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Characterization of zirconia powders prepared by high temperature hydrolysis (HTH)
Powder Technology, 27 (1980) 23 - 28 0 EtsevierSequoia S-A., Lausanne- Printedin the Netherlands
Characterization of Zirconia Hydrolysis (HTH) HijSEYIN Gatim,
Powders
Prepared
23
by High Temperature
SARI~MEN* Miidiirii
(Turkey)
(ReceivedFebruary26,198O)
SUMMARY In this article the hydrolysis of zirconium nitrate solutions of varying concentrations at high temperature and characterization of the precipitates obtained is described. The solutions were prepared from 99% pure zirconium nitrate compound and they were hydrolysed at 150”, 180”, 220” and 240 “C in a laboratory type autoclave for 6 hours. The precipitates obtained were characterized using (i) the Debye-Scherrer X-ray method, (ii) the Transmission Electron Microscope, (iii) the B.E.T. surface area measurement method, and the supernatants were analysed for their zirconium ion content using mandelic acid. It was found that during hydrolysis, the zirconium ions in solution were precipitated as monoclinic zirconia particles of varying size depending on the concentration of solution and temperature of the reaction. 1. INTRODUCTION Zirconium appears in aqueous solutions as zirconic ions Zr4+, and in low pH, around pH 2 - 3, the basic zirconyl ion ZrOOW becomes the prevailing ion. Zirconic ion forms a continuous series of polymers on dilution of their solution or on increasing their pH and each of the polymers is hydrolysed to a different degree [l - 33 _ Alexander and Bugosh [4, 51 hydrolysed acidic solutions of zirconium oxychloride at 120” and 150 “C for 1 h and obtained zirconium dioxide particles of 50 - 200 A size and 5 - 400 mz/g surface area_ Particles obtained at above 150 OCwere of monoclinic crystalline structure. *Presentaddress:TUEITAK, Ataturk Bulvarl,221, Ravakhdere,Ankara,Turkey.
Scott [S] hydrolysed acidic zirconium sulphate solutions at high temperature. The result. was a precipitate of monoclinic zirconium dioxide particles. Stambaugh [7] studied the precipitation of zirconia particles from stock solutions containing hydrochloric acid by hydrolysis at 180 OCunder 150 psi pressure_ He obtained monoclinic zirconium dioxide particles of 0.5 - 20 pm average size with quite a high recovery. In this work aqueous solutions of zirconium nitrate compounds were prepared at various concentrations and hydrolysed at 150”) 180”, 220” and 240 “C for 6 h under high pressure in a laboratory type autoclave
C812.
EXPERIMENTAL
2.1 Preliminary
tests Zirconiur=1 nitrate was 99%
pure as obtained from Johnson-Matthey Chemicals Ltd., 74, Hatton Gardens, London, and consisted of moist crystals of 0.1 - 1.0 mm average size. When calcined it produced 38.2% by weight monoclininc zirconium dioxide. 2.2 Hydrolysis of solutions Zirconium nitrate was dissolved in distilled water at amounts to give solutions of precalculated concentrations in grams of ZrGz/l. The solutions were highly acidic as prepared and they were hydrolysed under acidic conditions. The equipment used for the hydrolysis experiments was a Iaboratory type autociave of 1 gallon (4-4 1) capacity_ It was made of stainless steel casing fitted with a thermocouple (pt-Pt/lO% Rh), a gas line connected to a nitrogen gas cylinder, a pressur e gauge and a stirrer and heated electrically from outside the casing. Solutions were placed
24
into the autoclave in a glass tube and the space between the autoclave wall and the glass tube was fiied with Ca(OH)z solution to protect the autoclave wall from acidic corrosion. After closing the autoclave lid tightly it was pressurized to 200 psi (14.3 atm) at 18 “C by gaseous nitrogen and heated gradually up to a predetermined reaction temperature. During the heating-up period, the temperature of the solution and the pressure inside the autoclave were recorded as seen in Figs. 1 and 2. The whole system was held at the hydrolysis temperature for 6 h for completion of reactions_ At the end of 6 h the system was left to cool down to room temperature. Then the autoclave was depressurized and opened. The products of the hydrolysis experiments were usually whitish precipitates settled at the bottom of the glass tubes. They were separated i?om their supernatants by a centrifuge and prepared for characterization tests. 2.3 Characterizationofprec5pitates The precipitates obtained were separated from their supematants by a Rotafix 2800 model laboratory type centrifuge_ A portion of each precipitate was then washed three r
Fig.2.Hydrolysisof
I- ‘c
240
-
or;-_---___A
u,lL
6
4
Tml.
Fig.1_HydrolysisofZr(NOa)4
me&s 3,4,5,6,7;*expts. 7 expt.13;~expt.l.
B
TO
tnrr,
solutions. 0 JZxperi8,9,10,11,12;~expt_2;
Zr(N03)4
sohtions.
times with distilled water and dried in a petri dish under an infrared lamp. The dry precipitate was pressed into a fine powder with the tip of a spatula. A qualitative analysis of the powder was carried out by the Debye-Scherrer (D/S) X-ray diffraction method [9]. Thus it was shown that all the powders were composed of monoclinic zirconia crystals_ The particle size and shape of the precipitates were studied by an AEI EM 6 G electron microscope at transmission conditions [lo] _ Since the size and shape of the particles as precipitated were of particular interest, a suspension of the particles was used for the analysis. The micrographs taken were evaluated for particle size measurement and thus particle size distribution curves and mean particle diameters of precipitates were obtamed. The supematants of the precipitates were analysed for their zirconium content quantitatively. using mandelic acid [ II]. The analysis showed that all of ttie zirconium ions in solution had precipitated during the hydrolysis.
3. EXPERIMENTAL
RESULTS
As seen in Table 1, thirteen hydrolysis experiments were done with zirconium nitrate solutions_ Concentrations of the solutions were varied from 10 to 100 g of Z&s per Iitre and the pH was highly acidic, less than unity. The period of hydrolysis was 6 h for ali the experiments to ensure a complete hydrolysis of zirconium ions in solution. Most of the experiments were done at 220” and 240 “C except the first two, which were carried out at 150” and 180 “C respectively. The precipitates obtained at 150” and 180 “C consisted of very fine (sub-micron size) zirconia particles. A typical electron-micrograph and the size distribution curve of the precipitate obtained at 150 OCare seen in Figs. 4(a) and 3(a)_ The mean particle size as m easured horn electron-micrographs was found to be 243 + 5 A for the precipitate. As the hydrolysis temperature and the -once&ration of the solutions were increased, the zirconia particles became more crystahine and coarser. This is understandable from the electron-micrographs of various zirconia precipitates obtained as seen from Fig. 4(a - d). The size distribution curve of precipitate 13 as measured from its eIectronmicrographs is seen in Fig. 3(b). Its mean particle size was found to be 1722 f 9 A. The specific surface area of the precipitate was measured by the B-E-T. method and found to be 92.2 m”/g, which is equivalent to the surface area of spherical particles of 123 A diameter. TABLE
Fig. 3. Size distribution in zirconia precipitates_ Precipitate 2, (b) precipitate 13_
1
Hy-‘rolysis
resulk of zirconium
nitrate solutions
Expt. No_
ZrO* concentration (g/l)
pH at 25 “C
Temperature (C” )
Time (h)
Debye-Scherrer results
1 2 3 4 5 6 7 8 9 10 11 12 13
10 10 10 20 25 30 40 20 40 60 SO 100 100
1 1 1 1 1 1 1 1 1 1 1 1 1
150 180 240 240 240 240 240 220 220 220 220 220 240
6 6 6 6
Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monochnic Monoclinic Monoclinic Monoclinic
6
6 6 6 6 6 6 6 6
ZrOa ZrOa ZrOz ZrOa ZrOz ZrOa ZrOz ZrOs ZrOp ZrOs ZrOs ZrOa ZrOs
(a)
26
(b)
Fig. 4. Electron-micrographs of zirconia precipitates_ a: T = 150 “C, cont. = 10 g ZrOz/l (hf = x 64,000); b: T = 220 “C, cont. = 20 g ZrO*/l (M = X 70,200); c: T = 240 “C, cont. = 40 g ZrOZ/l (M = X 31,000); d: T = 240 “C, cont. = 100 g ZrOz/I (Al= x 9,000); e: T = 240 “C, COIIC. = 100 g Zr02n (M = x 28,000); f: T = 240 “C, cont. = 100 g ZrOl/l (.?I = x 65,000).
D/S diffraction patterns of the precipitates were composed of broad diffraction lines indicating crystallite size effect. The line broadening disappeared and the lines became sharper when the precip%ates were cakined at about 900 “C. All the diffraction patterns taken were identified as those of monoclinic zirconia. The analysis of the diffraction pattern of precipitate 13 is given in Table 2.
4_ DISCUSSION
The hydrolysis of zirconium nitrate solutions between 150” and 240 “C in highly acidic environment produced monoclinic zirconia precipitates of increasing crystallinity depending on temperature of reaction and concentration of solutions. Similar results were reported with the hydrolysis of zirconium
27
TABLE
2
Results of the X-ray analysis of precipitate
13
Line
d (A) measured
A.S.T.M. data for monoclinic Zr02, d (a)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
-
5.036 3.690 3.630 3.157 2.836 2.617 2.598 2.538 2.488 2.328 2.285 2.252 2.212 2.182 2.015 1.989 1.845 1.818 1.801 1.780 1.691 1.656
3.65 3.16 2.83 2.62 2.54 -
2.25 2.02 1.85 1.82 l-70 1.66
The missing diffraction measure.
Iines above wete too weak to
zirconium oxynitrate and zirconium sulphate solutions [4 - 73 . Likewise the hydrolysis period of 6 hours was found to be quite adequate for hydrolysis reactions to come to completion as shown by the analysis of the supematants with mandelic acid [ll] The effect of hydrolysis temperature and concentration of solution is readily observable from the electron-micrographs and size distribution curves obtained from the micrographs as seen in Figs. 4(a - d) and 3(a,b) respectively. The precipitates produced at 150” and 180 “C were yellowish in colour with a gelatinous appearance and were separated from their supematants with difficulty by centrifuging. As the temperature was raised from 150” to 240 “C and the concentration from 10 g ZrOJl to 100 g ZrOJl, the mean size of zirconia particles changed from 243 t 5 A to 1722 f 9 A. At the same time the particle shape also changed. At lower concentration and temperature the particles appeared to have round shapes with smooth surfaces_ With increasing temperature and
oxychloride,
concentration not only did the size of individual particles gro-N !arger, but they also formed triangular-shaped clusters of varying sizes. This may be explained as follows: at low concentration the particles form nearly independently of one another and remain separate as seen in Fig. 4(a). At higher concentrations, however, i.e. 20 g ZrOJl, particles form on top of one another, growing sideways or in end-to-end fashion. At the same time the size of each individual particle increases with increasing concentration and temperature_ Thus, clusters of zirconia particles, which first become apparent at 20 g ZrOP/l concentration, are obtained irrespective of the temperature of hydrolysis. These clusters assume a variety of shapes as can be observed on close examination of Fig. 4(a - d). As seen in Fig. 4(d) the dominating appearance of the clusters in precipitate 13 is triangular. Clusters of this shape were in fact first observed forming a major proportion of the precipitate at a concentration of 60 g ZrO& They appear to have a very rough surface in the micrograph. This is confirmed by the large surface area associated with the powder. In the case of precipitate 13 it was measured to be 92.2 m2/g. The metastable tetragonal and cubic forms of zirconia mentioned by Garvie [12] were not observed with zirconia precipitates produced by HTH in this work, although the particle sizes in most cases were within the size range reported for metastable zirconia to exist [12]_ This may be attributable to either the different method of preparation of zirconia, such that the high temperature thermodynamics in aqueous systems may not be favourable for the formation of the metastable forms, or the high purity and lack of lattice defects, which one may expect to be associated with particles produced by this method, prevents the form_ation of the metastable forms of zirconia in spite of the favourable particle size.
LIST OF SYMBOLS
d M T
distance between atomic total magnification temperature, “C
planes
REFERENCES
7
E. P. Stambaugh, (1968)
1 A. I.Vogel,A Textbook of Qucntitative Inorganic Analysis. Longmans, London, 3rd edn., p_ 546. 2 M. Pourbaix et al_. Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966, p_ 223. 3 J_ Brokington. Student’s Guide to Inorganic Chemistry, Buttenuorths, London, 1966. 4 B. AIexander and J. Bugosh, US. Pat. 2.984.628, May 16.1961_ 5 Ibid.. 2.984.576. May 16,196l. 6 T. R_ Scott, in M_ E_ Wadsworth and F_ T. Davis (Eds.), Unit Processes in Hydrometallurgy, Gordon and Breach, New York, 1964, p_ 169.
Battelle
Tech.
Rev..
27 (2)
3 - 7.
H. Saricimen, PhD Thesis, Univ of London, 1972. 9 B. D. Cuihty, Elements of X-Ray Diffraction, Addison-Wesley, Reading, Mass., 1959. 10 E. H. Cecil, Introduction to Electron Microscopy, McGraw-Hill, New York, 1953. 11 F. A. Cotton and G. Wilkinson, Advanced Zn8
organic
12
Chemistry,
A
Comprehensive
Text,
Inter-science, New York, 2nd edn., 1966, p_ 913. R_ C_ Garvie. in M_ A_ AIper (Ed_)_ High Temperature Oxides. Academic Press, New York, 1970, pp_ 118 - 164.
Characterization of Zirconia Hydrolysis (HTH) HijSEYIN Gatim,
Powders
Prepared
23
by High Temperature
SARI~MEN* Miidiirii
(Turkey)
(ReceivedFebruary26,198O)
SUMMARY In this article the hydrolysis of zirconium nitrate solutions of varying concentrations at high temperature and characterization of the precipitates obtained is described. The solutions were prepared from 99% pure zirconium nitrate compound and they were hydrolysed at 150”, 180”, 220” and 240 “C in a laboratory type autoclave for 6 hours. The precipitates obtained were characterized using (i) the Debye-Scherrer X-ray method, (ii) the Transmission Electron Microscope, (iii) the B.E.T. surface area measurement method, and the supernatants were analysed for their zirconium ion content using mandelic acid. It was found that during hydrolysis, the zirconium ions in solution were precipitated as monoclinic zirconia particles of varying size depending on the concentration of solution and temperature of the reaction. 1. INTRODUCTION Zirconium appears in aqueous solutions as zirconic ions Zr4+, and in low pH, around pH 2 - 3, the basic zirconyl ion ZrOOW becomes the prevailing ion. Zirconic ion forms a continuous series of polymers on dilution of their solution or on increasing their pH and each of the polymers is hydrolysed to a different degree [l - 33 _ Alexander and Bugosh [4, 51 hydrolysed acidic solutions of zirconium oxychloride at 120” and 150 “C for 1 h and obtained zirconium dioxide particles of 50 - 200 A size and 5 - 400 mz/g surface area_ Particles obtained at above 150 OCwere of monoclinic crystalline structure. *Presentaddress:TUEITAK, Ataturk Bulvarl,221, Ravakhdere,Ankara,Turkey.
Scott [S] hydrolysed acidic zirconium sulphate solutions at high temperature. The result. was a precipitate of monoclinic zirconium dioxide particles. Stambaugh [7] studied the precipitation of zirconia particles from stock solutions containing hydrochloric acid by hydrolysis at 180 OCunder 150 psi pressure_ He obtained monoclinic zirconium dioxide particles of 0.5 - 20 pm average size with quite a high recovery. In this work aqueous solutions of zirconium nitrate compounds were prepared at various concentrations and hydrolysed at 150”) 180”, 220” and 240 “C for 6 h under high pressure in a laboratory type autoclave
C812.
EXPERIMENTAL
2.1 Preliminary
tests Zirconiur=1 nitrate was 99%
pure as obtained from Johnson-Matthey Chemicals Ltd., 74, Hatton Gardens, London, and consisted of moist crystals of 0.1 - 1.0 mm average size. When calcined it produced 38.2% by weight monoclininc zirconium dioxide. 2.2 Hydrolysis of solutions Zirconium nitrate was dissolved in distilled water at amounts to give solutions of precalculated concentrations in grams of ZrGz/l. The solutions were highly acidic as prepared and they were hydrolysed under acidic conditions. The equipment used for the hydrolysis experiments was a Iaboratory type autociave of 1 gallon (4-4 1) capacity_ It was made of stainless steel casing fitted with a thermocouple (pt-Pt/lO% Rh), a gas line connected to a nitrogen gas cylinder, a pressur e gauge and a stirrer and heated electrically from outside the casing. Solutions were placed
24
into the autoclave in a glass tube and the space between the autoclave wall and the glass tube was fiied with Ca(OH)z solution to protect the autoclave wall from acidic corrosion. After closing the autoclave lid tightly it was pressurized to 200 psi (14.3 atm) at 18 “C by gaseous nitrogen and heated gradually up to a predetermined reaction temperature. During the heating-up period, the temperature of the solution and the pressure inside the autoclave were recorded as seen in Figs. 1 and 2. The whole system was held at the hydrolysis temperature for 6 h for completion of reactions_ At the end of 6 h the system was left to cool down to room temperature. Then the autoclave was depressurized and opened. The products of the hydrolysis experiments were usually whitish precipitates settled at the bottom of the glass tubes. They were separated i?om their supernatants by a centrifuge and prepared for characterization tests. 2.3 Characterizationofprec5pitates The precipitates obtained were separated from their supematants by a Rotafix 2800 model laboratory type centrifuge_ A portion of each precipitate was then washed three r
Fig.2.Hydrolysisof
I- ‘c
240
-
or;-_---___A
u,lL
6
4
Tml.
Fig.1_HydrolysisofZr(NOa)4
me&s 3,4,5,6,7;*expts. 7 expt.13;~expt.l.
B
TO
tnrr,
solutions. 0 JZxperi8,9,10,11,12;~expt_2;
Zr(N03)4
sohtions.
times with distilled water and dried in a petri dish under an infrared lamp. The dry precipitate was pressed into a fine powder with the tip of a spatula. A qualitative analysis of the powder was carried out by the Debye-Scherrer (D/S) X-ray diffraction method [9]. Thus it was shown that all the powders were composed of monoclinic zirconia crystals_ The particle size and shape of the precipitates were studied by an AEI EM 6 G electron microscope at transmission conditions [lo] _ Since the size and shape of the particles as precipitated were of particular interest, a suspension of the particles was used for the analysis. The micrographs taken were evaluated for particle size measurement and thus particle size distribution curves and mean particle diameters of precipitates were obtamed. The supematants of the precipitates were analysed for their zirconium content quantitatively. using mandelic acid [ II]. The analysis showed that all of ttie zirconium ions in solution had precipitated during the hydrolysis.
3. EXPERIMENTAL
RESULTS
As seen in Table 1, thirteen hydrolysis experiments were done with zirconium nitrate solutions_ Concentrations of the solutions were varied from 10 to 100 g of Z&s per Iitre and the pH was highly acidic, less than unity. The period of hydrolysis was 6 h for ali the experiments to ensure a complete hydrolysis of zirconium ions in solution. Most of the experiments were done at 220” and 240 “C except the first two, which were carried out at 150” and 180 “C respectively. The precipitates obtained at 150” and 180 “C consisted of very fine (sub-micron size) zirconia particles. A typical electron-micrograph and the size distribution curve of the precipitate obtained at 150 OCare seen in Figs. 4(a) and 3(a)_ The mean particle size as m easured horn electron-micrographs was found to be 243 + 5 A for the precipitate. As the hydrolysis temperature and the -once&ration of the solutions were increased, the zirconia particles became more crystahine and coarser. This is understandable from the electron-micrographs of various zirconia precipitates obtained as seen from Fig. 4(a - d). The size distribution curve of precipitate 13 as measured from its eIectronmicrographs is seen in Fig. 3(b). Its mean particle size was found to be 1722 f 9 A. The specific surface area of the precipitate was measured by the B-E-T. method and found to be 92.2 m”/g, which is equivalent to the surface area of spherical particles of 123 A diameter. TABLE
Fig. 3. Size distribution in zirconia precipitates_ Precipitate 2, (b) precipitate 13_
1
Hy-‘rolysis
resulk of zirconium
nitrate solutions
Expt. No_
ZrO* concentration (g/l)
pH at 25 “C
Temperature (C” )
Time (h)
Debye-Scherrer results
1 2 3 4 5 6 7 8 9 10 11 12 13
10 10 10 20 25 30 40 20 40 60 SO 100 100
1 1 1 1 1 1 1 1 1 1 1 1 1
150 180 240 240 240 240 240 220 220 220 220 220 240
6 6 6 6
Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic Monochnic Monoclinic Monoclinic Monoclinic
6
6 6 6 6 6 6 6 6
ZrOa ZrOa ZrOz ZrOa ZrOz ZrOa ZrOz ZrOs ZrOp ZrOs ZrOs ZrOa ZrOs
(a)
26
(b)
Fig. 4. Electron-micrographs of zirconia precipitates_ a: T = 150 “C, cont. = 10 g ZrOz/l (hf = x 64,000); b: T = 220 “C, cont. = 20 g ZrO*/l (M = X 70,200); c: T = 240 “C, cont. = 40 g ZrOZ/l (M = X 31,000); d: T = 240 “C, cont. = 100 g ZrOz/I (Al= x 9,000); e: T = 240 “C, COIIC. = 100 g Zr02n (M = x 28,000); f: T = 240 “C, cont. = 100 g ZrOl/l (.?I = x 65,000).
D/S diffraction patterns of the precipitates were composed of broad diffraction lines indicating crystallite size effect. The line broadening disappeared and the lines became sharper when the precip%ates were cakined at about 900 “C. All the diffraction patterns taken were identified as those of monoclinic zirconia. The analysis of the diffraction pattern of precipitate 13 is given in Table 2.
4_ DISCUSSION
The hydrolysis of zirconium nitrate solutions between 150” and 240 “C in highly acidic environment produced monoclinic zirconia precipitates of increasing crystallinity depending on temperature of reaction and concentration of solutions. Similar results were reported with the hydrolysis of zirconium
27
TABLE
2
Results of the X-ray analysis of precipitate
13
Line
d (A) measured
A.S.T.M. data for monoclinic Zr02, d (a)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
-
5.036 3.690 3.630 3.157 2.836 2.617 2.598 2.538 2.488 2.328 2.285 2.252 2.212 2.182 2.015 1.989 1.845 1.818 1.801 1.780 1.691 1.656
3.65 3.16 2.83 2.62 2.54 -
2.25 2.02 1.85 1.82 l-70 1.66
The missing diffraction measure.
Iines above wete too weak to
zirconium oxynitrate and zirconium sulphate solutions [4 - 73 . Likewise the hydrolysis period of 6 hours was found to be quite adequate for hydrolysis reactions to come to completion as shown by the analysis of the supematants with mandelic acid [ll] The effect of hydrolysis temperature and concentration of solution is readily observable from the electron-micrographs and size distribution curves obtained from the micrographs as seen in Figs. 4(a - d) and 3(a,b) respectively. The precipitates produced at 150” and 180 “C were yellowish in colour with a gelatinous appearance and were separated from their supematants with difficulty by centrifuging. As the temperature was raised from 150” to 240 “C and the concentration from 10 g ZrOJl to 100 g ZrOJl, the mean size of zirconia particles changed from 243 t 5 A to 1722 f 9 A. At the same time the particle shape also changed. At lower concentration and temperature the particles appeared to have round shapes with smooth surfaces_ With increasing temperature and
oxychloride,
concentration not only did the size of individual particles gro-N !arger, but they also formed triangular-shaped clusters of varying sizes. This may be explained as follows: at low concentration the particles form nearly independently of one another and remain separate as seen in Fig. 4(a). At higher concentrations, however, i.e. 20 g ZrOJl, particles form on top of one another, growing sideways or in end-to-end fashion. At the same time the size of each individual particle increases with increasing concentration and temperature_ Thus, clusters of zirconia particles, which first become apparent at 20 g ZrOP/l concentration, are obtained irrespective of the temperature of hydrolysis. These clusters assume a variety of shapes as can be observed on close examination of Fig. 4(a - d). As seen in Fig. 4(d) the dominating appearance of the clusters in precipitate 13 is triangular. Clusters of this shape were in fact first observed forming a major proportion of the precipitate at a concentration of 60 g ZrO& They appear to have a very rough surface in the micrograph. This is confirmed by the large surface area associated with the powder. In the case of precipitate 13 it was measured to be 92.2 m2/g. The metastable tetragonal and cubic forms of zirconia mentioned by Garvie [12] were not observed with zirconia precipitates produced by HTH in this work, although the particle sizes in most cases were within the size range reported for metastable zirconia to exist [12]_ This may be attributable to either the different method of preparation of zirconia, such that the high temperature thermodynamics in aqueous systems may not be favourable for the formation of the metastable forms, or the high purity and lack of lattice defects, which one may expect to be associated with particles produced by this method, prevents the form_ation of the metastable forms of zirconia in spite of the favourable particle size.
LIST OF SYMBOLS
d M T
distance between atomic total magnification temperature, “C
planes
REFERENCES
7
E. P. Stambaugh, (1968)
1 A. I.Vogel,A Textbook of Qucntitative Inorganic Analysis. Longmans, London, 3rd edn., p_ 546. 2 M. Pourbaix et al_. Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966, p_ 223. 3 J_ Brokington. Student’s Guide to Inorganic Chemistry, Buttenuorths, London, 1966. 4 B. AIexander and J. Bugosh, US. Pat. 2.984.628, May 16.1961_ 5 Ibid.. 2.984.576. May 16,196l. 6 T. R_ Scott, in M_ E_ Wadsworth and F_ T. Davis (Eds.), Unit Processes in Hydrometallurgy, Gordon and Breach, New York, 1964, p_ 169.
Battelle
Tech.
Rev..
27 (2)
3 - 7.
H. Saricimen, PhD Thesis, Univ of London, 1972. 9 B. D. Cuihty, Elements of X-Ray Diffraction, Addison-Wesley, Reading, Mass., 1959. 10 E. H. Cecil, Introduction to Electron Microscopy, McGraw-Hill, New York, 1953. 11 F. A. Cotton and G. Wilkinson, Advanced Zn8
organic
12
Chemistry,
A
Comprehensive
Text,
Inter-science, New York, 2nd edn., 1966, p_ 913. R_ C_ Garvie. in M_ A_ AIper (Ed_)_ High Temperature Oxides. Academic Press, New York, 1970, pp_ 118 - 164.