MIXED OXIDES OF ALUMINA AND ZIRCONIA CONSISTING ESSENTIALLY OF SPHERICAL PARTICLES HAVING A NARROW GRANULOMETRIC DISTRIBUTION AND PROCESS FOR THEIR PREPARATION

Abstract: Mixed oxides of alumina and zirconia having a controlled granulometric distribution, consisting essentially of spheric­ al, submicronic particles, wherein zirconia is distributed uni­ formly in alumina and may reach 38% by weight, said zirconia being in the stabilized form at room temperture in the tetra­ gonal phase after having subjected the amorphous hydrated composite oxides to thermal treatments, wherein the transi­ tion occurs of zirconia to the tetragonal phase.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to mixed oxides of alumina and zirconia consisting essentially of spherical particles having submicronic size. In particular it relates to mixed oxides of alumina and zirconia consisting essentially of spherical, non-aggregated particles having controlled granulometric distribution. 2. The Prior Art It is known that alumina in its different crystalline forms may be used in various ways, for instance such as material for electronics in the preparation of semiconductors substrates and in integrated circuits, packing ceramic material for the preparation of cutting tools or as catalysts support in heterogeneous catalysis. Alumina-based sintered materials do not exhibit, however, very high mechanical strength, for instance cutting tools tend to lose their characteristics during working. Furthermore such materials have a low tensile strength as well. It is well known that the low strength characteristics mostly come from the lack of hardness of the sintered material. A sensible improvement of the aforesaid properties is obtained by using alumina in its different crystalline forms, in the form of monodispersed or polydispersed, non-agglomerate, spherical particles with narrow granulometric distribution, having a submicronic size or a size up to a few microns. Several methods are known to obtain hydrated alumina in the form of spherical particles, such as, for instance, the process described in "Aluminum Hydrous Oxide Sols - II - Preparation of Uniform Spherical Particles by Hydrolysis of Al-sec-Butoxide" P.L.Catone e E.Matijevi , Journal of Colloid Interface Science, Vol. 48, No.2, August 1974. page 291. A process allowing to obtain hydrated alumina with a very high productivity was described in a preceding patent application of the applicant (Italian Patent Application No. 19959 A/85). The hydrated alumina thus obtained can be subjected to thermal treatments in order to obtain gamma-alumina which retains the same morphologic and granulometric characteristics of the starting precursor. With regard to this, is cited, for instance, the article of E.Matijevi et al. "Heterogeneous Atmospheric Chemistry Geophysical Monograph Series", Vol. 26 , pages 44-49 (1982). Furthermore, in a preceding patent application of the applicant - Italian Patent Application No. 19142 A/85) - the possibility is described of obtaining alpha-alumina or mixtures of crystalline phases comprising alpha-alumina with theta-, and/or eta- and/or delta-alumina consisting essentially of spherical particles wich retain the granulometric and morphologic characteristics of the starting precursor. In fact, it is known that the morphology and the granulometric distribution exert a great influence on the mechanical, thermal, electrical properties of the materials obtained from said aluminas, in the application set forth hereinbefore, improving remarkably said properties. Furthermore it was proved that the tensile strength of the alumina may be improved by dispersing zirconia particles in alumina itself. In the article "Preparation of Alumina-Zirconia Powders by Evaporative Decomposition of Solution", Comm.of the Amer.Ceram.Society, May 1984, pages C-92, by D.W.Sproson and G.L.Messing, the authors assert that the strength improvement is due to the presence of zirconia stabilized in the tetragonal phase. In fact it is known that zirconia can exist in different crystalline phases, the monoclinic one up to about 1200°C, the tetragonal one from 1200° to 2400°C and the cubic one at temperatures over 2400°C. The improvement of the mechanical properties of alumina is obtained only if the zirconia distributed in the alumina retains the tetragonal form at room temperature, and if it does not convert into the monoclinic phase after having subjected the material based on aluminum oxide, to thermal treatments between 1200° and 2400°C. Namely the alumina should stabilize the zirconia in the tetragonal phase and it should hinder its transition into the monoclinic phase after the product, subjected to thermal treatments for the sintering, has been brought to room temperature. Zirconia stabilization in the tetragonal phase may, however, be only obtained if the crystallite size of zirconia dispersed in alumina ranges within some critical values, usually between 0,2 and 0,6 µm. With regard to this we can cite, for instance, the article by E.P.Butler and A.H.Hener "X-Ray Microanalysis of ZrO₂ Particles in ZrO₂ - Toughened Al₂O₃". Communications of the Amer.Ceram.Soc., December 1982, page C-206 or issued European patent application EP 119028. The uniform distribution of the zirconia particles in the alumina is essential to obtain the described improvements of mechanical strength and hardness since it is well known that if zirconia particles are distributed in a non-uniform way, dishomogeneities form in the product which give rise to cracks during the sintering process. This phenomenon is described for instance in "Processing-Related Fracture Origins III, Differential Sintering of ZrO₂, Agglomerates in Al₂O₃/ZrO₂ Composite" by F.F.Lange and B.I.Davis, Journal Am.Ceram.Soc. 66 (6), 1983, page 407. Alumina-zirconia mixed oxides so far known having the described dispersion characteristics are not in the form of spherical particles, monodispersed or polydispersed with a low polydispersion index but they are in the form of agglomerate particles and above all they are lacking in sphericity. Although the zirconia is uniformly distributed in alumina, the product shows under electron microscope the lack in any geometric form and it is present in the form of agglomerates. Therefore in these mixed oxides, all the property improvements mentioned hereinbefore get lost, namely the improvement due to sphericity, non-agglomeration and granulometric distribution as set forth hereinbefor for alumina. In fact, by using the methods known in the prior art for preparing zirconia-toughened alumina (ZTA) as described hereinbefore, although one works under conditions wherein alumina is obtained in the form of spherical particles by means of the cited processes, alumina-zirconia mixed oxides are obtained which do not present anymore the morphologic and granulometric characteristics of the alumina as such. Though DE-A1-3 347 450 discloses powdery ceramic compositions containing fine alumina particles and zirconia dispersed in these particles - the cristalline phase of the zirconia having exclusively tetragonal symmetry and the contact of zirconia amounting to 10 to 25% by weight - this document fails to disclose particles with a size of at least 0.2 µm endowed with such disaggregation and polydispersion value as explained below in connection with the present invention. Therefore the necessity to have mixed oxides of alumina and zirconia which could combine the improvements of the strength characteristics of alumina obtainable by means of zirconia stabilized in the tetragonal form with the ones obtainable from alumina in the form of spherical particles and having a narrow granulometric distribution. THE PRESENT INVENTION Now it was found surprisingly that it is possible to obtain alumina containing zirconia stabilized in the tetragonal phase at room temperature consisting essentially of non-agglomerate particles having a controlled morphology and a controlled granulometric distribution which allow to improve considerably the mechanical, thermal and electrical characteristics of the alumina given by zirconia. Therefore an object of the present invention consists essentially of amorphous mixed oxides of alumina and zirconia in the form of non-agglomerate, submicronic, spherical particles having a number average diameter ranging between 0.2 and about 1 µm having a polydispersion index dw/dn lower than or equal to 2.0 wherein dw is the weight average diameter and dn is the number average diameter and wherein the zirconia, distributed uniformly in the alumina, is at the most 38% by weight and is stabilized in the tetragonal phase at room temperature after having subjected said amorphous mixed oxides to thermal treatments for their transformation into crystalline mixed oxides. The alumina particles comprising zirconia may be monodispersed or polydispersed. The term"monodispersed particles"in the present invention means particles of mixed oxides according to the invention having a polydispersion index indicated by a dw/dn lower than or equal to 1.20 wherein dw is the weight average diameter of the particles and dn is the number average diameter. The term "polydispersed mixed oxides" according to the invention means particles having a dw/dn ratio lower than or equal to 2; in particular if dw/dn is lower than or equal to 1.30, the particles are considered as having a low polydisperion index. Preferably the mixed oxides according to the invention have a linear diameter ranging between 0.2 and 0.5 µm, more preferably between 0.3 and 0.4 µm. A further object of the invention consists in ZTA obtained from the amorphous mixed oxides of the invention after thermal treatments. All the possible combinations of the tetragonal phase stabilized at room temparature with the different known crystalline phases of the alumina fall within the object of the present invention. In fact, subjecting the mixed oxides according to the invention to known thermal treatments, combinations may be obtained of tetragonal zirconia with alumina in gamma, theta, eta, delta, alpha phase. Combinations may be also obtained wherein alumina is present in various further crystalline phases, for instance gamma-theta, gamma-theta-delta, delta-alpha phase. Accordingly, the invention is directed also to mixed oxides as defined above obtained by subjecting to thermal treatments the amorphous hydrated mixed oxides and wherein zirconia is stabilized in the tetragonal crystalline form and alumina may be in the gamma and/or alpha crystalline phase and/or in the different phases that may be present singly or in combination between gamma and alpha phases consisting essentially of spherical particles having a narrow granulometric distribution, the diameter of said particles being 10-15% lower as compared with the particles of the amorphous hydrated mixed oxides. The zirconia contained in ZTA is preferably lower than or equal to 25% and especially lower than or equal to 11% by weight. The different zirconia-toughened aluminas according to the invention are characterized as to the morphologic and granulometric characteristics by means of a transmission electron microscope (TEM) and as to the determination of the amorphous or crystalline phases of zirconia and alumina by means of X-rays difraction analysis. The J.C.P.D.S. standards for the different utilized phases are the following: gamma-alumina No 14-425; alpha-alumina No 10-173; delta-alumina No 16-394; theta-alumina No 11-517; zirconia-tetragonal No 17-923. On the products, the size of the zirconia crystallites is determined as well in order to control if said size is lower than the critical one above indicated for obtaining the stabilized zirconia in the tetragonal phase. The average size D(111) of the crystallites was determined by X-rays according to Sherrer equation: wherein K = constant, λ = wavelength of the radiation, B = wideness (by degrees) at half-height of reflection (111), ϑ = Bragg's diffraction angle. When zirconia is concerned, the equation turns out to be: The term "average D(111) means the D(111) value coming out from the measurement of the half-height wideness of reflection D(111) that appears in the X-ray diffraction spectrum of ZTA calculated according to equation (2). The X-ray spectrum is carried out by means of a device equipped with a X-ray generator with anode tube made of Cu by using Cu K alpha radiation scintillation goniometer, voltage 36 KV, current 18 mA, Ni filter. The average size of crystallites D(111) in ZTA turns out to be always much lower than the critical limits for the stabilization of the tetragonal phase; it generally ranges between 15o and 800 A°. The zirconia-toughered alumina according to the invention may also comprise stabilizers such as for instance MgO, NiO, Y₂O₃,Cr₂O₃, CeO₂, CaO. The stabilizers may be used alone or in combination. The amounts that may be used, range between 1 and 10% for Y₂O₃ between 3.3 and 4.7% for CaO between 2.8 and 4% for MgO (all the percentages are by weight). Similar percentages may be used for the other oxides above cited. The ZTA aluminas according to the invention can be obtained by a precipitation from solutions by using polyelectrolytes as indicated hereinafter. In fact, a further object of the invention consists in a process for preparing amorphous mixed oxides of alumina and zirconia according to the invention starting from aqueous solutions comprising an aluminum salt, the Al³⁺ concentrations being up to 0,3 moles/l, preferably 0.2 moles/l, said aluminum salt coming for at least 80% by weight,of the total Al present in the system, from aluminum sulphate, a zirconium salt soluble under the reaction conditions, the Zr⁴⁺ concentration being such as to reach at the most 38% by weight as ZrO₂ calculated on the Al₂O₃ and ZrO₂ weight of the product subjected to calcination at 1000°C for 2 hours and by carrying out the precipitation of the hydrated mixed oxides of alumina and zirconia in presence of cationic polyelectrolytes, soluble under the reaction conditions, having an average molecular weight higher than 1 million and ionicity of at least 3 milliequivalents/g (meq/g) said precipitation being carried out under such conditions as to obtain a homogeneous precipitation in the presence of substances capable of releasing OH⁻ ions and furthermore by working in the presence of H₂SO₄ in such amounts as to redissolve the precipitate which occurs during the preparation of the starting solution at room temperature. An alternative way consists in preparing the starting solution by adding to the aluminum salt under the reaction conditions the polyelectrolyte and the basifying substance capable of releasing OH⁻ ions, in acidifying the solution up to at least pH 3, afterwards in adding the zirconium salt and a sulphate. The sulphate is added in an equimolar amount with respect to the sulphuric acid used in the method indicated hereinbefore. In this case, however, lower amounts of sulphate may be used as well depending on the used amount of zirconium. A person skilled in the art will be able to determine easily the lowest amount of sulphate at the different used concentrations of zirconium in order to obtain hydrated mixed oxides of aluminum and zirconium according to the invention, having a spherical morphology and the indicated granulometric distribution. The cationic polyelectrolytes have preferably ionicity higher than 3.5 meq/g and a molecular weight ranging between 3 and 6 millions. The polyelectrolytes of the invention are used in amounts higher than 2.5% by weight with respect to the theoretic amount of Al₂O₃ corresponding to the starting aluminum present in the solution. Preferably amounts of polyelectrolyte are used, higher than 3% by weight, in particular of 3.3% by weight. Especially the cationic polyelectrolyte is used in amounts higher than or equal to 10% by weight. The polyelectrolytes of the invention, before being used, are dissolved preferably in water in concentrations of a few g/l, usually of about 1 or 2 g/l. Amounts of polyelectrolyte may be used, higher than the ones indicated hereinbefore, for instance of about 7 or 20% by weight, in this case no significant variation is obtained as to the morphology and the granulometric distribution of the particles up to about 7.5% by weight. If an amount of polyelectrolyte higher than 7.5% by weight is used, a decrease of the particle diameter is obtained; for instance for concentrations of 20% by weight of polyelectrolyte, the diameter changes from 0.3 µm to 0.2 µm. Usually higher concentration of polyelectrolyte may be used as well, provided that under the reaction conditions, the polyelectrolyte does not separate into a phase different from the aqueous one. In this case, the effect on the diameter decrease is more considerable. The ionicity of the polyelectrolytes according to the invention is related with the number of cationic groups distributed on the whole length of the polymeric chain. In order to obtain the results according to the invention, the cationic groups have to be distributed regularly on the polymeric chain, as a grouping of theirs only on a few zones of the chain leads to a decrease in the polyelectrolyte activity. We have found that the best results are obtained with concentrations of ionic groups corresponding to at least 3 meq/g. The amount of cationic groups is preferably higher than 4 meq/g. The quantitive determination of the ionicity of the cationic polyelectrolyte is carried out according to known methods by using an anionic polyelectrolyte as titrant. The method used in the present invention is set forth in "Polyelectrolyte Determination at Low Concentration" L.K. Wang and W.W.Shuster, Ind. Eng.Chem.,Prod. Res.Dev., Vol.14, No 4,1975, pages 312-314 and it allows to calculate the milliequivalents per g. (meq/g) of positive charge present in the polyelectrolyte. The anionic polyelectrolyte used as titrant is a commercial product named PVSAK consisting of the potassium salt of polyvinylsulphonic acid. Furthermore we have found that the more linear the polyelectrolyte structure, the greater its performances. Molecular weight being the same, a polyelectrolyte having a branched structure presents a lower activity. However, branched cationic polyelectrolytes may be used as well, provided, they are soluble under the reaction conditions and they are introduced in greater amounts. The polyelectrolytes which proved to be particularly suitable for the process according to the invention are polymers comprising repeating unities based on substituted acrylamide having general formula: wherein R₁, R₂, R₃, T are equal or different from each other and are selected from the group consisting of H, an aliphatic hydrocarbon radical having from 1 to 4 carbon atoms; Y is H or -CH₃; Z is H or -CH₃; X is an anion, in particular chloride or sulphate; n is an integer. The polymers comprising the above mentioned unities are obtained from nonionic polyacrylamide by means of known reactions, for instance through the Mannich reaction. Other cationic polyelectrolytes that may be used are polyvinylamines which may be obtained from the polyacrylamides through the Hoffmann degradation reaction and subsequent quaternization of the nitrogen atom according to known techniques or through polymerization according to known processes of the vinylamines and subsequent nitrogen quaternization having general formula: wherein R₄, R₅, R₆ have the same meaning of R₁ as above indicated; X,Y and n have the meaning specified hereinbefore. The preferred compounds are the ones wherein: Y,Z,T,R₃ and R₄ are H and R₁,R₂,R₅,R₆ equal or different from each other are selected from the group consisting of H, -CH₃ or -C₂H₅. The polymers comprising the indiciated cationic unities may also contain neutral unities of non-substituted acrylamide having general formula: wherein m is an integer and wherein Y has the given meaning. The neutral and cationic unities are statistically distributed along the polymer chain ionicity and molecular weight assuming the given values. Use may be also made, however, of copolymers wherein the neutral unity may be selected among the following: wherein R = H or an aliphatic hydrocarbon radical con= taining from 1 to 4 carbon atoms; wherein R₇ has the same meaning of R; wherein R₈ has the meaning of R; wherein R₉ and R₁₀ have the meaning of R; unities coming from the polyvinylpyridines; m in the different classes has the meaning as given hereinbefore. The copolymers containing the cationic unities of acrylamide and the indicated neutral groups are obtained according to processes known to the prior art. In particular the polymers comprising the indicated ionic unities and the groups of class a1) are obtained through polymerization of a CH₂=CHR olefin with acrylamide; the ones of class a2) through polymerization of acrylamide with an ester of acrylic acid; the ones of class a3) through copolymerization of vinylether with acrylamide; the ones of class a4) through copolymerization of maleic acid with acrylamide; the ones of class a5) through copolymerization of acrylamide with vinylpyridine. The different neutral unities indicated in the different groups may be also present contemporaneously in the polymeric chain if more monomers are polymerized at the same time provided that the value of molecular weight and ionicity indicated hereinbefore, is always retained. The combinations of the different radicals that appear in the neutral and cationic groups indicated hereinbefore are, however, selected in such a way that they lead to soluble polymers under the employ conditions of the present invention. In particular, in the different classes one prefers the hydrocarbon radicals, having 1 or 2 carbon atoms. The polyelectrolytes of the class of polyacrylamides are commercial products named Ecoclar, Praestol, Separan which are available both in the solid state and in emulsion. Preferably they are utilized by preparing aqueous solutions of the commercial polyelectrolyte at concentrations of 1-4 g/l at the time of their use and by drawing the necessary amount. In fact, the aqueous solutions of these electrolytes are stable for a short time, usually for a few days only. The homogeneous precipitation is carried out by introducing into the reaction medium substances capable of releasing OH⁻ in a gradual and uniform way so that local oversaturations of the solution may be avoided. Any substance may be used capable of releasing OH⁻ ions as indicated hereinbefore. Urea and formamide, for instance, may be cited as preferred compounds. In the process according to the invention one works at temperatures at which the substance is capable of releasing OH⁻. However, to obtain quantitative yields in hydrated alumina comprising zirconia in a short time, it is necessary to work at such temperature at which the OH⁻ release occurs as quickly as possible. Urea, for instance, starts to release OH⁻ ions already at relatively low temperatures, at about 60°C. However, by working at temperatures of about 80°C as well, quantitative yields in alumina comprising zirconia should be obtained in an exceedingly long time of about several hours, or it should be necessary to use a very high amount of urea with consequent increase in the costs and difficulties in solubilization both of urea and of polyelectrolyte to obtain homogeneous solutions. One prefers to work at temperatures ranging between 90 and 100°C in order to complete the precipitation in a very short time and to avoid possible decompositions of the polyelectrolyte as well. Under these conditions, use may be made of molar ratios urea/Al higher than or equal to 10; preferably higher than 13. Under these conditions, the times required for a quantitative precipitation of the composite hydrated oxide of the invention amounts to a few hours, usually it ranges between 1 and 4 hours and the size of the spherical particles turns out to be practically independent of the used ratios urea/Al. It is possible to work at atmospheric pressure or under pressure, in the latter case the time is further lowered. As aluminum salt use is made preferably of sulphate,, use may be also made of mixed sulphates such as for instance the potassium, sodium and ammonium ones. Usually a mixture of the mentioned aluminum salts may be used as well; it is also possible to work in the presence of an excess of sulphate ions, obtained by addition for instance of sodium, ammonium and potassium sulphate. Mixtures may be also used of aluminum sulphate with aluminum nitrate, chloride, perchlorate, acetate, formate, provided the Al³⁺ amount coming from the salts which differ from sulphate, is about 5-10%, generally not higher than 20% by weight as compared with the total aluminium that has been introduced. As zirconium compound, any compound may be used provided it is soluble under the reaction conditions. For instance we may cite ZrOCl₂, sulphate, nitrate, preferably use is made of ZrOCl₂ or of sulphate. As sulphates to be used, if no use is made of H₂SO₄ in order to dissolve the precipitate again but if one works with acidified starting solutions at a pH lower than 3, we may cite Na₂SO₄, (NH₄)₂SO₄, K₂SO₄ or a composite sulphate. The addition order of the reactants does not affect the morphologic and granulometric characteristics of the final mixed oxide. One prefers, however, to use the above mentioned method consisting in acidifying the starting solution containing the Al³⁺ salt, urea, polyelectrolyte up to pH <3 in order to avoid that the addition of the zirconium salt causes the formation of a filamentary precipitate that does not dissolve by heating, and afterwards in adding the SO₄ = amount as indicated hereinbefore. Should the other method be used, namely without previous acidification, the filamentary precipitate may be treated with acids such as HCl or H₂SO₄ in order to be dissolved again. However, by working according to the latter case, the precipitate may be dissolved again rather with difficulty. The hydrated mixed oxides of the invention appear in the amorphous form under X-ray powder analysis. Crystalline mixed oxides having the same morphology and granulometric distribution as the amorphous mixed oxides of the invention may be obtained by subjecting the hydrated mixed oxides of the invention to thermal treatments according to known processes. By carrying out a particular thermal treatment as described in a preceding patent application of the applicant (Italian Patent Application No. 19142 A/85), the alpha phase of alumina may be also obtained, having the same granulometric and morphologic characteristics as the amorphous precursor. The average size (dw and dn) and the morphologic and granulometric characteristics are determined by analyses. The method for determining dw and dn is described by E.A.Collins, J.A.Davidson and C.A.Daniels in "Review of Common Methods of Particles Size Measurement", Journal of Paint Technology, vol.47, No.604, May 1975. By the process of the invention high productivities are obtained, usually of about 10 g/l. hour or even higher. The composite oxides obtained by the process of the invention turn out to be very pure. The greatest impurity that, desirably, should be as low as possible, is due to sulphur coming from the employ of sulphates which are necessary to obtain the results of the process, object of the invention. However, there are several methods suitable to eliminate this impurity. In particular we have found that the sulphur amount depends in a considerable way on the final pH of the suspension at the end of the precipitation. If the solution of Al³⁺ and Zr⁴⁺ is heated in the presence of urea at a temperature of about 100°C. a slow but progressive increase of pH is noted from the starting value, usually about 1, up to values about 3-3.5. During this phase a precipitate forms that consists of hydrated alumina comprising zirconia; successively a quick increase of pH is noted up to values of 4.5-5.5, afterwards the pH increase rate gets considerably lower. At a pH value of about 4 the Al³⁺ and Zr⁴⁺ precipitation has to be considered as complete and therefore at this point, the precipitate may be separated from the mother liquors by any technique suitable to this purpose such as for instance centrifugation or filtration. Although the Al³⁺ and Zr⁴⁺ precipitation is already over at pH 4, one prefers sometimes to reach pH values higher than 4 as we have found that the higher the suspension pH, the higher the purity referred to the sulphur content of the precipitated composite hydroxides. Obviously it is possible to let the pH rise further by extending the heating time in the presence of urea; the same effect can, however, be obtained by adding alkali directly to the suspension. In fact, as at pH 4 the Al³⁺ and Zr⁴⁺ precipitation is over, the alkali addition does not cause at this point any phenomenon of uncontrolled nucleation but only an increase of the suspension pH. The final pH has not to be over value 6 since higher pH values give rise to phenomena of particle agglomeration and of loss of sphericity. The sulphur content in the hydroxide can be further lowered by suspending said hydroxide after having removed the mother liquors in alkaline solutions at a pH ranging between 8 and 9.5 and at a temperature ranging between 15° and 50°C. Solutions containing 1-3% by weight of NH₄HCO₃ prove to be alkaline solutions advantageous to this purpose when an ammoniacal solution is added to these solutions in such amount that the suspension pH of the hydrated composite oxide may reach the selected value. The obtained final sulphur after the described treatment reaches values < 1% by weight. Moreover, the sulphur content may be further lowered by carrying out thermal treatments. We have found, however, that the almost total elimination of sulphur may be obtained if the amorphous hydrated composite oxide, used as precursor of the crystalline phases of the corresponding oxides, contains a sulphur amount that is as little as possible. A few examples are given hereinafter only by way of illustration but not of limitation of the present invention. EXAMPLE 1 0.6 g of a commercial polyelectrolyte, Ecoclar 8017®, having a molecular weight of 3-4 millions and a positive ionicity measured by titration with PVSAK® of 4.94 milliequivalents/g (meq/g) were dissolved under stirring in 330 ml of H₂O. Then 190 ml were added of a solution containing 2oo g/l of Al₂(SO₄)₃.18H₂O (0.6 moles/l of Al³⁺), 13 ml of concentrated H₂SO₄ (96%), 96 g of urea and finally 10 ml of a solution containing 171.73 g/l of a ZrOCl₂.6H₂O (0.6 moles/l.). The final volume of the solution was about 600 ml. The solution was then poured into a 1 l flask equipped with reflux cooler, thermometer and electrode for the pH measurement and heated without stirring to 100°C. The pH increased from 1 .3 to 5.5 over about 135 minutes: at about pH 3.4 an opalescence appeared whose intensity increased quickly while pH increased. The obtained precipitate after having been filtered was washed and dried at 120°C over a few hours. 12.5 g of hydrated mixed oxide of aluminum and zirconium were obtained; under TEM analyses such compound consisted essentially of spherical, uniform and nearly monodispersed particles (dw/dn = 1.19) having an average diameter equal to 0.3 µm. A photograph of the sample at an magnification of 9700 X is set forth in figure 1. The X-ray powder spectrum showed that the sample was amorphous. Under the example conditions total [SO₄²⁻] / [Zr⁴⁺]= 67.5; ZrO₂ represented 11.30% by weight (nominal) of the final product. Comparative EXAMPLE 1 13 ml of 96% H₂SO₄, 330 ml of a solution containing 96 g of urea and 10 ml of a solution containing 171.73 g/l of ZrOCl₂. 6H₂O (0.6 moles/l of Zr) were added to 190 ml of a solution containing 200 g/l of Al₂ (SO₄)₃.18H₂O (0.6 moles/l of Al₃+). One heated without stirring to 100°C; at such temperature the pH was 1.4. After having heated over 85 minutes at about pH 3.7, appearance was noted of a turbidity whose intensity increased quickly. After having heated for 120 minutes when pH was 5.6, the obtained precipitate was filtered, washed and then dried at 120°C for a few hours. 12.3 g were obtained of a product which, under X-ray analysis, consisted of particles which had different sizes and were agglomerated and lacking of well defined morphologic characteristics. (Figure 2, 9700 X). In particular, the presence of spherical particles was not noted. EXAMPLE 2 0.3 g of Ecoclar 8017® were dissolved in 330 ml of H₂O under stirring. One added 200 ml of a solution containing 200 g/l of Al₂(SO₄)₃.18H₂O and 96 g of urea. Then the solution was heated to 100°C without stirring till pH reached the value of 5.5. The TEM analysis of the precipitate indicated that it consisted essentially of spherical particles having an average diameter of 0.5 µm and dw/dn = 1 .28. A TEM photograph of the obtained product is set forth in figure 3 (magnification 9.700 X). EXAMPLE 3 180 ml of a solution containing 200 g/l of Al₂(SO₄)₃.18H₂O, 18 ml of 96% H₂SO₄, 96 g of urea and 2o ml of a solution containing 171.73 g/l of ZrOCl₂.6H₂O were added to 0.6 g of Ecoclar 8017® dissolved under stirring in 330 ml of H₂O. The solution was heated up to 100°C by using the apparatus described in example 1. At the start, the pH value was 1.6 and it reached value 3.3 after 80 minutes. At this point the appearance was noted of a slight opalescence whose intensity increased quickly while the solution acidity lessened. When after about 135 minutes pH reached value 5.5, the formed precipitate was filtered, washed and then dried in an oven at 120°C. 13.60 g of a product were obtained which under X-ray analysis turned out to be amorphous and under TEM analysis turned out to consist essentially of spheroidal, uniform particles, having a narrow granulometric dispersion and an average diameter of about 0.25 µm. Under the reported condition, [SO₄ ²⁻]/ [Zr⁴⁺] = 40.5, whereas ZrO₂ was nominally 21.20% by weight. A photograph of the sample at an magnification of 9700 X is set forth in figure 4. EXAMPLE 4 (comparative example) A comparison test with example 3 was carried out while keeping unalterated all the conditions but while replacing the solution of Al₂(SO₄)₃.18H₂O with 180 ml of a solution containing 21.14 g/l of (NH₄)₂.SO₄: in such a way the ratio: total [SO₄ ²⁻] / [Zr⁴⁺] remained unchanged, pH changed from 2.0 to 5.5 by heating at 100°C for 85 minutes; at pH = 3.2 one began noting the formation of a precipitate which after having been filtered and washed was dried at 120°C. 2.45 g were obtained of hydrated zirconium oxide, a photograph of which is shown on figure 5, at a magnification of 9700 X. The presence is noted of agglomerated, non-spherical particles which are very different from the ones of example 3, from a morphological point of view. EXAMPLE 5 The test described in example 3 was repeated with the only variant that 0.3 g (0.5 g/l ) of Ecoclar 8017® were used. The TEM analysis showed that the product still consisted of spheroidal particles which had a diameter slightly higher than the ones of example 3 (about 0.4 µm) and were a little more agglomerated. EXAMPLE 6 The test described in example 3 was repeated by using 1.2 g (2 g/l) of Ecoclar 8017®. At TEM examination the obtained product, a photograph of which is set forth in figure 6 with a magnification of 9700 X proved to consist of uniform spheroidal particles that were slightly agglomerated and had a narrow granulometric dispersion and an average diamter equal to 0.2 µm. EXAMPLE 7 The test of example 3 was repeated with the only variant that 48 g of urea were used instead of 96 g. The pH increase by heating to 100°C turned out to be, in this case, much slower so that only after 200 minutes the pH value was over 1.6. By carrying on the reaction over 6 hours up to pH 5.5, a hydrated mixed oxide was obtained having the same characteristics as the one of example 3. EXAMPLE 8 The test described in example 3 was repeated by using 20 ml of a solution containing 0.6 moles/l of Zr(SO₄)₂.H₂O instead of ZrOCl₂.6H₂O. Total [SO₄ ²⁻] / [Zr⁴⁺] = 43.3. The obtained precipitate, after having been dried at 120°C (14 g) did not show, either from the morphologic point of view or from the granulometric one, substantial differences as compared with the one of the test of example 3. E XAMPLE 9 180 ml of a solution containing 2oo g/l of Al₂(SO₄)₃.18H₂O, 96 g of urea and 42.82 g of (NH₄)₂SO₄ were added to 0.6g of Ecoclar 8017® dissolved under stirring in 300 ml of H₂O. The solution pH that turned out to be 3.5 was brought to 2.8 by means of 0.5 ml of H₂SO₄, then 20 ml were added of a solution containing 171.73 g/l of ZrOCl₂.6H₂O and the volume was brought to 600 ml by means of H₂O. Under these conditions total [SO₄ ²⁻] / [Zr⁴⁺] = 40.1 By heating to 100°C in the apparatus described in example 1, pH increased from 2.63 to 5.5 over 55 minutes. The obtained precipitate did not present, at TEM analyses, any substantial difference as to morphology and granulometry of the particles forming said precipitate as compared with the one obtained in the test described in example 3. EXAMPLE 10 A test was carried out according to the operative conditions described in example 6 except that a rotor blade stirrer, having a speed of 260 rounds/minute, was added to the described apparatus. The TEM analyses of the obtained precipitate showed that neither morphology nor granulometric distribution of the particles forming said precipitate were affected. EXAMPLE 11 One dissolved under stirring 0.6 g of Ecolar 8017® in 200 ml of H₂O and one added 170 ml of a solution containing 200 g/l of Al₂(SO₄)₃.18H₂O, 176 ml of a solution containing 96 g of urea, 23.5 ml of 96% H₂SO₄ and 30 ml of a solution containing 171.73 g/l of ZrOCl₂.6H₂O. By heating to 100°C, pH changed from 1.8 to 5.5 over 170 minutes and a precipitate formed that after filtration, washing and drying at 120°C turned out to consist under TEM analysis of particles having characteristics similar to the ones described in the preceding examples. Under the above mentioned conditions ZrO₂(nominal) represented 29.3% by weight; Furthermore the ratio total [SO₄ ²⁻] / [Zr⁴⁺] turned out to be equal to 31.6. X-ray powder spectrum showed that the product was amorphous. EXAMPLE 12 The test described in example 11 was repeated by using 2.5 ml of 96% H₂SO₄ instead of 23.5 ml Under these conditions total [SO₄²⁻] / [Zr⁴⁺] = 11. The TEM analysis of the obtained product showed that it consisted of polydispersed, agglomerated and morphologically heterogeneous particles: anyhow, the presence of spherical particles was not observed. EXAMPLE 13 One dissolved 1.2 g of Ecolar 8017® in 350 ml of H₂O and then one added 96 g of urea, 27 cc of 96% H₂SO₄, 160 ml of a solution containing 200 g/l of Al₂(SO₄)₃.18H₂O and 40 ml of a solution containing 171.73 g/l of ZrOCl₂.6H₂O. At the end of the test that was carried out according to the already described method, 14.58 g were obtained of a precipitate consisting of spheroidal particles having an average diameter of 0.2 µm. X-ray powder spectrum showed that the sample was amorphous. Under the test conditions, the ZrO₂ concentration (nominal) was equal to 37.7% by weight; the ratio total [SO₄ ²⁻] / [Zr⁴⁺] was 26.25. EXAMPLE 14 Example 11 was repeated but use was made of a molar ratio total [SO₄ ²⁻] / [Zr⁴⁺] = 4o by utilizing (NH₄)₂SO₄ to reach this value. The obtained product showed the same morphologic and granulometric characteristics as the one of example 11. EXAMPLE 15 18 ml of 96% H₂SO₄, 96 g of urea, 140 ml of a solution of 0.3 M Al₂(SO₄)₃.18H₂O and 60 ml of a solution of 0.6 M ZrOCl₂.6H₂O were added to 0.6 g of Ecolar 8017® dissolved in 330 ml of H₂O. Under these conditions the nominal concentration of ZrO₂ was 50.89% by weight and total [SO₄²⁻] / [Zr⁴⁺] = 12.5. The test was over (pH = 5.5) after heating at 100°C for 115 minutes. After drying of the obtained precipitate (14.9 g), the product analysed at TEM turned out to consist of particles having a very irregular morphology, anyhow such particles were not spherical and were partly agglomerated. A photograph of the sample at a magnification of 9700 X is set forth in figure 7. EXAMPLE 16 The product obtained in example 3 was subjected to thermal treatment at 900°C for 60 minutes. The obtained product analysed at TEM showed that the particles remained spherical and that the presence of agglomerates was not noted. Morphology was maintained, the particle diameter turned out to be 10% lower as compared with the one of example 1. X-ray powder spectrum showed the presence of gamma and delta Al₂O₃ and ZrO₂ in the tetragonal crystalline phase. EXAMPLE 17 Example 16 was repeated while carrying out thermal treatment at 700°C for 3 hours. Use was made of a diagram of heating slope of 250°C/hour. The particles remained spherical with a diameter decrease of 5% as compared with the product of example 3. The presence of aggregate was not noted and the granulometric distribution was unvaried as compared with the product of example 3. The presence of aggregate was not noted and the granulometric distribution was unvaried as compared with example 16. X-ray powder spectrum of the thus treated sample showed that ZrO₂ was in the tetragonal phase and Al₂O₃ in the gamma phase. EXAMPLE 18 Example 17 was repeated by carrying out the thermal treatment always at the same temperature and over the same period of time with the only variant that the sample was introduced directly into the oven from room temperature to treatment temperature. The results were similar to the ones of example 17. EXAMPLE 19 The product obtained after treatment of example 16 was subjected to a thermal treatment at 1140°C for 15 minutes. Use was made of a diagram of instantaneous heating slope as the one used in example 18. The product maintained the same morphological characteristics as the one of example 16. This product, analysed at X-rays, turned out to consist of a mixture of delta- and theta-Al₂O₃ and of ZrO₂ in the tetragonal crystalline phase. The yield as obtained crystalline mixed oxide, turned out to be about the theoretic one (98%) as compared with the amorphous precursor obtained in example 3. The average size of the t.ZrO₂crystallites drawn from the X-ray spectra turned out to be 0.0245 µm (245A°). EXAMPLE 20 The product obtained in example 1 was subjected to the same thermal treatment as set forth in example 16. Morphology and granulometric distribution of the particles were alike to the one of example 1. The particle diameter was reduced of about 8%. A photograph taken under TEM of the sample obtained at a magnification of 9700 X is set forth in figure 8. X-ray powder analysis showed the presence of the same crystalline phases of example 16 as to Al₂O₃; ZrO₂ was present in the tetragonal phase. EXAMPLE 21 The obtained product in example 20 was subjected to thermal treatment at 1250°C for 30 minutes: X-ray analysis showed the presence of Al₂O₃ in the alpha phase and of ZrO₂ in the tetragonal phase. The average size of the t.ZrO₂ crystallites drawn from the X-ray spectra turned out to be 365A°. The yield in obtained crystalline composite oxide turned out to be about the theoretical one as compared with the amorphous precursor obtained in example 1. EXAMPLE 22 Example 1 was repeated while using 0.6 g of Ecolar 8337®, a cationic polyelectrolyte having an average molecular weight of 4-5 millions, ionicity of 4.99 meq/g. The precipitated mixed oxide had characteristics similar to the ones of example 1. EXAMPLE 23 Example 22 was repeated while using 0.6 g of Praestol 444K®, a cationic polyelectrolyte having an average molecular weight of 4 millions and ionicity of 4.24 meq/g. The precipitated mixed oxide had characteristics similar to the one of example 1. EXAMPLE 24 Example 22 was repeated while using 0.6 g of Praestol 334K®, a cationic polyelectrolyte having an average molecular weight of 3 millions and ionicity of 3.97 meq/g. The obtained composite oxide had characteristics similar to the ones of example 1.

Claims: Amorphous mixed oxides of alumina and zirconia consisting essentially of spherical submicronic non-agglomerated particles, having an average diameter ranging between 0.2 and about 1 µm, having a polydispersion index dw/dn lower than or equal to 2.0 wherein dw is the weight average diameter and dn is the number average diameter and wherein zirconia is uniformely distributed in alumina, said zirconia being at the most 38% by weight and being stabilized in the tetragonal phase at room temperature after having subjected said amorphous mixed oxides to thermal treatments for their transformation into crystalline mixed oxides. Amorphous mixed oxides of alumina and zirconia according to claim 1, wherein dw/dn is lower than or equal to 1.30 and the number average diameter of the particles ranges between 0.2 and 0.5 µm. Amorphous mixed oxides of alumina and zirconia according to claim 2, wherein dw/dn is lower than or equal to 1.20 and the average diameter of the particles ranges between 0.3 and 0.4 µm. Mixed oxides according to claim 1 obtained by subjecting to thermal treatments the amorphous hydrated mixed oxides and wherein zirconia is stabilized in the tetragonal crystalline form and alumina may be in the gamma and/or alpha crystalline phase and/or in the different phases that may be present singly or in combination between gamma and alpha phases consisting essentially of spherical particles having a narrow granulometric distribution, the diameter of said particles being 10-15% lower as compared with the particles of the amorphous hydrated mixed oxides. Mixed oxides according to claim 4 wherein the percentage of zirconia distributed in the alumina is lower than or equal to 25% by weight. Mixed oxides according to claim 5 wherein the zirconia is lower than or euqal to 11% by weight. A process for the preparation of mixed oxides of alumina and zirconia according to claim 1, starting from aqueous solutions comprising an aluminum salt having an Al³⁺ concentration up to 0.3 moles/l, said aluminum salt coming for at least 80% by weight of the total Al present in the system from aluminum sulphate,a zirconium salt soluble under the reaction conditions, the Zr⁴⁺ concentration being such to reach at the most 38% by weight as ZrO₂ calculated on the weight of Al₂O₃ and ZrO₂ of the product subjected to calcination at 1000°C for 2 hours and by carrying out the precipitation in the presence of cationic polyelectrolytes, soluble under the reaction conditions, having an average molecular weight higher than 1 million and ionicity of at least 3 milliequivalents/g (meq/g), said precipitation being carried out under such conditions as to obtain a homogeneous precipitation in the presence of substances capable of releasing OH⁻ ions and moreover by working in the presence of sulphuric acid in such amount as to redissolve the precipitate which forms during the preparation of the starting solution at room temperature. A process for the preparation of mixed oxides of alumina and zirconia according to claim 7, wherein the Al³⁺ concentration is 0.2 moles/l. A process for the preparation of mixed oxides of alumina and zirconia according to claim 7, wherein the starting solution of aluminum salt, polyelectrolyte, substance capable of releasing OH⁻ ions under the reaction conditions is acidified up to pH below 3, then the zirconium salt is added and a sulphate is added in an amount equimolecular to the sulphuric acid of claim 7. A process according to claim 7, wherein the cationic polyelectrolyte has ionicity higher than 3.5 meq/g and an average molecular weight of 3 million. A process according to claim 7, wherein the cationic polyelectrolyte is used in amounts higher than or equal to 3.3% by weight as compared with the theoretical alumina corresponding to the starting aluminum present in the solution. A process according to claim 11, wherein the cationic polyelectrolyte is used in amounts higher than or equal to 10% by weight. A process according to claim 7, wherein the cationic polyelectrolyte comprises repeating units based on substituted acrylamide, having general formula: wherein R₁, R₂, R₃, T, equal or different from each other, are selected from the group comprising hydrogen, an aliphatic hydrocarbon radical having from 1 to 4 carbon atoms; Z,Y = -CH₃, H X is an anion, n is an integer. A process according to claim 13, wherein Y,Z and R₃ are H, and R₁ and R₂, equal or different from each other, are selected from the group comprising H, -CH₃ and -C₂H₅. A process according to claim 7, wherein the cationic polyelectrolyte comprises repeating units based on vinylamine, having general formula: wherein R₄, R₅, R₆ have the same meaning of R₁ as given hereinbefore, Y₁ has the same meaning of Y and m the meaning of n. A process according to claim 15, wherein R₄ is H and R₅ and R₆, equal or different from each other, are selected among H, -CH₃, -C₂H₅. A process according to claim 13, wherein the polyelectrolyte also comprises besides the ionic units neutral units of non-substituted acrylamide having general formula: wherein m and Y₂ have the same meaning of Y and n and the ionic and neutral units are distributed statistically along the polymer chain. A process according to claim 13, wherein the cationic polyelectrolyte consists of a copolymer comprising the above mentioned positively charged units and neutral units different from the ones of acrylamide,statistically distributed along the polymer chain. A process for the preparation of hydrated mixed oxides of alumina and zirconia according to claim 7, wherein the homogeneous precipitation is carried out in the presence of urea or formamide. A process according to claim 19, wherein the reaction temperature ranges between 90° and 100°C.

Inventors: MONTINO, FRANCO, SPOTO, GIUSEPPE
Applicants: MONTEDISON S.P.A.
Application year: 1986
Main classification: C01G25
IPC codes: C01G25, C01F7, C04B35