A METHOD OF MAKING PRECISION SHAPED SINTERED ALUMINA ABRASIVES AND A SHAPING DEVICE FOR MAKING IT
FIELD OF THE INVENTION:
The present invention is related to a process of making precision shaped sintered alumina abrasive. In particular, the invention pertains to a 'near net shape' manufacturing process to shape the sintered alumina abrasive grains which in turn are expected to enhance the performance multifold over conventional abrasives grains. The shaping device used for the purposes of the present invention plays a pivotal role to achieve precision shape of the grain. Therefore, another embodiment of the present invention pertains to the design of the shaping machine. This invention simplifies the design of the shaping machine as well as process of making precision shaped sintered alumina abrasives.
BACKGROUND:
The sintered alumina abrasives/ sol gel alumina abrasives / microcrystalline alumina abrasives were introduced in 1980s and gained popularity in the abrasive industry for their superior properties like self-sharpening, cooler cutting, longer life etc. These salient features help to perform better in high pressure precision grinding and bridge the wide gap between conventional fused abrasives and super abrasives. The regular sintered alumina manufacturing process uses conventional crushing which generates irregular shapes grains and grading to match the standard size distributions i.e., FEPA, ANSI, JIS etc. specification for abrasives. The aspect ratio of the grain is also limited to the range of 1.5 to 2.5 where blocky shape one is preferred in bonded abrasive and sharper/ acicular shape for coated abrasive.
In order to enhance the performance of the sintered alumina abrasives further, the concept of structured grain was introduced in early 1990s. In this process, the precursor material was shaped into a definite configuration and processed subsequently. Here each grain is identical and the shape of the grain varies depending on the manufacturer. It is seen that

the shaped abrasives provide superior performance than the irregular shaped sintered alumina abrasive of same compositions. The definite grain shape provides the necessary cutting edges for efficient grinding.
'Near net shape' manufacturing is one of the key areas of 21st century as it provides the dimension needed by the final product. This eliminates the requirement of additional machining as well as generation of waste or co-generated products thus providing process advantages.
Most of the design of shaped abrasives has at least one sharp corner followed by a wider base and they are intended to be used in coated abrasives. The wider base is generally attached to the backing of the coated abrasives while sharp corners are away from backing. The 'sharp corner with wider base' grain goes wear flat after certain grinding duration and efficiency drops significantly. Moreover, initially grains are in point contact with the work piece which increases progressively as the grain wears out. The grinding force required to maintained the micro fracturing characteristics of sintered alumina abrasives also increases with the grinding time. The unit pressure acting on the grain decreases progressively as the contact area of the grain increases. This becomes major drawback for this 'sharp corner with wider base' grain design. The present invention addressed this draw back as well as simplifies the manufacturing process.
BRIEF DESCRIPTION OF FIGURES:
Figure 1: Schematic diagram of continuous precision shaped grain manufacturing machine.
SUMMARY OF INVENTION:
One aspect of the present invention provides a method for making precision shaped sintered alumina abrasives comprising the steps of: a) providing a precursor of aluminium oxy-hydroxide;

b) dispersing the said precursor in a solvent by mechanical agitation;
c) peptizing the precursor to obtain an alumina gel;
d) drying the alumina gel into powder;
e) forming a dough from the gel powder;
f) processing the dough to obtain precision shaped sintered alumina abrasives.
Another aspect of the present invention pertains to a shaping device designed to produce precision shaped abrasive grain near to its final shape. The shaping device of the present invention enables to produce precision shaped abrasive grain near to its final shape. This eliminates the process of crushing and grading, cutting/chopping to the grain into desire length/dimension etc.
The shaping device of the present invention comprises:
a) a shaping tool (11) in the form of a belt fitted with rollers, driven by a control roller (12);
b) a motor drive attached to the control roller through a gear box;
c) a piston extruder (21) to prepare a thin gel sheet and an extruder die (23) through which the gel comes out as a thin sheet;
d) a pressing roller 17 to press the gel sheet against the shaping tool (11)
e) drying chambers 31 and 32 for drying of the projected precisions shaped grain;
f) a scrapper (33) to remove the dried grains from the shaping tool.
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.
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.
Microcrystalline sintered alumina abrasives are generally manufactured through advanced sol gel processing. Aluminium oxy-hydroxide i.e., boehmite (AlOOH) powder is the prime precursor for this process which is dispersed in suitable solvent and peptize subsequently to form sol. The water, alcohol etc., can be used as solvent whereas mineral acid as well as organic acid such as HN03, HCl, HCOOH etc., can be used as peptizing agent. Hot water of 60 to 90°C is the preferred solvent for this process. The pH of the dispersion can be maintained at 1-4 more preferably 2-3. Dispersion generally takes place by the action of mechanical agitation followed by chemical attack of the acid. Basically boehmite particle are agglomerates of it primary crystallites and the crystallite size is typically in the range of 2 to 50 nm more preferably 10 to 20 run. The mechanical agitation breaks the agglomerates partially which is later de-agglomerated by chemical attack of acid. This process ensures the dispersion of the boehmite crystallites to their primary size. The quality of dispersion has significant effect of the final micro structure as well as performance of the sintered alumina abrasive grain. The conventional high speed stirrer, attrition mill, colloidal mill can be used for the dispersion. The boehmite powder to water ratio can be varied from 1:3 to 1:8 for better dispersion.
Optionally, additives of various metal salts are also added to the dispersion which plays a major role to control the properties of sintered alumina abrasive grains at later stage. The additives are primarily magnesium nitrate, manganese nitrate, cobalt nitrate, zirconium oxy chloride, lanthanum nitrate, neodymium nitrate, cerium nitrate, gadolinium nitrate etc. Single as well as combination of them can be used as additives. Small amount of organic additives also known as drying control chemical additives such as glycerol,

glycol etc., can also be added to the dispersion. The boehmite hydrosol is converted to gel once the mechanical agitation is stopped.
The alumina gel is then dried into powder or flakes form. In case of powder, instantaneous drying such as spray drying, spin-flash drying etc., can be used. The temperature of the drying chamber can be maintained at 180°C to 280°C, more preferably 200 to 250 °C. On the other hand, tray drier, tunnel drier etc., are used to get flakes of dry gel. This gel flakes can be ground further to form powder.
The gel powder is then mix with required amount of binder solution to form a dough. The particle size of the powder can be less than 200 micron more preferably less the 150 micron. The binder solution can be only water, water solution of poly ethylene glycol, water solution of poly vinyl alcohol etc. The binder solution can be used from 30 to 60 %, more preferably 40 to 50% of the total mass. The dough is prepared using suitable mixers such as planetary mixer, spiral mixer, vertical cutter mixer etc. The dough is then processed in precision shaped grain manufacturing machine to prepare the desired output. The design and working principle of the shaping machine is disclosed here in detail. The schematic of continuous precision shaped grain manufacturing machine is shown in figure 1A).
The machine consists of shaping tool 11 in the form of belt fitted with a series of rollers 12-17. The shaping tool is basically a wire mesh/flexible belt which is driven by the control roller 12. A motor drive is attached to the control roller through a gear box. The piston extruder 21 is used to prepare thin gel sheet where the gel dough is fed through the feeding hoper 22 and compressed against the extruder die 23. Extruder die has thin slot through which gel comes out in the form of thin sheet 41. The gel sheet gets pressed against the shaping tool using the pressing roller 17. The gel is forced to pass through the opening of the shaping tool and it remains projected 42 on the other side of the shaping tool. Two drying chambers 31-32 are attached to the machine which can be operated as

single drying zone of uniform temperature or can be set as two different drying zones by setting different temperature to ensure complete drying of the projected precision shaped grain. A scraper 33 is attached to the machine which helps to remove the dried grains 43 form the shaping tool. Cleaning of the shaping tool takes place in cleaning zone 34 which is equipped with mechanical cleaning followed by vacuum cleaning. The cleaning zone ensures the availability of the shaping tool for continuous operation.
The shaping tool is basically a wire mesh/ flexible metal sheet with multiple opening which shapes the gel into a definite form. The geometry of the opening determines the diameter as well as cross sectional geometry of the precision shaped grain i.e., circular, square, star etc. Thin gel sheet gets converted into precision shaped grain when it gets pressed against this multiple opening wire mesh i.e., shaping tool.
Function of the rollers 12-17 is to guide the shaping tool and keeps it in stretched condition. The control roller 12 drives the belt continuously in given direction. Rollers 13-16 are support roller to hold the wire mesh tight. Roller 17 is the pressing roller which basically does the casting job. It presses the gel sheet, which is held between the roller and shaping tool, out through the opening of the shaping tool.
The angle a between the shaping tool and pressing roller is important to ensure the complete casting of the gel sheet. The control roller 12 also serves dual purpose. Firstly, ensure complete casting of gel in case any gel leftover on the shaping tool even after the pressing roller. Secondly, the shaping tool takes convex profile 51-52 where the projected grains get apart which ensures removal of sticking of green projected grain if any. Top view of the shaping tool 61 is shown in figure IB) where the shaping tool is a flexible one and has multiple opening. The opening can be of any geometrical shape 62-67. Precision shaped grain with different geometric cross section is shown in figure 1C) which is basically a cylindrical mass 71-72. The length of the grain is generally multiple times of its diameter.

The dough of the gel powder is filled in a sheet making machine where flat gel sheet is produced. Sheet making machine can be an extruder, rolling mill etc. In case of extruder, piston type as well as screw extruder can be used. The thickness of the sheet also plays important role to control the final dimension of the precision shaped grain. The sheet is directly fed to the shaping tool of the machine and pressed against the shaping tool using a series of rollers. The gel sheet can also be prepared separately and feed to the machine. The shaping tool is like a flexible belt which can be a stainless steel wire mesh, perforated metal sheet etc. The dimension of the grain is determined by the tool as well as the thickness of the gel sheet. The opening of the tool provides the diameter of the grain whereas the sheet thickness determines the length of the grain. Feeding of the gel sheet to the shaping tool should be done uniformly in order to avoid over lapping. Overlapped or wrinkle gel sheet can cause uneven length of the grain. The pressing roller ensure near 100% pass through the shaping tool. The contact angle of the shaping belt with the pressing roller determines the effectiveness and it should be maintained at 30° to 70°.
The aspect ratio of the grain is determined during pressing stage which completely eliminates the requirement of cutting/chopping the grain into the required length. The grain takes shape of its final form i.e., 'near net shape' in the pressing stage itself. The aspect ratio of the grain can be varied from 1:3 to 1:12, more preferably 1:4 to 1:7. The cross section of the grain can be definite geometrical shape such as circle, half circle, triangular, squared, star, hexagon etc., depending on the shaping tool profile. Since the cross section is uniform throughout the length, the contact angle will remain same even after grain wear. The grinding pressure requirement will also remain same. High aspect ratio of the grain can help to maintain a high toughness. In case of bonded abrasive, random distribution of high aspect ratio grain results in low packing density as well as better reinforcement of the wheel. Inter grain space of high aspect ratio grain can help to create desired porosity in the wheel which in turn provide better permeability of the coolant. All these features would help to enhance the grinding efficiency.

The size of the precision shaped grain is derived and standardized based on FEPA guide line. The target diameter of the grain is the opening size of the control mesh (3rd mesh). Therefore opening of the shaping tool can be selected according to get the desired diameter of the final product. The provision of drying as well as sintering shrinkage is also to be taken in to account to select the suitable opening of the shaping tool.
The grains remain projected on the surface of the shaping tool and progress towards the drying chamber which is maintained at 110 to 300°C, more preferably 150 to 250°C. Projected green grain are very prone to stick each other. In order to avoid the sticking of the grains, the shaping tool takes almost 90° turn on control roller prior to entering the drying chamber i.e., the shaping tool takes convex shape and the projected grins gets separated from each other which eliminates sticking. The control roller ensures the complete passing of the gel sheet i.e., any residual mass present in the pressed side of the shaping tool. The grains are directly exposed to the heat source and get dried completely inside the drying chamber. The source of heat can be electrical heating, microwave heating, infrared heating etc. The scraper is fixed at the end of the drying chamber which helps to remove the dried grain from the shaping tool. The shaping tool is cleaned thoroughly by mechanical means such as brush, vibration, ultrasonic, air jet, and vacuum cleaning etc., in the cleaning zone. The shaping tool proposed here is a continuous one which can be cleaned thoroughly prior to reuse.
The dry shaped grain is calcined to remove the chemically bound water and other volatiles. The calcination can be done in a stationary calcined or in a rotary calciner. The calcination temperature can be varied from 450 °C to 850 °C, more preferable 550 °C to 750 °C and the soaking time/ residence time can be from 30 minute to 5 hours. The calcined grain is then sintered at elevated temperature. The sintering can be performed in shaft kiln, tunnel kiln, rotary furnace etc. The sintering temperature can be in the range of

1400 °C to 1650 °C with a soaking time 15 minutes to 2 hours. Rotary furnace would be the preferred choice as it provides rapid sintering which helps to control the properties as well as micro structure of the grain. The mode of heating can be electrical, microwave, fuel fired etc.
During sintering boehmite transforms into alpha alumina and the additives of metal salts get converted into their oxide form. The additives react with alumina and form various secondary phases such as spinel, rare earth hexaaluminate etc. These phases are randomly distributed in the alumina matrix and reinforced to achieve superior properties. The calcined grain shrinks and attains at least 95% of its theoretical density during sintering. Sintering shrinkage can be up to 40 % more preferably up to 25%. The surface of the sintered grain can be rough i.e., matt finished which can enhance the adherence between the grain and bond thus can influence the grinding performance.
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 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:
25 kg boehmite was dispersed to 150 liter of hot water (80 °C) using stator rotor type high speed disperser for 15 minutes. 4 liter of dilute nitric acid (20 % concentration) was added to the dispersion under continuous stirring where acid peptizes the slurry. A water solution of lanthanum nitrate, yttrium nitrate and magnesium nitrate was added to the

dispersion. The quantity of these additives was 800 gram each. 150 ml of poly ethylene glycol and 300 gram of alpha alumina (d50 is ~ 0.5 urn) slurry were added to the dispersion. The dispersion was homogenized for another 25 minutes where it forms a 'sol'. The sol was filled into stainless steel tray where it got converted into 'gel'. The gel was then dried in tunnel tried where the temperature was maintained at 110 °C. The dry gel flakes were crushed in roll crusher and graded to get the -100 mesh powder (<150 urn). This powder was mixed with sufficient amount of water to form dough using planetary mixer. The moisture content of the dough was maintained at 45 to 55 % of the total mass. The dough was then filled into a piston extruder where it was converted into 2 mm continuous gel sheet. The gel sheet was fed to the shaping tool of precision shaped grain processing machine. The shaping tool was basically a wire mesh of desired size opening (450 urn). The gel sheet was pressed against the wire mesh where gel was passed through the mesh opening and projected on the other side of the mesh. The contact angle between the roller and mesh was maintained at 45°. The projected cylindrical shaped gel was moved into the drying chamber to get dried completely. The temperature of the drying chamber was maintained at 180 °C and was heated by IR heaters. The dry precision shaped grain was removed from the mesh belt using a scrapper and the mesh was cleaned thoroughly for next cycle. It was then calcined in a stationary calciner at 650 °C for 2 hours and sintered in a rotary furnace at 1550 °C with a 45 minutes soaking time. The sintered grain surface was matt finished. The properties of the sintered material is measured and reported in table 1.

25 kg boehmite was dispersed to 150 liter of hot water (80 °C) using stator rotor type high speed disperser for 15 minutes. 4 liter of dilute nitric acid (20 % concentration) was added to the dispersion under continuous stirring where acid peptizes the slurry. A water solution of lanthanum nitrate, yttrium nitrate and magnesium nitrate was added to the dispersion. The quantity of these additives was 800 gram each. 150 ml of poly ethylene glycol and 300 gram of alpha alumina (d50 is ~ 0.5 urn) slurry were added to the dispersion. The dispersion was homogenized for another 25 minutes where it forms a 'sol'. The sol is then spray dried to form fine powder. The temperature of the spray drying chamber was maintained at 220 °C. The gel powder was then mixed with sufficient amount of water to form dough using planetary mixer. The moisture content of the dough was maintained at 45 to 55 % of the total mass. The dough was then filled into a piston extruder where it was converted into 2mm continuous gel sheet. The gel sheet was fed to the shaping tool of precision shaped grain processing machine. The shaping tool was basically a wire mesh of desired size opening (450 urn). The gel sheet was pressed against the wire mesh where gel was passed through the mesh opening and projected on the other side of the mesh. The contact angle between the roller and mesh was maintained at 45°. The projected cylindrical shaped gel was moved into the drying chamber to get dried completely. The temperature of the drying chamber was maintained at 180 °C and was heated by IR heaters. The dry precision shaped grain was removed from the mesh belt using a scrapper and the mesh was cleaned thoroughly for next cycle. It was then calcined in a stationary calciner at 650 °C for 2 hours and sintered in a rotary furnace at 1550 °C with a 45 minutes soaking time. The sintered grain surface was matt finished. The properties of the sintered material is measured and reported in table 2

Example 3:
25 kg boehmite was dispersed to 150 liter of hot water (80 °C) using stator rotor type high speed disperser for 15 minutes. 4 liter of dilute nitric acid (20 % concentration) was added to the dispersion under continuous stirring where acid peptizes the slurry. A water solution of lanthanum nitrate, yttrium nitrate and magnesium nitrate was added to the dispersion. The quantity of these additives was 800 gram each. 150 ml of poly ethylene glycol and 300 gram of alpha alumina (d50 is ~ 0.5 urn) slurry were added to the dispersion. The dispersion was homogenized for another 25 minutes where it forms a 'sol'. The sol is then spray dried to form fine powder. The temperature of the spray drying chamber was maintained at 250 °C. The gel powder was then mixed with sufficient amount of water to form dough using planetary mixer. The moisture content of the dough was maintained at 45 to 55 % of the total mass. The dough was then filled into a piston extruder where it was converted into 1 mm continuous gel sheet. The gel sheet was fed to the shaping tool of precision shaped grain processing machine. The shaping tool is basically a wire mesh of desired size opening (450 urn). The gel sheet was pressed against the wire mesh where gel was passed through the mesh opening and projected on the other side of the mesh. The contact angle between the roller and mesh was maintained at 45°. The projected precision shaped gel was moved into the drying chamber to get dried completely. The cross section of the grain is kind of square shape. The temperature of the drying chamber was maintained at 180 °C and was heated by electrical heaters. The dry precision shaped grain was removed from the mesh belt using a scrapper and the mesh was cleaned thoroughly for next cycle. The dry precision shaped grain was then calcined in a stationary calciner at 650 °C for 2 hours and sintered in a rotary furnace at 1550 °C with a 45 minutes soaking time. The sintered grain surface was matt finished. The properties of the sintered material is measured and reported in table 3.
Table 3: properties of the precision shaped grain of example 3

Example 4:
25 kg boehmite was dispersed to 150 liter of hot water (80 °C) using stator rotor type high speed disperser for 15 minutes. 4 liter of dilute nitric acid (20 % concentration) was added to the dispersion under continuous stirring where acid peptizes the slurry. A water solution of lanthanum nitrate, yttrium nitrate and magnesium nitrate was added to the dispersion. The quantity of these additives was 800 gram each. 150 ml of poly ethylene glycol and 300 gram of alpha alumina (d50 is ~ 0.5 urn) slurry were added to the dispersion. The dispersion was homogenized for another 25 minutes where it forms a 'sol'. The sol is then spray dried to form fine powder. The temperature of the spray drying chamber was maintained at 250 °C. The gel powder was then mixed with sufficient amount of water to form dough using planetary mixer. The moisture content of the dough was maintained at 45 to 55 % of the total mass. The dough was then filled into a piston extruder where it was converted into 1 mm continuous gel sheet. The gel sheet was fed to the shaping tool of precision shaped grain processing machine. The shaping tool was basically a wire mesh of desired size opening (320 urn). The gel sheet was pressed against the wire mesh where gel was passed through the mesh opening and projected on the other side of the mesh. The contact angle between the roller and mesh was maintained at 45°. The projected cylindrical shaped gel was moved into the drying chamber to get dried completely. The temperature of the drying chamber was maintained at 180 °C and was heated by electrical heaters. The dry precision shaped grain was removed from the mesh belt using a scrapper and the mesh was cleaned thoroughly for next cycle. The dry precision shaped grain was then calcined in a stationary calciner at 650 °C for 2 hours and sintered in a rotary furnace at 1550 °C with a 45 minutes soaking time. The properties of the sintered material is measured and reported in table 4.

Example 5:
25 kg boehmite was dispersed to 75 liter of hot water (80 °C) using stator rotor type high speed disperser for 15 minutes. 4 liter of dilute nitric acid (20 % concentration) was added to the dispersion under continuous stirring where acid peptizes the slurry. A water solution of lanthanum nitrate, yttrium nitrate and magnesium nitrate was added to the dispersion. The quantity of these additives was 800 gram each. 150 ml of poly ethylene glycol and 300 gram of alpha alumina (d50 is ~ 0.5 urn) slurry were added to the dispersion. The dispersion was homogenized for another 25 minutes where it converted into thick gel. The gel was put under vacuum to remove entrapped air and then filled into a piston extruder where it got converted into 2 mm continuous gel sheet. The gel sheet was fed to the shaping tool of precision shaped grain processing machine. The shaping tool was basically a wire mesh of desired size opening (450 urn). The gel sheet was pressed against the wire mesh where gel was passed through the mesh opening and projected on the other side of the mesh. The contact angle between the roller and mesh was maintained at 45°. The projected cylindrical shaped gel was moved into the drying chamber to get dried completely. The temperature of the drying chamber was maintained at 180 °C and was heated by electrical heaters. The dry precision shaped grain was removed from the mesh belt using a scrapper and the mesh was cleaned thoroughly for next cycle. The dry precision shaped grain was then calcined in a stationary calciner at 650 °C for 2 hours and sintered in a rotary furnace at 1550 °C with a 45 minutes soaking time. The properties of the sintered material is measured and reported in table 5.

We Claim:
1. A method for making precision shaped sintered alumina abrasives comprising the steps
of:
a) providing a precursor of aluminium oxy-hydroxide;
b) dispersing the said precursor in a solvent by mechanical agitation;
c) peptizing the precursor to obtain an alumina gel;
d) drying the alumina gel into powder;
e) forming a gel dough from the gel powder;
f) preparing a thin gel sheet from the gel dough; and
g) processing the sheet to obtain precision shaped sintered alumina abrasives.

2. The method as claimed in claim 1, wherein the precursor is boehmite powder.
3. The method as claimed in claim 1, wherein the solvent for dispersion is selected from water and alcohol.
4. The method as claimed in claim 1, wherein the dispersion of precursor is in hot water.
5. The method as claimed in claim 1, wherein the hot water used for the dispersion can be at a temperature ranging from 60 C to 90 C.

4. The method as claimed in claim 1, wherein mechanical agitation can be generated by high speed stirrer, attrition mill, colloidal mill can be used or combination of them.
5. The method as claimed in claim 1, wherein peptisation can be done by adding mineral acid as well as organic acid and the pH of the dispersion can be maintained at 1-4.
6. The method as claimed in claim 1, wherein the precursor to water ratio can be 1:3 to 1:8.

7. The method as claimed in claim 1, wherein a solution of additives in water is added to the dispersion in an amount of up to 10 wt%.
8. The method as claimed in claim 1, wherein additives can be magnesium nitrate, manganese nitrate, cobalt nitrate, zirconium oxy chloride, lanthanum nitrate, neodymium nitrate, cerium nitrate, gadolinium nitrate etc.
9. The method as claimed in claim 1, wherein the alumina gel can be dried in stationary and fast instantaneous drier to form gel flakes and fine powder respectively.

10. The method as claimed in claim 1, wherein gel flakes can be crushed and graded to obtain desired powder.
11. The method as claimed in claim 1, wherein fast instantaneous drying such as spray drying, spin-flash drying etc., can be used and the drying chamber can be maintained at 180°C to 280°C, more preferably 200 to 250 °C.
12. The method as claimed in claim 1, wherein the dough is prepared by mixing the gel powder with binder solution where the moisture content can be varied from 30 to 60 %, of the total mass.
13. The method as claimed in claim 1, wherein the binder solution can be only water, water solution of poly ethylene glycol, water solution of poly vinyl alcohol etc.
14. The method as claimed in claim 1, wherein the gel dough is converted into thin sheet using sheet making machine such as an extruder, rolling mill etc.
15. The method as claimed in claim 1, wherein the method comprises calcination, and sintering of the precision shaped grain.

16. The method as claimed in claim 15, wherein calcination can be done in stationary or rotary calciner at temperature of 450 °C to 850 °C with a 30 minutes to 5 hours soaking time.
17. The method as claimed in claim 15, wherein the precision shaped calcined grain is sintered in shaft kiln, tunnel kiln, rotary furnace etc.
18. The method as claimed in claim 15, wherein the sintering temperature can be in the range of 1400 °C to 1650 °C with a soaking time 15 minutes to 2 hours.
19. A shaping device for making precision shaped sintered alumina abrasives comprising:

a) a shaping tool (11) in the form of a belt fitted with rollers, driven by a control roller (12);
b) a motor drive attached to the control roller through a gear box;
c) an extruder (21) to prepare a thin gel sheet and an extruder die (23) through which the gel comes out as a thin sheet;
d) a pressing roller 17 to press the gel sheet against the shaping tool (11)
e) drying chambers 31 and 32 for drying of the projected precisions shaped grain;
f) a scrapper (33) to remove the dried grains from the shaping tool.

20. The shaping device as claimed in claim 19, wherein the extruder can be piston type, screw type etc.
21. The shaping device as claimed in claim 19, where the shaping tool is like a flexible belt which can be a stainless steel wire mesh, perforated metal sheet etc.
22. The shaping device as claimed in claim 19, where the contact angle of shaping tool and pressing roller can vary from 30° to 70°.

23. The shaping device as claimed in claim 19, wherein gel sheet can be fed directly as well as separate piece.
24. The shaping device as claimed in claim 19, wherein the shaping tool has multiple opening of definite geometrical shape such as circle, half circle, triangular, squared, star, hexagon etc.,
25. The shaping device as claimed in claim 19, wherein heat source in the drying chamber is selected from electrical heating, microwave heating, infrared heating etc.
26. Precision shaped sintered alumina grain obtained using the method of claim 1.
27. The sintered alumina grain as claimed in claim 1, wherein the surface of the sintered grain is rough i.e., matt finished.
28. The sintered alumina grain as claimed in claim 1, wherein the aspect ratio of the grain ranges from 1:3 to 1:12, more preferably 1:4 to 1:7.