GB2168334A - Production of complex metal hydroxide powders - Google Patents

Abstract

The invention provides a method for the production of complex metal hydroxide and/or hydrous oxide particles, such as those of titanium and zirconium, useful for obtaining the oxide powders thereof, for use in ceramics. The production of spherical monosized particles is promoted and the method involves mixing a solution (12) of an alkoxide of the metal in question in a solvent such as an alcohol with a solution (14) of water in a similar solvent under turbulent conditions (16), quickly to form a homogeneous mixture to cause hydrolysis of the alkoxide. As soon as the mixture has been formed, the mixture is caused or permitted to assume a state of relatively gentle motion (18), to promote condensation polymerization of the hydrolysis product, to form the complex metal hydroxide and/or hydrous oxide product. <IMAGE>

Description

SPECIFICATION Production of complex metal hydroxide powders This invention relates to the production of complex metal hydroxide and/or hydrous oxide particles. More particularly, the invention relates to the production of complex metal hydroxide and/or hydrous oxide particles from metal alkoxides, these particles being useful in powder form in the production of metal oxides.

According to one aspect of the invention a method for the production of complex metal hydroxide and/ or hydrous oxide particles comprises the steps of: mixing a solution of a metal alkoxide in a suitable solvent with a solution of water in a suitable solvent under conditions of relatively high turbulence, rapidly to form a substantially homogeneous mixture; and as soon as the mixture has been formed, causing or permitting the mixture to assume a state of relatively gentle motion to permit the formation in the mixture of complex metal hydroxide and/or hydrous oxide particles, the solvents for the metal alkoxide and for the water being selected, and the proportions thereof used in the mixture being selected, so that they are miscible with each other to form a mixture which is a solution in which the metal alkoxide and water are soluble and in which the complex metal hydroxide and/or hydrous oxide produced is insoluble.

It is believed that the reaction whereby the particles are produced takes place in two stages, namely an initial hydrolysis stage in which individual alkoxide molecules react with water to form an at least partially hydrolysed intermediate, followed by a condensation polymerization of the intermediate to form the complex metal hydroxide and/or hydrous oxide particles, as the case may be. As will emerge hereunder, one of the advantages of the present invention is that it can in certain situations lead to the production of particles which are more or less spherical and more or less monosized, having a narrow particle size distribution. As mentioned above, these particles can be useful in the production of oxides, which oxides can be obtained by heating the particles in powder form.Whereas in certain cases the particles produced by the method of the invention can best be characterised as complex metal hydroxides, they can in other cases better be characterised as complex metal hydrous oxides, or indeed mixtures or conibinations of hydroxides and hydrous oxides, depending on process conditions and starting materials. Their exact characterisation (i.e. whether they are hydroxides, hydrous oxides or some combination) is however a matter of indifference as regards their primary intended use in the production of metal oxides.

The initial hydrolysis step can be represented by reaction (I), for an alkoxide of the formula M(OR)4, where M is a metal which has a valency of 4 and R is an alkyl group:

(more generally

the final condensation polymerization step can be represented by reaction (ill): EM -OR+ =-M-OH =-M-O-ME +ROH (II).

(more generally m(M-OR)+m--M-OH) < =M-O-ME) + mROH).

The hydrolysis reaction (I) is believed to be repeated for most (but not necessarily all) the -OR groups of the metal alkoxide, depending on factors such as the nature of the -OR group, the degree of mixing and speed at which it is effected, the concentration of the starting solutions respectively of alkoxide and water in solvent and their relative proportions, etc. As regards the condensation polymerization reaction (II), this takes place with repeated cross-linking and the formation of oxo-bridges as typically spherical polymer particles are built up in three-dimensional fashion from successive molecules produced by reaction (I).

Reaction (I) can also be represented more generally by:

where the metal has a valency of n, and reaction (II) will correspondingly be: (RO)n,MOR + (RO)n.,M-OH > (RO),,M-O-M(OR),., + ROH (IV) (more generally m((RO)n ,M-OR) + m((RO),,M-OH) ((RO),., M-O-M(OR),,), + mROH The product of reaction (I) will contain some residual -OR groups and the product of reaction (II) will be an amorphous hydroxide and/or hydroux oxide complex containing residual -OH groups (terminal -OH groups and/or -OH groups on the particle surface) and some residual -OR groups.This complex can be converted to the refractory oxide of the metal by heating according to reaction (V):

whereby the hydroxide and/or hydrous oxide is converted to the oxide and any residual -OR groups and/ or water are driven off.

The solvent in which the metal alkoxide is dissolved may be an organic solvent, the solvent in which the water is dissolved also being an organic solvent. The solvent in which the metal alkoxide is dissolved may be selected from the group comprising acetone, ether, the alcohols and mixtures of two or more of the aforegoing, the solvent in which the water is dissolved also being selected from the group comprising acetone, ether, alcohols and mixtures of the aforegoing.

Preferably the organic solvent in which the metal alkoxide is dissolved is selected from the group comprising alcohols and mixtures thereof and the organic solvent in which the water is dissolved is also selected from the group comprising alcohols and mixtures thereof.

Conveniently, the solvent in which the metal alkoxide is dissolved and the solvent in which the water are dissolved are substantially the same. If the metal alkoxide is represented by M(OR) where M is the metal and R is an alkyl group, eg ethyl or propyl, then a solvent may be chosen which is typically an alcohol according to the formula ROH where R is the same alkyl group as the R in the metal alkoxide.

Naturally, the solvent for the starting metal alkoxide in question should be selected so that this alkoxide is sufficiently soluble therein, and, similarly, the solvent for the water should be selected so that water is sufficiently soluble therein.

In accordance with the method the mixing step should take place sufficiently rapidly to avoid any substantial degree of agglomeration of the particles formed after the condensation polymerization has taken place. Such agglomeration is undesirable and leads to a relatively non-uniform particle size in the product of reaction (II). Such agglomeration is believed possibly to result in part from reaction (I) taking place side-by-side and simultaneously with reaction (II) in the event of inadequate or insufficiently fast mixing, ie from the reactions taking place in a relatively inhomogeneous mixture.

Accordingly the starting materials should in their entirety be subjected to a thorough and rapid mixing, with no dead spots if possible, over a time interval which is sufficiently short for no appreciable condensation polymerization to have taken place before mixing is complete and the mixture is homogeneous.

The mixing should thus take place by subjecting the starting solutions to a sufficient degree of turbulence in the mixing step, to obtain the desired homogeneity rapidly for producing, if possible, monosized particles. It should however be noted that the method of the present invention is useful for producing hydroxides and hydrous oxides which in turn are useful in the production of metal oxides, even if the particles produced are not necessarily monosized nor substantially spherical.

Similarly, agglomeration can arise when partially formed condensation polymer particles collide sufficiently violently with one another in a turbulent mixture, again leading to an undesirable particle size distribution with a range of relatively non-uniform particle sizes, and non-spherical particles.

It follows that, as soon as mixing is complete and the mixture has reached a desired degree of homogeneity, the turbulence in the mixture should immediately be reduced to a sufficient extent, although motion in the mixture should not be eliminated entirely, a state of relatively gentle motion being necessary to promote the formation of the complex metal hydroxide and/or hydrous oxide particles by means of the condensation polymerization.

The degree of turbulence desireable for the mixing step, the gentle motion desirable for the polymerization stage, and the length of time for which the mixing step should last (at maximum) will depend on the kinetics of reactions (I) and (II) above, particularly the kinetics of reaction (I), and these kinetics will in turn depend on the exact nature of the starting materials, ie the nature of the metal alkoxide in question, on the solvents used and on the relative concentrations in the starting solvents of the metal alkoxide and the water respectively.In other words, for particular alkoxide and water starting solutions, the turbulence in the mixing step and the duration thereof, and the nature of the relatively gentle motion in the mixture after mixing, should be determined by routine experimentation so as to obtain the desired hydrous oxide and/or hydroxide product, and in particular so as to obtain or promote a a desirable sphericity in the particles produced, and a desirable narrow particle size distribution, both as regards particle size uniformity and ultimate particle size, with as little agglomeration as possible.

From the aforegoing it will be appreciated that the turbulance in the mixing step may thus be selected to be sufficiently high, the duration of the mixing step being selected to be sufficiently low, and the movement of the mixture after mixing being selected to be sufficiently gentle, for no substantial degree of aggolomeration of the particles to take place.

The method of the invention may thus involve passing the starting solutions through a mixer, into which they may be fed simultaneously, separately or in a single stream, and from which they issue after a suitable interval mixed to a sufficient degree of homogeneity. The mixing step may be effected by means of a static mixer.

The turbulence in the static mixer may be sufficiently high to provide a Reynolds number (Re) for the static mixer of between 500 and 3 000. This Reynolds number will be the Reynolds number represented by the following equation: Re = pw6.

in which p = liquid density (g/cm3) = = mean flow velocity referred to the empty mixer(cm/s) 3, = hydraulic diameter of the mixing element channels (cm) N = dynamic viscosity (g/cm.s) The above equation is that given by Sultzer Technical Publication No. 19 for static mixers.

The Applicant has found that, typically, a mixing step carried out under conditions of turbulence above the lower Reynolds number specified above leads to adequately thorough and rapid mixing for the desired homogeneity to be obtained in the mixing step, after a sufficiently short duration, in a static mixer.

The upper limit of turbulence, given by the upper Reynolds number specified above is to avoid possible cavitation or bubble formation, as discussed hereunder. The duration of the mixing step in a static mixer should preferably be no longer than t the reaction time for the hydrolysis step, as defined hereunder, otherwise there may be a danger of agglomeration.

The step of relatively gentle motion may take place in a stirred vessel, and the gentle motion in the stirred vessel may be such as to provide a Reynolds number of between 50 and 300, preferably about 100, in accordance with the following equation: Re - (D,) 2N p in which: N = revolutions per second p = fluid density (g/cm3) ii = viscosity (g/cm.s) Da = impeller diameter (cm).

The above equation is that given by Perry, Chemical Engineers' Handbook, 5th Edition, page 19-6.

Accordingly, in accordance with the invention, the mixing step and the step of relatively gentle motion preferably takes place in physically separate zones. Thus the mixing step may take placd in a mixing zone defined by a static mixer, the step of relatively gentle motion taking place in a stirred vessel, the static mixer discharging directly into the stirred vessel.

The method may be carried out as a continuous process, the metal alkoxide starting solution and the water starting solution being fed continuously into the mixing step and the mixture passing continuously from the mixing step to the step of relatively gentle motion from which it is continuously withdrawn. It will however be appreciated that the method may instead be carried out as a semi-continuous or batchtype process, wherein batches of starting solutions are fed through the static mixer and into the stirred vessel, in which they are stirred until condensation polymerisation has taken place.

It should be noted that, while every attempt should be made to have the condensation polymerization according to reaction (II) take place only after mixing is complete and after the relatively high turbulence has been reduced to the relatively gentle motion. While it is believed that most of the hydrolysis according to reaction (I) takes place during the mixing when turbulence is high, it is possible that some of the hydrolysis according to reaction (I) can continue to take place after mixing is complete and turbulence has been reduced to the relatively gentle motion, but this can be tolerated if agglomeration caused thereby is not excessive.

Preferably the starting materials should be as clean and pure as practicable, and the alkoxide starting solution as dry as practicable, to avoid solid impurities which can form undesirable nucleation sites in the reaction mixture, and to avoid premature hydrolysis of alkoxide in the alkoxide starting solution. The Applicant has found that sufficiently pure starting materials can be obtained, for example, when the invention includes the preliminary step of filtering the starting solutions, prior to the mixing step, through a filter having a filter size of no greater than 0,3 microns, preferably no greater than 0,22 microns.Similarly, an adequately dry alkoxide starting solution can be obtained by selecting or dehydrating a metal alkoxide starting solution so that it has a water content of no greater than 250 parts per million, prefera bly no more than a 100 parts per million, on a volume basis.

Various metal alkoxides have been found (as reported by Yoshiharu Ozaki in an article entitled "Ultrafine Electroceramic Powder Preparation from Metal Alkoxides" Ferroelectronics, 1983, Vol 49, pages 285296) to be capable of hydrolysis to the corresponding hydroxides and/or hydrous oxides. Such alkoxides include those of the metals which are members of the group of metals comprising: Li Ba Co Si Te Na Ti Cu Ge Y K Zr Zn Sn La Be Nb Cd Pb Nd Mg Ta Al As Sm Ca Mn Ga Sb Eu and Sr Fe In Bi Gd.

The Applicant believes that difficulties may be encountered with excessive solubility of the hydrolysis and condensation polymerization products of the alkoxides of alkali metals and alkaline earth metals; and difficulties may arise from instability of alkoxides of lead. Accordingly, the Applicant believes that the method of the present invention would be more appropriate for alkoxides of the metals of the group of metals comprising: Be Fe In Te Sr Co Si Y Ti Cu Ge La Zr Zn Sn Nd Nb Cd As Sm Ta Al Sb Eu and Mn Ga Bi Gd.

Of the above, the Applicant believes that the present invention to be particularly promising for alkoxide starting materials selected from the alkoxides of metals of the group of metals comprising Ti, Zr, Si and Y, in the fields of structural and electronic ceramics, as relatively spherical particles can be produced therefrom, the alkoxides of titanium and zirconium being of particular commercial importance.

The alkyl group of the alkoxide in the metal alkoxide starting material may be selected from branched and straight chain saturated unsubstituted alkyl groups having from 1 to 6 carbon atoms, and mixtures thereof. Preferably said alkyl group is selected from branched and straight chain saturated unsubstituted alkyl groups having from 2 to 3 carbon atoms, and mixtures thereof, with the most preferred alkoxide being the ethoxide, in which said alkyl group is the ethyl group. In this regard, the larger/longer and/or more branched the alkyl group is, and the more carbon atoms it has, the more possible that steric factors can in general hinder the reactions involved in the method of the present invention.On the other hand, if the alkyl groups are too small, reactions may become too rapid for easy control, and it is expected that routine experimentation will be employed to determine the optimum size and configuration for the alkyl group or groups in an alkoxide for a particular reaction.

Although mixing in a static mixer and gentle motion in a stirred vessel have been emphasised above, any suitable apparatus can be used to carry out the method of the present invention. Indeed, flow of starting materials at high turbulence through a narrow conduit which expands into a broad conduit in which the relatively gentle motion takes place, can in principle be employed for the present invention, provided that the appropriate degree of turbulence is obtained for the appropriate period in the narrow conduit, followed by appropriately gentle motion in the broad conduit.In this situation the Reynolds number to be employed is calculated according to the equation: Dip in which: D = diameter of conduit (cm) X = mean flow velocity (cm/s) p = liquid density (g/cm3) ffi = viscosity (g/cm.s) In this case the turbulence as expressed by the Reynolds number should be between 2 500 and 5 000 in the mixing step in the narrow conduit, and should be between 200 and 800 in the broad conduit for the step involving gentle motion.

When the mixing step takes place in a static mixer it should be noted that, for certain starting solutions it can be desirable to promote rapid flow through the static mixer by means of a suitable pressure drop over the static mixer, to provide, in addition to the required overpressure at the feed to the static mixer, a suction or vacuum at the outlet of the static mixer. Furthermore, for the purpose of the method of the present invention, cavitation or the formation of bubbles in the static mixer is undesirable for the purpose of obtaining a homogeneous mixture. These condensations, particularly when a suction or vacuum is employed, and the fact that cavitation or the formation of bubbles tends to take place at high Reynolds numbers, can impose an upper limit to the turbulence which is desirable or acceptable in the static mixer.In practice, turbulence in the static mixer as reflected by a Reynolds number of between 500 and 3 000 has been found to be acceptable for static mixers, and for a narrow conduit expanding into a broad conduit as mentioned above, turbulence in the narrow conduit as expressed by a Reynolds number of between 2 500 and 5 000 is believed to be acceptable, for the same reasons (cavitation and bubble avoidance).

The Applicant believes, from experiments which the Applicant has conducted in accordance with the invention, that the hydrolysis reaction which occurs in the mixing step can be expressed by the reaction rate equation: 1/t, = k[Alk.]a [H2O]b in which: t, = the reaction time in seconds for the hydrolysis to run essentially to completion and before significant condensation polymerisation starts k = constant a = constant b = constant [Alk] = concentration of the metal alkoxide in the alkoxide starting solution (moles') [H2O] = concentration of water in the water starting solution (moles/t).

The abovementioned routine experimentation canoe conducted, on the basis of this rate equation, to obtain a suitable duration for the mixing step, to ensure that it does not continue for too long. Accordingly, very small batches of starting solutions can be tested in test tubes, in which vigorous shaking is adequate to form a homogeneous mixture for the hydrolysis step, before the condensation polymerization starts. In these test tube-scale experiments, various concentrations of the metal alkoxide starting solution and the water starting solution can be tested, in varying proportions. For the starting point of such tests, a stoichiometric excess of water relative to the metal alkoxide can be employed of about 100% 200% by mass.

The interval after the start of the test mixing (by shaking the test tubes) before hydrolysis is complete (determined visually by noting the formation of a visible product in the test tubes) gives an approximate indication of the reaction time (t,).

Concentrations of the metal alkoxide and water reagents in the starting solutions are varied, as are their relative proportions, in these tests, to obtain values which provide particles of the product metal hydroxide and/or hydrous oxide of a desired uniform size and of a desired uniform morphology (sphericity). Then, in the vicinity of these concentrations and proportions, further tests, in which the quantity and concentration of one of the reagents is held constant and the quantity and concentration of the other is varied, and vice versa, are then used to determine the constants in the above rate equation.Once the constants in the rate equation have been determined, for particles of a desired size and morphology, and the reaction time (t,) can be confirmed with greater accuracy and can then be used for scaling up the reaction for use in the method of the present invention.

Thus, once the reaction time for the hydrolysis has been determined, this sets a maximum or upper limit for the duration of the mixing step of the method of the present invention. The static mixer, or indeed any other mixer so employed, will accordingly be selected so that mixing to homogeneity is complete and the mixture has issued from the mixer, before this reaction time has elapsed.

In the particular case of a static mixer, once desirable proportions of the alkoxide and water reagents, and their concentrations in their starting solutions have been determined by the test tube experiments described above, a static mixer will be selected through which the starting solutions can pass with a predetermined desired degree of turbulence, sufficiently quickly for the mixture to issue from the static mixer before the predetermined reaction time has been exceeded and with adequate homogeneity.

Put differently, a static mixer should be selected of a volume such that the starting solutions can pass through the static mixer at a sufficient rate to provide the desired turbulence, to issue therefrom before the reaction time has been exceeded, ie in accordance with the equation: Volume of Mixer (me) flow rate of combined starting solutions (mf/sec)tr The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a schematic flow diagram for the method of the present invention; and Figure 2 shows, for one of the Examples, a plot of the concentration of the alkoxide solution against average particle size of the product particles obtained.

In Figure 1 of the drawings, reference numeral 10 generally designates a flow diagram of apparatus for carrying out the method of the present invention. The apparatus 10 comprises an alkoxide reservoir 12, a water reservoir 14, a static mixer 16 and a reaction chamber 18.

The reservoirs 12, 14 are arranged to hold solutions respectively of metal alkoxide in alcohol and water in alcohol, and are connected by flow lines 20 and 22 to a three-way tap 24 from which a common flow line 26 extends to the mixer 16, the mixer in turn being connected by a flow line 28 to the reaction chamber 18 which is provided with a stirrer 30 so that it constitutes a stirred mixing vessel.

The apparatus 10 is designed for a semi-continuous process to be operated on a laboratory scale with litre-quantities of reagents, the reservoirs 12 and 14 being constructed to hold the starting solution under pressure under an inert (argon) atmosphere.

In use, the apparatus will be purged continuously with clean and dry argon and the starting solutions charged under argon which is clean and dry and under pressure, into the reservoirs 12, 14. These reagents will then be introduced through the static mixer 16 into the reaction chamber by opening the valve 24, argon in the chamber 18 being bled out by a suitable non-return bleed valve (not shown).

As the starting solutions pass through the static mixer 16 they are thoroughly mixed into a homogeneous mixture which passes into the reaction chamber 18, where the mixture is subjected to gentle agitation by the stirrer 30.

It should be noted that the argon pressure in the reservoirs 12, 14 and in the chamber 18 is essentially the same, so that flow of liquids from the reservoirs 12, 14 into the chamber 18 takes place essentially by gravity and is not affected by the argon pressure.

For reservoirs 12, 14 charged respectively with volumes of starting solutions of 250 me, the starting solutions are typically fed through the mixer 16 at a rate determined by prior test tube experiments, the mixer 16 having a volume of 30 mt and the residence time of the liquids therein being determined by their flow rate and being less than the reaction time of the hydrolysis reaction, likewise determined beforehand by test tube experiments.Turbulence in the mixer 16 will conveniently be expressed by a Reynolds number, as described hereinabove for static mixers of the order of about 1000, the stirrer 30 being operated such that the turbulence in the chamber 18 can be expressed by a Reynolds numbers as described hereinabove for mixed vessels of about 100, sufficiently merely to keep such particles as form in the chamber 18 in suspension.

It should further be noted that the flow line 28 is short and narrow, ie its volume is relatively low, so that the time which the mixture issuing from the mixer 16 spends in the flow line 28 before it reaches the chamber 18 is negligible, and the mixer 16 in effect discharges directly into the chamber 18. Were the flow line to have too large a volume and be sized so that turbulent flow takes pice therein, the time spent by the mixture therein could be long enough for condensation polymerization to take place therein in turbulent conditions, thus possibly leading to undesirable particle agglomeration.

The invention will now be described, by way of non-limiting example, with reference to the following worked Examples in terms of which the method of the present invention was carried out by the apparatus 10 of the drawing.

Example 1 Solutions were prepared respectively of titanium tetraethoxide in ethanol and water in ethanol. These solutions were filtered through filters having a filter size of 0,22 microns to ensure that they had no undesired particles present therein, and care was taken to ensure that the titanium tetraethoxide solution had a water content of less than 100 parts per million. The apparatus 10 of the drawing was used, and was continually purged with clean dry argon.

Two 250 mt aliqugts of the respective solutions of titanium tetraethoxide in ethanol and water in ethanol were charged into the reservoirs 12, 14 under the argon. The concentration of the water solution in the ethanol solvent was 1,5 moles/ and the concentration of the titanium tetraethoxide solution was 0,044 moles/4. These solutions could, if desired, be stirred in the reservoirs 12, 14 for example by a suitable stirrer (not shown), to ensure thorough mixing if freshly prepared.

The starting solutions were fed under gravity at 20"C through the mixer 16 into the chamber 18 via the valve 24, passing through the mixer, which had a volume of 30me, and through the flow line 28, in less than about 10 seconds, which period had been determined previously by test tube experiments to be adequately less than the reaction time for the hydrolysis. In the chamber 18 the argon was bled out through a non-return bleed valve.

The stirrer 30 was operated gently (no more vigorously than was necessary to keep the eventual particles produced in suspension) and the first sign of a reaction, namely cloudiness in the solution, was observed after about 120 seconds. Gentle stirring was continued for 5 minutes after the solution became opaque.

The solid product obtained from the reaction was analysed, and was found to comprise substantially spherical monosized particles of a sub-micron size, in the range of 0,5 - 1,0 (about 0,7) microns (ie about 700 nm). The size ratio of the largest particles to the smallest particles was less than 3 and the size ratio of the largest particles to the particles of mean size was less than 2. The particles accordingly had a narrow size distribution, and agglomeration appeared to be absent.

A yield of about 80% by mass, based on titanium dioxide content, of condensation polymer product was obtained.

In contrast, when similar aliquots of the starting solution were merely mixed in a beaker under argon by means of a magnetic stirrer, substantial agglomeration appeared to take place and the resulting particles deviated substantially from ideality, having irregular shapes, and a substantially greater particle size distribution, with the largest particles having a size ratio to the smallest particles of greater than 20.

Subsequent experiments were conducted on the basis described above, but with changes in the concentration of the initial titanium tetraethoxide starting solution. Changes in this concentration had an influence on the mean particle size of the resulting powders. The change appeared to be approximately linear, and is shown in Figure 2, which is a plot of mean size in microns against the concentration of titanium tetraethoxide in the starting solution in ethanol, in moles/4. The mean particle diameter or size increased with increasing ethoxide concentration, and demonstrates that changes in the ethoxide concentration can be used to synthesize powder batches of particles of a predetermined or desired mean diameter.

The product of the reaction is an alcoholic suspension of particles which can be broadly described as complex titanium hydroxide particles (although they may in part contain titanium hydrous oxide) containing some residual -OH groups, and indeed, although fewer, some residual ethoxide groups. It is possible from these particles to form a quasi-stable mono-dispersed suspension in ammoniacal water at a pH of about 10 with the use of ultrasonic energy to break up any agglomerates formed. From this suspension in ammoniacal water compacts can be formed gravitationally or centrifugally, which can densify to high final densities as green compacts. The compacts can then be dried and sintered to remove any residual water, -OH groups and ethoxide groups, and thereby produce a ceramic artifact.

Example 2 Solutions were prepared respectively of zirconium tetra-n-propoxide and water in ethanol. These solutions were filtered as described for Example 1, and the propoxide solution again had a water content of less than 100 ppm. The apparatus 10 of the drawing was again used, and was continually purged with clean dry argon. As in Example 1, the method was carried out at 20"C. As in Example 1, the starting solutions were passed through the apparatus 10 under gravity, so as to avoid excessive turbulence and possible bubble formation and cavitation.

Once again 250 me aliquots of the respective starting solutions were used, having been charged into the reservoirs 12, 14 under argon. The concentration of the water solution in the ethanol solvent was 10,0 moles/t and the concentration of the zirconium tetra-n-propoxide solution was 1,0 moles/f.

Once again the starting solutions passed through the mixer 16 in less than 10 seconds, and an alcoholic product suspension was obtained, being in this case gently stirred for 15 minutes by means of the stirrer 30 in the chamber 18, to ensure that equilibrium had been reached.

Tests were conducted to determine the average particle size in this suspension. A plurality of 1,3 mt aliquots of the alcoholic suspension were added to 10m4 quantities of a series of aqueous solutions of different pH's, followed by ultrasonic aggitation for 15 seconds, the resulting dispersed suspensions then being evaluated for particle size using a Coulter Microsizer.

The remainder of the alcoholic product from which said aliquots were taken was divided into two portions and centrifuged. One portion was washed twice in ethanol, the other being washed twice in water.

In each case the sediment obtained from washing was redispersed in an aqueous ammonium hydroxide solution at a pH of 10, particle size evaluations again being made using the Coulter Microsizer. Results are set out in the following Table in which particle size is indicated in nanometers and in which PDF refers to the polydispersivity factor which gives a qualitative indication of the size distribution about the measured mean, on a scale of 0 - 10, wherein 0 represents mowosized particles, the Dispex referred to being a proprietary ammonium salt of polyacrylic acid:: TABLE Sample Particle size PDF pH 9 (Borax) 251 3 (Dispex-suspension in ethanol) 458 1 pH 9 (ammonium hydroxide) agglomerated pH 10 (ammonium hydroxide) 331 2 pH 11 (ammonium hydroxide) 271 2 pH 5 (acetic acid) 302 3 pH 5 (hydrochloric acid) 312 4 pH 4 (acetic acid) 281 3 pH 4 (hydrochloric acid) 285 1 pH 3 acetic acid) 317 0 pH 3 (hydrochloric acid) 273 1 pH 10 (centrifuged and water washed and in an aqueous amonium hydroxide solution) 243 1 pH 10 (centrifuged and ethanol washed and in an aqueous ammonium hydroxide solution) 252 2 Scanning electron micrograph tests were also carried out on the particles of Example 2, and an approximate particle size of 254 nanometers was obtained.

Examples 1 and 2 demonstrate the utility of the invention which is in principle applicable to any metal which has a suitably stable alkoxide which is sufficiently soluble to provide a starting solution for the method of the invention in a suitable solvent, and which after the method of the invention has been carried out, produces a metal hyroxide or hydrous oxide which is sufficiently insoluble in the reaction mixture.The method of the invention has the advantage that it provides a method of producing metal hydroxides and/or hydrous oxides suitable for the production of metal oxides, the method being capable, at least in certain cases, of having its process parameters selected to promote the production of relatively monosized relatively spherical particles of the hydroxide and/or hydrous oxide, from which similar monosized spherical metal oxide powders of a small and even particle size can be obtained. Oxides with this morphology and particle size are useful in the field of structural and electronic ceramics, because of their ability to be formed into dense homogeneous compacts in the green state. This can be achieved in suitable cases without unacceptable agglomeration and without an unacceptable particle size distribution. Furthermore, the invention in particular provides a method which can produce these particles and powders using reagents in litre quantities or more, which method lends itself to scaling up to industrial scale on a continuous, semi-continuous or batch-type basis, as desired, eg by employing a plurality of static mixers in parallel.

Claims (25)

1. A method for the production of complex metal hydroxide and/or hydrous oxide particles which comprises the steps of: mixing a solution of a metal alkoxide in a suitable solvent with a solution of water in a suitable solvent under conditions of relatively high turbulence, rapidly to form a substantially homogeneous mixture;; andí as soon as the mixture has been formed, causing or permitting the mixture to assume a state of relatively gentle motion to permit the formation in the mixture of complex metal hydroxide and/or hydrous oxide particles, the solvents for the metal alkoxide and for the water being selected, and the proportions thereof used in the mixture being selected, so that they are miscible with each other to form a mixture which is a solution in which the metal alkoxide and water are soluble and in which the complex metal hydroxide and/or hydrous oxide produced is insoluble.

2. A method as claimed in claim 1, in which the solvent in which the metal alkoxide is dissolved is an organic solvent and the solvent in which the water is dissolved is also an organic solvent.

3. A method as claimed in claim 2, in which the solvent in which the metal alkoxide is dissolved is selected from the group comprising acetone, ether, the alcohols and mixtures of two or more of the aforegoing, and the solvent in which the water is dissolved is also selected from the group comprising acetone, ether, alcohols and mixtures of the aforegoing.

4. A method as claimed in claim 3, in which the organic solvent in which the metal alkoxide is dissolved is selected from the group comprising alcohols and mixtures thereof, and the organic solvent in which the water is dissolved is also selected from the group comprising alcohols and mixtures thereof.

5. A method as claimed in any one of the preceding claims, in which the solvent in which the metal alkoxide is dissolved and the solvent in which the water is dissolved are substantially the same.

6. A method as claimed in any one of the preceding claims, in which the turbulence in the mixing step is selected to be sufficiently high, the duration of the mixing step is selected to be sufficiently low, and the movement of the mixture after mixing is selected to be sufficiently gentle for no substantial degree of agglomeration of the particles produced to take place.

7. A method as claimed in any one of the preceding claims, in which the mixing step is effected by means of a static mixer.

8. A method as claimed in claim 7, in which the turbulence in the static mixer is sufficiently high to provide a Reynolds number for the static mixer as defined herein of between 500 and 3 000.

9. A method as claimed in any one of the preceding claims, in which the step of relatively gentle motion takes place in a stirred vessel.

10. A method as claimed in claim 9, in which the gentle motion the stirred vessel is such as to provide a Reynolds number as defined herein of between 50 and 300.

11. A method as claimed in any one of the preceding claims, in which the mixture step and the step of relatively gentle motion take place in physically separate zones.

12. A method as claimed in claim 11, in which the mixing step takes place in a mixing zone defined by a static mixer and the step of relatively gentle motion takes place in a stirred vessel, the static mixer discharging directly into the stirred vessel.

13. A method as claimed in claim 11 or claim 12, which is carried out as a continuous process, the metal alkoxide starting solution and the water starting solution being fed continuously into the mixing step and the mixture passing continuously from the mixing step to the step of relatively gentle motion from which it is continuously withdrawn.

14. A method as claimed in any one of the preceding claims, which includes the preliminary step of filtering the starting solutions, prior to the mixing step, through a filter which has a filter size of no greater than 0,3 microns.

15. A method as claimed in any one of the preceding claims, which comprises selecting or dehydrating the metal alkoxide starting solution so that it has a water content of no greater than 250 parts per million on a volume basis.

16. A method as claimed in any one of the preceding claims, in which the metal of the metal alkoxide starting material is selected from the group of metals comprising: Li Ba Co Si Te Na Ti Cu Ge Y K Zr Zn Sn La Be Nb Cd Pb Nd Mg Ta Al As Sm Ca Mn Ga Sb Eu and Sr Fe In Bi Gd.

17. A method as claimed in claim 16, in which the metal of the metal alkoxide starting material is selected from the group of metals comprising: Be Fe In Te Sr Co Si Y Ti Cu Ge La Zr Zn Sn Nd Nb Cd As Sm Ta Al Sb Eu and Mn Ga Bi Gd.

18. A method as claimed in claim 17 in which the metal of the metal alkoxide starting material is selected from the group of metals comprising Ti, Zr, Si and Y.

19. A method as claimed in claim 18, in which the metal alkoxide is titanium alkoxide.

20. A method as claimed in claim 18, in which the metal alkoxide is zirconium alkoxide.

21. A method as claimed in any one of the preceding claims, in which the alkyl group of the alkoxide in the metal alkoxide starting material is selected from branched and straight chain saturated unsubstituted alkyl groups having from 1 to 6 carbon atoms, and mixtures thereof.

22. A method as claimed in claim 21, in which said alkyl group is selected from branched and straight chain saturated unsubstituted alkyl groups having from 2 to 3 carbon atoms, and mixtures thereof.

23. A method as claimed in claim 22, in which said alkyl group is the ethyl group.

24. A method for the production of complex metal hydroxide and/or hydrous oxide particles substantially as described herein.

25. Complex metal hydroxide and/or hydroxide particles whenever produced according to any one of the preceding claims.