solution of magnesium in )'-A1203 is formed as a result of a solid-phase reaction between magnesium oxide and alumina. These data are in good agreement with the concept of the mechanism and kinetics of the solid-phase reaction of spinel formation, where t he basic diffusing component is magnesium oxide. The following confirms the fact that in this case a solid solution of the cationic type is formed. As is shown in ref. 11,
68 TABLE 2 Properties of alumina modified by magnesium Content of MgO ( wt.c~.~
723 K
97:] K
P,, ( MPa ~
Phase composition
a. I,~ p
b
P~, I MPa )
Phase composition
a. I.~ b
0 4.5 6.1 7.2
26 "2"2 26 31
)'-AI_,O~ i'-AI.O~~' )'-AI,O, e )'-AL,O,"
7.919 7.926 7.9:31 7.938
1.22 1.2,5 1.34 1.52
29 :32 31 32
;,'-AI_,O, i'-Al_)O,' i'-A],O~~ '-AI:O., ~
7.917 7.943 7.962 7.967
1.35 1.71 1.83 2.21
"Solid solution of the anionic t.x~leof the structure of ",'-aum na. ~Solid solution of the cationic t.xlaeof the structure of )'-alumina. if the c o n t e n t of MgO i s > 7 wt.-% in samples of series 1 a decrease in the a m o u n t of a n i o n s ( O H - a n d N O ; groups ) is observed, d e t e r m i n e d as the total loss in weight ( T L W ) at 1073 K, a n d as this takes place the coefficient b grows and the solid solution s t r u c t u r e is described by the formula: AI~.:~_, (Mg, Al)s.,-, [AI],, .... 4, O.,.2.~+, (OH)9..~_, i.e. the s t r u c t u r e of this c o m p o u n d is close to the spinel one. Analogous c h a n g e s take place in case of the series 3 samples. Indeed, a s h a r p increase of b a n d the lattice a p a r a m e t e r occurs upon increasing the a m o u n t of m a g n e s i u m , and especially upon increasing the t e m p e r a t u r e . T h e anion cont e n t , e s t i m a t e d by T L W , for the samples of series 3, is less t h a n that of the sample o f series 1 (at equal Mg c o n t e n t and at the same calcination t e m p e r a ture ). If the c o n t e n t of MgO is 4.5 wt.-% at 823 K a sample of series 1 has T L W 3.5%, while t h a t of series 3 is 2.5%. As m a y be seen from T a b l e s 1 and 2, t h e r e is no evidence of a n y considerable s t r e n g t h e n i n g of spherical granules produced by i m p r e g n a t i o n of )'-Al~O ~with a solution of Mg( N O , )2. F r o m the results discussed above one may conclude that, upon i n t r o d u c i n g m a g n e s i u m into a l u m i n a , solid p r o t o s p i n e l solutions of t h r e e types are formed: the anionic, cationic and c a t i o n i c - a n i o n i c . T h e s t r u c t u r e of these c o m p o u n d s d e p e n d s on the m e t h o d of i n t r o d u c t i o n of the modi~,ing additive a n d on the c o n d i t i o n s of the s u b s e q u e n t heat t r e a t m e n t . Any increase in the s t r e n g t h of the spherical granules is p r o b a b l y c o n n e c t e d with the f o r m a t i o n of" solid solutions of the m i x e d type (the a n i o n - c a t i o n i c ones ) at certain p a r t i c u l a r ratios b e t w e e n the m a g n e s i u m / a l u m i n i u m a n d the anions.
69 REFERENCES 1 2 3 4 5 6 7 8 9 10 11
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