CA2003526A1 - Ceramic microspheres - Google Patents

Abstract

ABSTRACT
Ceramic microspheres are produced by dispersing a bauxite or bauxitic clay material in an aqueous medium, spray drying the dispersion to produce green microspheres and then subjecting the microspheres to calcination and sintering in the temperature range of between 1100°C and 1600°C. The resultant ceramic microsphere product is used as a functional filler in composite materials to modify physical and chemical properties of the composite.

Description

~0~3s26 CERAMIC MICROSPHERES
This invention is concerned with the manufacture of ceramic microspheres and in particular to ceramic microspheres produced from bauxlte fines.
S Prior art processes for the production of ceramic microspheres are well known and may be classified broadly into two distinct process types.
In one type of process an aqueous colloidal sol of fine ceramic powder is dispersed in a non-aqueous liquid to precipitate, under the influence of gravity, green ceramic microspheres which are subsequently dried and calcined.
In the other type of process, green ceramic microspheres are formed mechanically by agglomeration in a rotary pelletizer or the like before drying and calcining.
The former processes relate to microspheres of small diameter in the range 40-60 micrometers whereas the latter processes are concerned with somewhat larger microspheres having a diameter in the range 0.25 to 5.0 millimeters.
Japanese Published Patent Application Number 57-84731 and United States Patent Number 4746468 describe processes wherein the colloidal sol is prepared by initially milling particulate ceramic materials or by precipitating the ceramic material from solution to produce a fine powder having a particle size in the range of say 0.005 micrometers to 0.05 micrometers.

~0035Z6 The sol is then dispersed in a high boiling point liquid having a specific gravity equal to or higher than the ceramic material and is allowed to settle under the influence of gravity to form microspheres. The microspheres are then S washed with solvent to remove adsorbed or occluded high boiling point liquid and then sintered in the range 1000C-1300C to produce the final product.
The above described processes are capable of producing very fine ceramic microspheres but such processes are inefficient and costly in terms of low productivity and materials usage.
United States Patent Numbers 3491492, 4068718, 4427068 and 4713203 all describe processes for manufacture from clay or bauxite materials æubstantially spherical ceramic particles or pellets in the size range 0.25 - 5.0 millimeters. Apart from United States Patent Number 4713203 which utilizes naturally occurring bauxite fines as feedstock the other prior art documents describe the use of relatively coarse clay or bauxite particles. United States patent Number 4427068 does imply however that expensive grinding of calcined clays or bauxites may be employed to produce particle sizes less than 15 micrometers.
Each of United States Patents 3491492, 4068718, 4427068 and 4713203 is concerned with the manufacture of proppants for hydraulic fracturing of subterranean formations and each requires the physical formation of pellets by ~0035;~

agglomeration in a rotary pelletizer or the like, with or without a binder. Subsequently calcining of the green pellets is usually carried out in a rotary calcining kiln.
Previous attempts to produce spherical proppants by spray drying have produced rounded non spherical particles characterized by a hollow recess similar in appearance to a mushroom cap. This shape has been ascribed to thermodynamic deformation of the slurry droplet in the hot gas stream before the particle dried.
Although the prior art manufacturing processes for spherical proppants are generally effective in producing relatively large, low density ceramic spheres, the processes are inherently unadapted to produce high density microspheres under controlled conditions within a particle size range of say 5-100 micrometers.
Accordingly, it is an aim of the present invention to provide a simple and economic method for the production of small diameter ceramic microspheres having a high degree of sphericity from naturally occurring ultrafine bauxites and bauxitic clays.
According to one aspect of the invention there is provided a method for the manufacture of ceramic microspheres, said method comprising the steps of :-preparing a dispersion of bauxite or bauxitic clay having a mean particle size substantially in the range 0.01 to 0.5 micrometers;

spray drying the dispersion to produce green microspheres of a predetermined mean particle diameter; and, subjecting said green microspheres to calcination and sintering. Preferably calcination and sintering i5 5effected in a stationary or gas suspension calciner.
Preferably said dispersion of bauxite or bauxitic clay comprises an aqueous slurry having a solids content in the range 10-60% w/w solids.
Suitably, spray drying is carried out by directing 10a fine spray of said slurry into a heated region under conditions adapted to produce green microspheres of predetermined mean particle size.
Preferably calcination and sintering is carried out in a temperature range between 1100C and 1600C.
15Most preferably calcination and sintering is carried out in a temperature range between 1300C and 1450C.
According to another aspect of the invention there is provided a ceramic microsphere product whenever made in accordance with the method described herein.
20According to yet another aspect of the invention there is provided a composite material comprising as a functional filler a ceramic microsphere product made in accordance with the invention.
In a further aspect of the invention, modifications 25to the chemical composition and microstructure of the microspheres may be provided by the addition of water soluble ;20~352~

inorganic salts, organo-metallic complexes or other mineral compositions. Addition of 0-10 wt~ of water soluble inorganic salts, organo-metallic complexes or other mineral compositions containing metallic elements selected from Groups 1, 2, 3, 3a of the Periodic Table as well as fluorine silicon and phosphorous may be added to the ultrafine bauxite slurry prior to the spray drying. Such additions allow careful control of the microsphere composition to suit specific end use applications.
In this aspect of the invention, the addition of certain water soluble inorganic salts, organo-metallic complexes or other minerals can modify the surface chemistry of the ceramic microspheres to better suit composite enforcement between the ceramic microspheres and the composite matrix and thus considerably improve reinforcing properties.
The present invention relates to ceramic microspheres made from ultrafine bauxite or bauxitic clay.
The relative ease of manufacture and the superior material properties of the product arise primarily from the choice of material, an ultrafine fraction of a naturally occurring bauxite. This ultrafine fraction has a particle size typically from about 0.02 to 0.3 micrometers with an associated surface area of about 34 square metres per gram.
This extreme fineness lends the following advantages to the manufacturing process.

- the need for expensive pregrinding is eliminated - the need for expensive precalcination is eliminated - high strength of the green microspheres is obtained without the addition of a binder - the very high surface area of the feed material makes it highly reactive. This leads to a reduction in sintering time, hence in energy consumption - an exceptionally high degree of uniformity in the composition of the microspheres The advantages of the product produced according to the invention may be summarized as follows:-1. The very fine particle size of the bauxite or bauxitic clay feedstock allows an exceptionally high degree of uniformity of blend in the manufacture of the green microsphere .

2. The green microsphere comprising a multitude of ultrafine particles has numerous points of contact between the particles, and it at these points that sintering will be initiated.

3. It has been found that the extremely intimate dispersion of minerals and thus of constituent elements is conducive to a high degree of reactivity within éach microsphere as it is heated. This leads to a correspondingly intimate dispersion of microscrystallites on sintering. The microsph~res thus Z~103526 exhibit very high strength. It is well known in the art that ceramic products with microcrystalline structure are extremely strong.

4. The ultrafine particle size of the constituted minerals, combined with the small target size of the manufactured green microspheres, permits the use of the simple spray drying process to form particles of substantially spherical shape. Previous experience with the same material in spray drying to form larger particle diameters resulted in rounded but not spherical particles which were characterised by a shape of mushroom cap appearance. This shape has been ascribed to the aerodynamic deformation of the slurry droplet before it dried in the hot gas stream of the spray dryer. It has now been found that when the droplet size is restricted to a particle size less than about 100 micrometers, these deformities in shape substantially disappear. Scanning electron microscopy reveals that sphericity of particles under about 62 micrometers improves dramatically, with particles in the range from about S to 45 micrometers being almost perfectly spherical. It is believed that the higher rate of surface tension to mass in the fine droplets overcomes the tendency to form non--spherical particles.

5. Since the only mechanism employed in the formation of the green microspheres from the ultrafine particles is the removal of hygroscopic moisture by drying, it will be apparent that the optimum benefit has been obtained from the extremely high interparticulate Van der Waal's forces. This leads to near maximum densification of the particles without the need of an expensive binder.

6. Spray drying process allows relatively simple control over the particle size of the green microspheres.
Choice of such appropriate spray drying parameters such as slurry solids concentration, slurry viscosity rate of introduction of slurry, flow rate of compressed air and spray head type and configuration permit this control to be achieved.

7. The choice of the calcination and sintering process is an important element in the large scale commercial production of ceramic microspheres from bauxite or bauxitic clay. Although the small scale production by firing in 1006ely packed beds is feasible, the process must be interrupted at a relat~vely high temperature to allow the bed to be stirred and to prevent the particles sintering together at the temperature chosen for optimum densification of the individual particles.
This is clearly not feasible on a commercial scale.
The small size of the green microspheres and their free-flowing nature precludes the economic and practical use of conventional rotary kilns as 20035;~6 unacceptable losses of finer microspheres would occur.
It was considered that the use of modern stationary calciners such as the so-called flash and gas suspension calciners and which are successfully used for the calcination of such materials as fine aluminium trihydroxide would not be suitable for the calcination and sintering of the ceramic microspheres. It was thought that the very steep temperature gradient and very short residence times typical of such apparatus would lead to the shattering of the microspheres due to the inability of hygroscopic and chemically bound moisture to diffuse from the microspheres at sufficiently rapid .rate. It was also thought that the turbulent nature of the hot gases in the calcination and sintering zone of the calciner that the microspheres would sinter together. Surprisingly, it was found that the amount of particle degradation in the product was low. It was also surprisingly found that few of the microspheres were sintered together.
In order that the invention may be more clearly understood reference is now made to preferred embodiments illustrated in the following examples.
Finely divided materials suitable for the production of ceramic microspheres according to the invention are readily obtained from bauxite deposits such as for example those of the type occurring at Weipa in Northern Queensland. In these and similar bauxites and bauxitic clays, a fine fraction exists which is easily separated, for example, by slurrying the bauxite in water in the presence of a dispersant. A variety of dispersants may be used, such as compounds based on phosphates or polyacrylates, but in the case of Weipa and related bauxites, adjustment of the pH in the range of approximately 10 to 11 by the addition of a base such as ammonia, sodium hydroxide or mixed bases is adequate to provide the required dispersion.
A simple separation step results in recovery of the fine fraction as a dilute slurry from coarse particulate matter. Satisfactory separation may be effected for instance using gravity settlers or hydrocyclones.
In order to render the process economical it is necessary to increase the solids concentration of the slurry of fine particleæ. This may be conveniently done by adjusting the pH of the slurry to about pH7 by the addition of acid. Sulphuric and hydrochloric acids are satisfactory for this purpose. The fine particles are then seen to agglomerate. These agglomerates are then observed to settle.
It will be recognised by those skilled in the art that flocculants may be used to assist in the more rapid settling and increasing the density of the settled solids.
The solids content o~ the settled slurry may still be too low for economical operation of the process, and it may be necessary to introduce a further dewatering step, such as centrifugation or filtration.
The thickened solids now have the consistency of a paste, and it is necessary to redisperse them to a suita~ly low viscosity before the next step. The pH of the thickened slurry is adjusted to pH 10 to 11 by the addition of a base, such as ammonia or sodium hydroxide or a mixture of bases.
More efficient dispersion may be facilitated by the addition of dispersants such as polyacrylates e.g. ammonium polyacrylate, sodium hexametaphosphate or sodium polyphosphate. The use of a high intensity mixer aids in the destruction of agglomerates and the dispersion of most of the particles in their liberated form.
A spray dryer, such as manufactured by the Niro company, or as manufactured by the Bowen-Stork company, is used to form the green microspheres. It is possible to exercise control over particle size distribution by controlling the preparation of the dispersed slurry and the conditions of operation of the spray dryer.
It has been found that the particular fraction of bauxite proposed i.n this document as a source of microspheres has in the Weipa deposits an alumi.na content of about 59%
with a typical range from 55-63%. Similarly, the silica content will a~erage about 10%, with a range typically 7-13%.
Substantially, all of this silica will be present as the mineral kaolinite, although small amounts of quartz may be present. Typically, the mineralogy will comprise 30-50%

20~3S~

gibbsite with 15-45% boehmite, 16-27% kaolinite with less than 0.2% quartz. Oxides of iron and titanium will total about 8-14~.
Electron microscope studies have confirmed that the mineral particles are typically from about 0.02 to 0.3 micrometers in diameter and the minerals gibbsite, boehmite, kaolinite, hematite and anatase are commonly present as liberated crystallites. In other words, the particles are frequently mono-minerallic in nature, and because of the large surface area rapid reaction between particles is facilitated at elevated temperatures, producing mainly corundum and mullite.
In the course of heating such a product to the typical range of 1300 - 1600C to produce the range of 1~ properties typically required in the product, a series of changes occurs in the microspheres.
The alumina minerals which are present, namely, gibbsite, boehmite and kaolinite progressively lose their combined water as the temperature is increased to about 600C.
$he crystal lattices become disordered and as the temperature is further increased the aluminium oxides undergo a series of phase changes. It is to be expected that the sequence of phase changes may include as transition states gamma, delta, theta, chi and kappa forms of alumina. At a temperature of approximately 950C the formation of mullite 20035Z~s commences and at about 1050C the alumina begins to covert to the alpha-phase mineral which is known as corundum. Further increase in temperature causes sintering of the mullite and corundum to form a ceramic body with high strength. The minerals corundum and mullite are the final major phases in the end product and as the temperature is increased the particles gain in compressive strength due to the development of an extremely fine intergrowth or network of these minerals.
During sintering, the pellets may shrink in diameter by up to 30%.
The high reactivity of the uncalcined bauxite and its very fine particle size facilitates the formation of the required phases.
The green microspheres are subjected to calcination and sintering to form the final product. On a laboratory scale, calcination is performed by placing the green microspheres in a suitable crucible and heating the crucible in a muffle furnace to about 900C. The heating rate should be slow enough to allow diffusion of the chemically bonded water to occur. This rate is suitably of the order of about 100C/hour.
After the initial temperature range of about 900C has been attained, heating is continued with a temperature increase rate of about 10C/minute until the crucible and its contents reach about 1300C. The process is then interrupted ~00352~

to prevent the sintering of the particles to each other.
The material is cooled, lightly crushed to break up any agglomerates and screened through a 106 ~m screen to ensure individual particles. The microspheres are then returned to the crucible and rapidly fired up to the final sintering temperature of between 1500C-1600C at a rate of about 20C/minute. It is apparent to those skilled in the art that the temperatures and times chosen for the sintering of the microspheres are dependent on the chemical composition of the raw material, as well as the desired properties of the product. For instance, density and porOsity of the product may be varied by altering the sintering temperature and time.
Generally, a sintering temperature between 1300C and 1600C
is used.
The economic production of ceramic microspheres on a large-scale industrial basis demands a continuous calcination process, rather than the batch process previously described. This is most satisfactorily accomplished in a stationary gas suspension calciner, in which the particles are transported by a moving stream of gas through the drying, calcination and cooling sections of the apparatus. The product may be separated by gravity, or by cycloning or f iltration.
For purposes of example, such devices are manufactured by the Deutsche-Babcock and Lurgi companies of the Federal Republic of Germany, the F.L. Smidth company of 200;~526 Denmark, or the Fuller company of the United States of America. As well as being a more economic process, improvements to the quality of the product are noted. This is particularly in respect to the microcrystallinity of the particles which is expressed in the surface and strength of the sintered particles.
Gas suspension calciners feature a particularly short residence time of particles in the sintering zone of the furnace. For instance, the High Temperature Mixer manufactured by the Deutsche Babcock company, features a residence time in the calcination zone estimated at about one quarter to one half of a second. As a consequence of this short residence time, crystal grain growth is inhibited, and a fine crystal microstructure is evident. The average crystal grain size has been estimated to be about one micrometer. As is known to those skilled in the art, a fine microstructure is conducive to obtaining strength in a ceramic object. The fine microstructure is also important in conveying a smooth surface to the microspheres. When used in composites which are to be subjected to such processes as injection moulding, wear of the surfaces of the forming equipment is reduced.

Z0035~6 1. PREPARATION OF SLURRY DISPERSION
1.1 Fine Bauxite tailings were dispersed with 0.2% sodium hydroxide and 0.15% sodium hexametaphosphate, both expressed as weight per weight on dry bauxite solids, basis.
1.2 Particles coarser than about 5 micrometers ~ere then separated by settling. Dispersed ultrafine particles were decanted.
1.3 The pH of the dispersed ultrafines suspension was corrected to pH7 by addition of sulphuric acid. The fines thereby coagulated. The coagulated fines were allowed to thicken for five days. The solids content of the thickened slurry was calculated to be 25% w/w.
2. PREPARA~ION OF GREEN MICROSPHERES
Spray drying was performed by feeding the above thickened slurry to Bowen Stork Laboratory Scale Spray Dryer, model BE1203.
Spraying to achieve microspheres was performed by an atomiser spray machine, model AT-4 spinning at about 50,000 revolutions per minute, driven by compressed air supplied at about 0.5 cubic metres per minute.
The spray drier was fired with liquid propane gas to achieve an inlet temperature of about 250C, and the outlet temperature was set to about 95-100C.
Production rate was about ~.6 kilograms per hour of dry solids.

~0035Z6 CHEMICAL ANALYSIS OF BAUXITE ULTRAFINES USED FOR
PREPARATION OF GREEN MICROSPHERES
% BY MASS
Al20 57-SiO2 Total 13.3 SiO2 Quartz 0.1 Fe 23 5-5 TiO2 2.6 Loss of ignition20.1 Specific surface area of the above bauxite ultrafines was 34 m2g~1 3. CALCINATION OF MICROSPHERES
This process is carried out by a laboratory scale process as previously described and the following steps are employed:-3.1 The green microspheres were calcined in laboratory muffle furnace in fire clay crucibles, open to air.
3.2 Calcination was carried out in two stages.
3.2.1 Precalcined at 900C for one hour. Temperature reduced to room temperature, and small agglomerates broken up.
3.2.2 Crucible returned to muffle, and the microspheres sintered to 1450C. The final product was free flowing, with no lumps apparent.
4. PARTICLE SIZE OF PRODUCT AS DETBRMINED BY LEEDS AND
NORTHRUP MICROTRAC APPARATUS
Particle diameter micrometers Cumulative % by mass passin~

31 91.6 22 77.2 16 57.5 11 38.8 7.8 24.9 5.5 14.4 3.9 6.8 2.8 2.2 EXAMPLE 2.
1. PREPARATION OF SLURRY DISPERSION

1.1 Raw material for production of green microspheres was prepared as for Example 1, except that the solids c~ntent of 20~35~

the slurry was 40% w/w.
2. CHEMICAL ANALYSIS OF ABOVE RAW MATERIAL
Percent by Mass Al 23 58.3 SiO2 Total 7.1 Fe23 11. 1 TiO2 3.2 Na2O 0.14 SO 0.28 Loss on ignition 19.8 3. PREPARATION OF GREEN MICROSPHERES
3.1 The solids were dispersed by the addition of an ammonium polyacrylate dispersant at the rate of 0.2%
w/w of dry bauxite solids, with the assistance of the addition of ammonia solution to achieve a solution pH
of 10.5.
3.2 The spray dryer used was a Niro Type P-6.3, fitted with a 4.5 inch vaned atomizer wheel rotating at 16000rpm.
The spray dryer was heated with natural gas and the inlet temperature was about 250~C. The evaporation rate was estimated to be 30 Kg water per hour.

.
Calcination of the green microspheres was carried in the same manner as in Example 1.
CHEMICAL ANALYSIS OF FIRED PRODUCT
% By Mass Al 23 73.0 sioz g . o Fe O 13.7 iO 3 4.03 Na2O 0.1 Loss on ignition 0.06 Particle size analysis of fired product, as determined by Leeds and Northrup Microtrac apparatus.
PARTICLE DIAMETER CUMULATIVE PERCENT PASSING
MICROMETERS

44 92.4 31 64.6 20(~1352~

22 32.4 16 13.3 11 5.5 7.8 1.7 5.5 0.3 3.9 0 Krumbien and Schloss sphericity: Greater than 90 percent.
Pycnometric Density 3.4 gcm~3 Surface Area 0.1m2/g The same green microspheres as prepared in Example 2 above were fed to a gas suspension calciner, model "High Temperature Mixer", manufactured by the Deutsche-Babcock company of the Federal Republic of Germany.
The calciner was fuelled by natural gas.
Temperature in the calcination zone was measured to be 1370C. Residence time of material through the whole calcination apparatus was estimated to be about four seconds.
Residence time in the calcination zone however, was estimated to be between 0.25 and 0.5 seconds.
The fired product was wet-sieved at 63 micrometers to remove contaminants such as small pieces of refractory and a small quantity of agglomerates.
Particle size of fired product, as determined by Leeds and Northrup Microtrac apparatus.
PARTICLE DIAMETER CUMULATIVE PERCENT
MICROMETERS _ASSING

44 88.3 31 58.0 22 28.3 16 10.8 11 4.6 7.8 1.6 5.5 0.3 3.9 ' 0.1 MEDIAN PARTICLE SIZE: 30 MICROMETERS
Krumbien and Schloss sphericity: greater than 95 percent Z0~3~,,i, .?`

Bulk (tapped) density 2.1 g cm-3 Pycnometric density 3.66 g cm~3 Porosity 2.9%, predominantly closed pores Packing fraction 0.6 (approximately) CHEMICAL ANALYSIS
PERCENT BY MASS
AL23 73.0 si2 9 . O
Fe O 13.7 ~io2 4.03 Na20 O. 1 Loss on ignition 0.06 APPROXIMATE MINERALOGICAL ANALYSIS
PERCENT BY MASS
Corundum 41 Mullite 51 Titanium-rich glass 8 Ceramic microspheres produced in accordance with the inven~ion may be used for a variety of purposes and in particular may be used as functional fillers in a variety of media. The ceramic composition of the particles may lend such properties as reinforcement, toughness, hardness wear and abrasion resistance, chemical resistance, resistance to weathering and the like in a composite material.
It will be apparent to a skilled addressee that many modifications and variations will be possible with the present invention without departing from the spirit and scope thereof.

Claims (13)

1. A method for the manufacture of ceramic microspheres, said method comprising the steps of :-preparing a dispersion of bauxite or bauxitic clay having a mean particle size substantially in the range of 0.01 to 0.5 micrometers;
spray drying said dispersion to produce green microspheres of a predetermined mean particle diameter; and, subjecting said green microspheres to calcination and sintering.

2. A method as claimed in claim 1 wherein said dispersion of bauxite or bauxitic clay comprises an aqueous slurry having a solids content in the range 10-60% w/w solids.

3. A method as claimed in claim 2 wherein said aqueous slurry has a solids content in the range 40-50% w/w solids.

4. A method as claimed in any preceding claim wherein said dispersion includes from 0-10wt% of a water soluble inorganic salt, an organo-metallic complex, or other mineral composition or a mixture thereof.

5. A method as claimed in claim 4 wherein said water soluble inorganic salt, said organo-metallic complex or said other mineral composition comprises metallic elements selected from groups 1, group 2, group 3 or group 3a of the Periodic Table of Elements or fluorine, silicon or phosphorous or any combination thereof.

6. A method as claimed in claim 5 wherein said metallic elements are selected from sodium, potassium, boron, aluminium, magnesium, calcium, zirconium or barium.

7. A method as claimed in any preceding claim wherein calcination and sintering is carried out in a temperature range between 1100°C - 1600°C.

8. A method as claimed in claim 7 wherein the temperature range is between 1300°C and 1450°C.

9. A method as claimed in any preceding claim wherein calcination and sintering is effected in a stationary calciner.

10. A method as claimed in claim 9 wherein said calciner comprises a gas suspension calciner

11. A method as claimed in claim 9 wherein said calciner comprises a flash calciner.

12. Ceramic microspheres whenever produced according to the method of any one of claims 1-11.

13. A composite material comprises as a functional filler a ceramic microsphere product made in accordance with any one of claims 1-11.