PARTIALLY STABILIZED ZIRCONIA AND ZIRCONIA TOUGHENED ALUMINA BASED CERAMIC MINIMEDIA, A PROCESS AND APPRATUS
FOR MANUFACTURING THE SAME
FIELD OF INVENTION
The present application relates to partially stabilized zirconia and zirconia toughened alumina based ceramic mini-media /beads for fine grinding applications and/or as a filler for composites. In particular, the present invention relates to partially stabilized zirconia and zirconia toughened alumina based ceramic beads or more specifically, partially stabilized zirconia and zirconia toughened alumina beads wherein the zirconia is partially stabilized with yttria (Y2O3), ceria (CeO2), magnesia (MgO), or calcia (CaO) . It also relates to a method for their manufacture by simple, inexpensive and environmentally friendly method and the development of the equipment for producing the same.
BACKGROUND OF INVENTION
Spherical ceramic beads/ mini-media are being used for fine grinding of minerals, food processing, paint industries. The ceramic beads are also being used as catalyst supports, metal matrix composites, shot-blasting operations and also for heat exchange media. For these applications, the shape, sphericity, fracture toughness and hardness are the required characteristics of the ceramic material. The beads and balls are preferably formed from ceramic materials those exhibit high fracture toughness, high hardness, good strength, and high wear resistance.
For most of the milling applications especially in the field of paint industries, food processing industries and pharmaceuticals, partially stabilized zirconia beads or zirconia toughened alumina beads are being used over glass beads and zirconium silicate beads. The high hardness, fracture toughness, strength and durability of partially stabilized
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zirconia and zirconia toughened alumina provides better performance than that of glass beads or zirconium silicate beads.
There are a number of known methods of forming ceramic beads, with the “spray dry” and/or “oil-drop” methods being most widely used. Representative examples of prior art methods of forming ceramic beads are disclosed in: Elliott, U.S. Pat. No. 3,457,335; Schoonover, U.S. Pat. No. 4,318,896; Hansson et al., U.S. Pat. No. 4,621,936; Green et al., U.S. Pat. No. 4,628,040; and Johns, U.S. Pat. No. 5,484,559.
In general, the sintered ceramic beads are obtained by cold-forming a ceramic powder and sintering the ceramic powder by sintering at a high temperature. On the other hand, a molten ceramic bead is formed by melting a ceramic component at a very high temperature and forming it into spherical droplets. Most of the beads are composed of zirconia having monoclinic crystallization and forming a vitreous silicate phase to have a composition of zirconia-silica (ZrO2 -SiO2) system, and have low abrasion resistance and optimum abrasion characteristics (mechanical strength, High density, chemical inertness). The limitations and drawbacks associated with prior art methods of forming ceramic beads are well known. One of the most significant drawbacks is the difficulty in obtaining ceramic beads in highly pure form and in the desired uniform shape.
It has been found that the alginate route is one of the best method for making ceramic beads. US6797203 discloses the process for making mini-media by alginate route wherein the solution of the said ceramic powders, e.g. alumina, Ce- TZP, YPSZ has been dissolved using Darvan C as a dispersant. The said ceramic solution is passed through CaCl2 solution. The droplets are dislodged using a nozzle tip into said immiscible fluid layer using said shear force to make a subsequently rigid bodies.
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Preparation of ceramic beads with mono-modal size distribution has been disclosed in US0007789 wherein ammonium alginate is being used as a sacrificial media. The beads are passed through a solution column consisting of two layers, the first being petroleum solvents/ glycols/ acetates and the second being water mixed with calcium chloride.
US2004/007789 discloses a method of forming alumina-zirconia beads, comprising (a) forming a slip comprised of water and a mixture of metal oxides, said mixture comprised of: alumina, ceria, and monoclinic zirconia, (b) milling said slip; (d) adding an ammonia-containing binding agent to said slip, (e) forming beads by dropping said slip into a calcium chloride solution wherein drops of said slip gel into beads containing calcium chloride; (f) washing said beads, (g) drying said beads; and (h) firing said beads at a temperature between 1,400 °C and 1,600 °C for 2 hours to 8 hours. The beads contain 0.5% CaO.
As is evident from the above, making ceramic beads using alginate as a sacrificial in-situ gelling technique in which Ca+2, NH4+ replaces Na+ in Na-alginate to form Ca or Ammonium alginate, which would be decomposed during subsequent heating process is widespread. However the use of the same is limited due to the further cleaning step which is required to remove the same. In most of the preparation techniques of ceramic micro-beads through alginate route, the stabilized zirconia or zirconia toughened alumina are being introduced through the metal precursor salt which is being dissolved in the alginate solution. The major disadvantage with the same is the use of expensive metal precursor salt. Sometimes during the direct processing there is a chance of generation of micro-bubble in the beads due to the evaporation of water. Additionally, powders prepared through sol-gel route can be used upon proper mixing. However the mixing of the powders with alginate having high specific surface area, as obtained by sol-gel route is difficult.
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It is therefore felt that a technique for production of spherical ceramic beads of stabilized zirconia, zirconia toughened alumina or any other material which does not require a further cleaning step to remove excess ammonia or calcia ions and which eliminates the use of high cost metal precursors or ceramic powders prepared with expensive route and which does not depend upon the production of stable hydrosols or emulsions or upon melting of the inorganic raw material will be an advancement in state of art of preparation of ceramic mini- media using alginate route.
OBJECT OF THE INVENTION
One object of the present invention is to provide partially stabilized zirconia toughened ceramic beads with good sphericity and hardness.
Another object of the present invention is to develop a technique for making partially stabilized zirconia/ zirconia toughened ceramic beads with good sphericity and properties by alginate route wherein no further cleaning process is required in order to remove the excess ions from the beads.
Yet another object of the present invention is to develop ceramic beads with partially stabilized zirconia and/ or zirconia toughened alumina wherein the ceramic powders are not prepared with expensive sol- gel/ co- precipitation route. The stabilization of zirconia is being achieved in-situ during sintering.
Yet another object of the present invention is development of simple apparatus which can be used to prepare the ceramic minimedia at a very low price.
BRIEF DESCRIPTION OF THE FIGURES
Fig. 1: Process Flowchart of preparing Ceramic Minimedia.
Fig. 2: Schematic cross-sectional view of a manufacturing apparatus of the present
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invention, the specific view of the equipment for drip casting.
Fig. 3: Sectional view of the solution drainage system and hot air conveyer for drying..
Fig. 4: Polished Surface of the partially stabilized zirconia Media showing fully dense
structure @ 100X.
Fig. 5: Indentation of partially stabilized zirconia Media @ 100X.
Fig. 6: XRD of partially stabilized zirconia Minimedia.
Fig. 7: c- DTA Curve showing transformation and crystallographic of tetragonal phase of
zirconia.
SUMMARY OF THE INVENTION
In one aspect, the present application provides stabilized zirconia toughened ceramic
beads having the following chemical composition by weight percentage:
about 3 wt% to about 100 wt % by weight ZrO2;
about 0.1 wt % to about 60 wt% by weight Al2O3;
about 1.0 wt % to about 13.75 wt% yttria
and/or about 1.0 to 25 wt% of Ceria
and/or about 1.0 to 5.0 wt% of MgO
and/or about 1.0 to 5.0 wt% of CaO as the zirconia stabilizing agent;
about 0.1% to about 1.0% by weight SiO2;
In another aspect the present application provides a process for the manufacture of the partially stabilized zirconia toughened ceramic beads or the partially stabilized zirconia toughened alumina beads by alginate route by using in-situ gelling technique wherein a single reaction column is employed.
Yet another aspect of the present application provides an apparatus for the manufacture of the partially stabilized zirconia toughened ceramic beads or the stabilized zirconia toughened alumina beads comprising a single reaction column.
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DETAILED DESCRIPTION OF THE INVENTION
For the purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification are to be understood as being modified in all instances by the term "about". It is noted that, unless otherwise stated, all percentages given in this specification and appended claims refer to percentages by weight of the total composition.
Thus, before describing the present invention in detail, it is to be understood that this invention is not limited to particularly exemplified systems or method parameters that may of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only, and is not intended to limit the scope of the invention in any manner.
The use of examples anywhere in this specification including examples of any terms discussed herein is illustrative only, and in no way limits the scope and meaning of the invention or of any exemplified term. Likewise, the invention is not limited to various embodiments given in this specification.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the case of conflict, the present document, including definitions will control.
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It must be noted that, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to a “polymer” may include two or more such polymers.
The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention.
As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.
In one aspect, the present application provides stabilized zirconia toughened ceramic
beads having the following chemical composition by weight percentage:
about 3 wt% to about 100 wt % by weight ZrO2;
about 0.1 wt % to about 60 wt% by weight Al2O3;
about 1.0 wt % to about 13.75 wt% yttria
and/or about 1.0 to 25 wt% of Ceria
and/or about 1.0 to 5.0 wt% of MgO
and/or about 1.0 to 5.0 wt% of CaO as the zirconia stabilizing agent;
about 0.1% to about 1.0% by weight SiO2;
In an embodiment, ceramic beads are partially stabilized zirconia toughened alumina or more specifically, yttria partially stabilized zirconia toughened alumina beads. The size of the beads may range from 0.5 to 5.0 mm and more preferably from 0.5 mm to 2.5 mm.
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The ceramic beads may further comprise of 0.1 wt% to 1 wt% MgO.
In another aspect the present application provides a process for the manufacture of the partially stabilized zirconia toughened ceramic beads or the partially stabilized zirconia toughened alumina beads by alginate route by using in-situ gelling technique wherein a single reaction column is employed.
In an embodiment, the zirconia stabilized ceramic beads are yttria partially stabilized zirconia toughened alumina or yttria partially stabilized zirconia beads.
In accordance with the present invention, the process for making stabilized zirconia/ zirconia toughened alumina beads by alginate route employs Na- Alginate as a sacrificial media and the process is an in- situ gelling technique. When Na-alginate comes in contact with ions having valency more than one, the Na+ ions would be replaced by the same. As a result the gelation happens due to cross-linking of the alginate. In the process of the present invention, fused monoclinic zirconia powder is employed which is stabilized in -tetragonal phase by yttria and/ or ceria in- situ during sintering. Further cleaning of ceramic micro- bubbles at green condition upon casting has been removed by using a column of liquid prepared with yttrium nitrate solution. Since oxides of yttria, ceria, magnesia, or calcium such as Y2O3, CeO2, MgO, CaO etc are being used in stabilization of zirconia phase no further cleaning process is being required.
The process of the present invention produces a ceramic mini- media prepared with stabilized zirconia and zirconia toughened alumina by Na- alginate as sacrificial template.
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The ceramic powder may consist of monoclinic zirconia prepared by fusion route, Yttrium oxide as stabilizing agent and/ or alumina powders. The fused zirconia powder has a size ranging from 1 micrometer to 15 micrometer.
The powders are milled together in a tube mill using water as a dispersing agent. The pH of the solution is controlled at 7- 8 which provides a stable solution of the ceramic powders. The slurry prepared with the solution is therefore mixed with Na-Alginate solution which is capable of gelation to form self supporting beads before contact with solid surface. The sodium alginate solution has a concentration of 0.1 to 2%.
A single reaction column, is employed. In a specific embodiment, the column is a liquid column prepared with yttrium nitrate precursors. The precursors include but are not limited to any yttrium containing salts. The slurry mixed with alginate binder is passed through the liquid column. The function of the yttrium nitrate liquid column is to convert the tear drop or other irregular shaped droplets into spherical form while dispersing through the salt solution and also promoting the gelation of the alginate binder into spherical droplets. The concentration of the yttrium salt is kept low in the range of 0.5 to 1 wt%. Since yttria is the stabilizing agent for zirconia, the final desired purity and phase compositions in the mini-media is being maintained.
The process parameters which are considered while preparing mini-media with alginate route is stabilization and viscosity of the slurry as a function of ceramic powder/ alginate being added to the slurry. The viscosity of the slurry is in the range of 100 cps to 500 cps by Brookfield viscometer.
The gelation process of sodium alginate mixed ceramic slurry in the presence of yttrium nitrate solution introduces ions after gelation and thus it influence the green strength of the ceramic micro bubble upon casting.
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The ion exchanging reaction which occurs during the gelation, is as follows: Y(NO3)3 + 3 (C6H7O6- Na) → 3Na+ + 3(NO3)- +(C6H7O6- Y- C6H7O6- Y- C6H7O6)
The beads upon casting are dried in a belt type air heater which upon drying result into hard micro- bubble. The further densification of same is achieved by sintering the same in a muffle furnace at a suitable temperature as per the composition. During sintering the zirconia which is present in the composition is transformed to the partially stabilized tetragonal phase by in- situ sintering process.
Yet another aspect of the present application provides an apparatus for the manufacture of the partially stabilized zirconia toughened ceramic beads comprising a a single reaction column.
In a specific embodiment, the reaction column is a liquid column prepared with yttrium nitrate precursors.
The apparatus of the present invention comprises:
a) a chamber for storing the ceramic slurry with alginate solution;
b) a casting tank through which the slurry is dropped into the reaction column;
c) a reaction column, wherein multiple number of reaction columns move on a conveyor; and
d) a hot air dryer.
The chamber is referred to as slurry tank for storing the ceramic slurry with alginate solution. The slurry tank is fitted with a stirrer, which is rotating at rpm in the range of 100 – 1000 rpm, and a connecting pipe with valve: The opening and closing of which is controlled by the level sensor in the casting tank (2).
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The slurry is dropped into the reaction column through casting tank which comprises of Level sensor to maintain the slurry height, and a nozzle system which is equipped with multiple numbers of nozzle (3 - 12). The system is kept under little vibration.
The reaction column has multiple number of reaction columns prepared with toughened glass or borosilicate glass or stainless steel vessel or fibre reinforced plastic material which would move on a conveyor. The reaction column is fitted with item as described below:
a) a level sensor in order to maintain the casting height. Upon achieving the height, the reaction column will move to the next stage.
b) a tap arrangement in order to drain the slurry after casting and ion exchange reaction (Typical reaction time being 30- 60 minutes).
The bottom of the reaction column may be removable. Upon drainage of the slurry the same may be removed and is sent through hot air dryer (4) for subsequent drying process. The above process is shown in Fig. 3.
The hot air dryer which comprises of a conveyor for conveying the trays. Hot air flows in counter-current direction by means of a motor, and aluminium heater plates are arranged below the conveyor. The inside temperature of the dryer is maintained at a temperature in the range of 500C to 1100C. The solution drainage system and hot air conveyor for drying is described in Fig. 3.
The features of the apparatus which are considered while preparing mini-media with alginate route are (1) Distance between the nozzle and reactor column is in the range of 0 mm to 500 mm, (2) Dimension of nozzle is in the range of 0.3 mm to 7.5 mm, and (3) Height of the reaction column is in the range of 100 mm and above.
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Typically three forces are acting while the slurry droplets are being passed through the reaction column, namely (1) surface tension reacted upon the droplet, (2) the gravitational force, (3) A third force which is pulling the droplet downwards, i.e. The inertial force. The surface tension are acting upwards while the inertia force and the gravitational force is acting downwards.
In order to maintain the equilibrium between the surface tension and gravitational force, the speed of the drops need to be maintained. This is maintained by maintaining the slurry level in the casting tank which again is controlled by introduction of a level sensor.
The casting height is in the range of 50 mm and above at the reaction column is also need to be maintained at a certain extent, otherwise there is a chance of shape change of the droplets due to the self load of the casting beads acting downwards. This again is maintained by introduction of a level sensor in the casting tank.
In one embodiment, in order to increase the rate of production, multiple reaction columns can be introduced which will travel in a conveyor belt. The yttrium nitrate solution is being removed after a certain time, since the complete ion- exchange operation is already completed.
The following examples are provided to better illustrate the claimed invention and are not to be interpreted in any way as limiting the scope of the invention. All specific materials, and methods described below, fall within the scope of the invention. These specific compositions, materials, and methods are not intended to limit the invention, but merely to illustrate specific embodiments falling within the scope of the invention. One skilled in the art may develop equivalent materials, and methods without the exercise of inventive
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capacity and without departing from the scope of the invention. It is the intention of the inventors that such variations are included within the scope of the invention.
EXAMPLES:
Example 1:
90 wt% of Alumina powder having a particle size of less than 5 um is mixed with 10 wt% fused monoclinic zirconia powder. Yttrium oxide at 5.35 wt% of that of zirconia powder is then added in the composition. The powders are milled in a tube mill up to D50 of 2+/-0.1 um size using water at 30 wt% as solvent. The resultant slurry contains a minor amount of dispersant in order to stabilize the slurry. Na- alginate solution of 1% concentration is prepared by adding the Na- alginate powder into hot water which is maintained at a temperature of 70 degree centigrade. The mixing is accomplished by thorough mixing of same using high speed stirrer. The final solid content in the slurry mixed with Na- Alginate is maintained at 60 wt%.
Yttrium Nitrate solution is prepared by dissolving 1 wt% of Yttrium Nitrate into water by occasional stirring. The ceramic slurry mixed with Na- Alginate solution is passed through a nozzle of size 2 mm size. The height at the reaction column is maintained at 1/3 rd of the total column height in order to maintain the static load. The resultant ceramic beads of 2.4 mm - 2.6 mm diameter are kept at reaction column for 30 mins in order to complete the ion- exchanging reaction. Upon removal of the solvent from the reaction column, the resultant beads are dried up in a hot air conveyor oven wherein the temperature is maintained at 70 Degree Centigrade. There is a reduction of 30% in the diameter of the beads upon drying. The final densification is achieved in a muffle furnace wherein the dried micro- bubble is fired at a temperature of 1600 Degree Centigrade. The resultant ceramic bubble is of 1.4 mm- 1.5 mm diameter in size and has suitable fracture toughness. The density of the resultant beads is almost 98.5% of the theoretical density.
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Example 2:
90 wt% of Alumina powder having a particle size of less than 0. 5 um is mixed with 10 wt% fused monoclinic zirconia powder having particle size of 5 micrometers. Yttrium oxide at 5.35 wt% of that of zirconia powder is added in the composition. The powders are milled in a tube mill up to D50 of 0.6+/-0.1 um size using water at 35 wt% as solvent. Na- Alginate of 1% concentration is prepared as per Example 1. Na- Alginate solution is mixed at a concentration in a way so that the solid content can be maintained at 65 wt%. The casting and drying operation is done in the same way as per Example 1. The final densification is achieved in a muffle furnace wherein the beads are kept at a temperature of 1550 Degree C. The resultant ceramic bubble is of 1.3 mm- 1.5 mm diameter in size. The density of the resultant beads is almost 99% of the theoretical density and the mechanical properties are also satisfactory.
Example 3:
75 wt% of Alumina powder having a particle size of less than 0. 5 um is mixed with 25 wt% fused monoclinic zirconia powder having particle size of 5 um. Yttrium oxide at 5.35 wt% of that of zirconia powder is then added in the composition. The powders are milled in a tube mill up to D50 of 1.90+/-0.1 um size using water at 35 wt% as solvent. Na- Alginate of 1% concentration is prepared as per Example 1. Na- Alginate solution is mixed at a concentration in a way so that the solid content can be maintained at 60 wt%. The casting and drying operation is done in the same way as per Example 1. The nozzle diameter is changed to 3 mm in this case. The final densification is achieved in a muffle furnace wherein the beads are kept at a temperature of 1650 Degree C. The resultant ceramic bubble is of 2.4- 2.5 mm diameter in size. The density of the resultant beads is almost 98.5% of the theoretical density and the mechanical properties are also satisfactory.
Example 4:
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94.65 wt% of Fused Zirconia powder of 5 micrometers particle size is mixed with 5.35 wt% of yttrium oxide. The slurry preparation, bead preparation and sintering is same as per the previous examples. The final product has a phase distribution of 75 wt% tetragonal and 25 wt% monoclinic as per the XRD. The resultant product is dense media of around 99% of theoretical density.
The process of the present invention is known to increase the sphericity. Sphericity can be increased by passing the droplet through an organic solution such petroleum solvents, glycols and acetates.
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We Claim:
1. Partially Stabilized zirconia toughened ceramic beads comprising by weight
percentage:
3 wt% to 100 wt % by weight ZrO2;
0.1 wt % to 60 wt% by weight Al2O3;
1.0 wt % to 13.75 wt% yttria
and/or 1.0 to 25 wt% of Ceria
and/or 1.0 to 5.0 wt% of MgO
and/or 1.0 to 5.0 wt% of CaO as the zirconia stabilizing agent;
0.1% to 1.0% by weight SiO2;
2. The partially stabilized zirconia toughened ceramic beads as claimed in claim 1, which are zirconia toughened alumina ceramic beads.
3. The alumina beads as claimed in claim 1, which are zirconia toughened alumina beads and comprise of MgO of 0.05 to 1 wt%
4. A process for the manufacture of the partially stabilized zirconia toughened ceramic beads or zirconia toughened alumina beads by alginate route by an in-situ gelling technique wherein a single reaction column is employed.
5. The process as claimed in claim 4, wherein the single reaction column is a liquid column prepared with yttrium precursors.
6. The process as claimed in claim 5, which comprises the steps of:

a) providing ceramic powder consisting of monoclinic zirconia, and yttrium oxide and/or alumina powders;
b) milling of the powders in a tube mill using water as dispersing agent;
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c) mixing the slurry obtained with sodium alginate solution to form self supporting beads;
d) passing the beads through a liquid column prepared with yttrium nitrate precursors;
e) drying and sintering the beads.

7. The process as claimed in claim 6, wherein the ceramic powder consists of monoclinic zirconia prepared by fusion route.
8. The process as claimed in claim 6, wherein the pH of the solution during milling is controlled at 6 to 9.
9. The process as claimed in claim 6, wherein the concentration of the salt solution in the column is kept at 0.5 to 1 wt%.

10. The process as claimed in claim 6, wherein multiple reaction columns are introduced which travel on a conveyer belt.
11. The process as claimed in claim 6, wherein the yttrium nitrate solution or yttrium slat solution has a concentration of 0.5 to 1 wt%.
12. The process as claimed in claim 6, wherein the sodium alginate solution has a concentration of 0.1 to 2%.
13. The process as claimed in claim 6, wherein the fused zirconia powder has a size ranging from 1 micrometer to 15 micrometer.
14. The process as claimed in claim 6, wherein the zirconia containing minimedia is partially stabilized at tetragonal phase by in-situ sintering process.
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15. An apparatus for the manufacture of the partially stabilized zirconia toughened
ceramic beads comprising a single reaction column.
16. The apparatus as claimed in claim 15, wherein the reaction column is a liquid column prepared with yttrium nitrate precursors.
17. The apparatus as claimed in claim 16, which comprises:

a) a chamber for storing the ceramic slurry with alginate solution;
b) a casting tank through which the slurry is dropped into the reaction column;
c) a reaction column, wherein multiple number of reaction columns move on a conveyor; and
d) a hot air dryer.