A PROCESS FOR PREPARING SINTERED ALUMINA ABRASIVES FROM SODIUM ALUMINATE

FIELD OF THE INVENTION:

This invention relates to abrasive particles and methods of making the same. Particularly, this invention relates to a process of making alpha alumina based sintered abrasive grain. Abrasive grains of this invention show improved properties such as high density, high micro-hardness, and moderate fracture toughness. This invention also discloses the integrated manufacturing process of sintered abrasives.

BACKGROUND OF THE INVENTION:

The present invention discloses the integrated manufacturing process of improved alpha alumina based abrasive starting from sodium aluminate.

Alpha alumina based sintered alumina abrasives have drawn attention of abrasive manufacturer during last decade due to its superior properties over conventional abrasives such as fused alumina abrasives. Microstructure of this grain is suitably tailored to obtain a microcrystalline sintered alumina grain. Controlled fracture of this microcrystalline alumina grain coupled with high hardness and high toughness provide distinct advantages over conventional abrasives.

The density of the commercially available sintered alumina abrasive is around 3.90g/cm3, micro hardness -2100 HV and toughness ~5 MPa.ml/2. With the advancement of grinding technology and requirement of improved efficiency for high pressure and precision grinding, next generation of high performance sintered alumina abrasive grains are a major demand of this decade.

Most of the manufacturing processes start with dispersing boehmite (aluminium oxy hydroxide) in acidified water. Various additives (metal oxide) are added to the dispersion in the form of their salt solution. Then the sol is processed subsequently to make grains. Alpha alumina seeds are added to the sol to obtain higher density of the grain at much lower temperature and also to refine the microstructure. Alpha alumina seed particle provide the heterogeneous nucleation site and lower the activation energy required for nucleation and also prevent the formation of vermiculite structure. As a result, higher sintered density obtained at lower temperature.

The basic requirement of boehmite used for manufacturing 'sol gel derived sintered alumina abrasives' are good acid dispersibility and lower soda content. Various precursor materials have been used to obtain boehmite. The most common precursors are sodium aluminate, aluminium salts, aluminium alkoxide etc. Depend on the method of processing and processing conditions determines the quality of boehmite. Physical properties of the boehmite like surface area, pore size and distribution, crystallite size, size distribution etc. influence on the final properties of sintered abrasives. Generally boehmite slurry is spray dried and characterized physically as well as chemically to meet the above mention requirements.

Even though there are a wide variety of abrasive particles known, including a number of sol-gel-derived abrasive particles, the abrasive industry continues to desire additional abrasive particles which may offer a performance advantage(s) in or more applications. It is also apparent that the processing steps of the conventional methods have certain inherent drawbacks that prevent implementing them. In view of the aforesaid problems, the inventors of the present invention have endeavored to make the process simpler and faster. The process as explicated in the present invention enhances the productivity phenomenally and also improves the product characteristics.

The present invention discloses the integrated process of manufacturing alpha alumina based sintered alumina abrasive grains starting from sodium aluminate. Intermediate stages such as boehmite powder preparation as well as its characterization is eliminated by this integrated manufacturing process. This invention is also provides the next generation alpha alumina based high performance sintered alumina grain with radically improved properties.

SUMMARY OF THE INVENTION:

In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a alpha alumina based sintered abrasive grain that shows improved properties such as high density, high micro-hardness, and moderate fracture toughness.

In accordance to one aspect, the present invention provides a method for making sintered alpha alumina-based abrasive particles the method comprising (a) Preparing sodium aluminate solution by dissolving sodium aluminate in water (b) Precipitating aluminium hydroxycarbonate by purging CO2 and the precipitates are added to demineralised water and stirred to get homogeneous slurry (c) Autoclaving the mixture (d) Preparing a dispersion of boehmite in demineralized water and peptizing the same using monoprotic acid (e) Converting the dispersion to particulate alpha alumina-based abrasive particle precursor material (f) Drying, compacting and grading the mixture (g) Calcining and sintering the graded gel.

The hardness of the sintered alumina produced according to the process of the present invention has micro-hardness of about 2500HV, fracture toughness of greater than 4.1MPa.m1/2 and true density of about 3.97g/cc.

Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior art methods. Other features and advantages of the present invention will become apparent as the following detailed description proceeds or may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and not restrictive of the invention.
DESCRIPTION OF DRAWINGS

The foregoing summary, as well as the following detailed description of preferred embodiments of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

Figure 1: Reactor for aluminium hydroxycarbonate precipitation.

DETAILED DESCRIPTION OF THE INVENTION

The terms used in this specification generally have their ordinary meanings in the art, within the context of the invention, and in the specific context where each term is used. Certain terms that are used to describe the invention are discussed below, or elsewhere in the specification, to provide additional guidance to the practitioner regarding the description of the invention.

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.

As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.

The use of numerical values in the various ranges specified in this application, unless expressly indicated otherwise, are stated as approximations as though the minimum and maximum values within the stated ranges were both preceded by the word "about". Thus, slight variations above and below the stated ranges can be used to achieve substantially the same results as values within the ranges. Moreover, in the disclosure of these ranges, a continuous range is intended, covering every value between the minimum and maximum values, including the minimum and maximum end points of the range.

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 "comprises", "comprising", "includes", "including", "containing", "characterized by", "having" or any other variation thereof, are intended to cover a non-exclusive inclusion.

"Sintered alpha alumina-based abrasive particle" as used herein refers to an alpha abrasive particle that has been sintered to a density of at least 85% by weight AI2O3.

"Boehmite" refers to alpha alumina monohydrate and boehmite commonly referred to in the art as "pseudo" boehmite (i.e., AI2O3.XH2O, wherein x=l to 2).

"Dispersion" or "sol" refers to a solid-in-liquid two phase system wherein one phase "comprises finely divided particles (in the colloidal size range) distributed throughout a liquid.

"Sintering" refers to a process of heating at a temperature below the melting temperature of the material being heated to provide densification and crystalline growth to provide a tough, hard and chemically resistant ceramic material.

The process of the present invention starts with dissolving sodium aluminate in water. Preferably, the solution can be little warmed up for faster dissolution. Slow stirring using suitable stirrer like magnetic stirrer, propeller type etc. may be provided to accelerate the dissolution rate fast. The solid to water ratio for this type of application can be maintained 1:2 to 1:10 (more preferably 1:6 to 1:8).

Sodium aluminate solution is then treated with various precipitating agents like sodium bicarbonate, CO2 gas etc. to obtain a precipitates of aluminium hydroxide or aluminium hydroxyl compounds. CO2 method shows promising results among the above mentioned methods to get high purity product. In case of CO2 purging method, Sodium aluminate solution is filled into reactor made of either stainless steel or glass in order to avoid contamination. Reactor is fitted with gas purging line and a vent for escaping gases after reaction (Figure 1). Carbon di oxide gases is then purge into the sodium aluminate solution to precipitate aluminium hydroxycarbonate and this process continued still the precipitation completed. The purging pressure of CO2 gas can vary for 0.5 bar to 5 bar (more preferably 1-3 bar). 50 to 60% volume of the reactor is filled with sodium aluminate solution in order to provide a space for expanded volume of sodium aluminate solution during gas purging.

Precipitates are collected from the reactor and placed in suitable hot water washing agreement to remove the sodium hydroxide from the precipitates. 70 to 90°C hot water can be used for washing. Washing arrangement can be filter press, vacuum belt filter etc. for repeated washing to remove the sodium hydroxide. The final pH of the aluminium hydroxycarbonate slurry and/or the electrical resistance gives the indication of sodium hydroxide level in the slurry.

Precipitates are added to the demineralized water and stirred to get homogeneous slurry. The ratio of precipitates to water maintain around 3:1 to 1:6 ( more preferably 1:1 to l:3).Little amount of dilute monoprotic acid such as nitric acid, hydrochloric acid, acetic acid etc. is added to the slurry to bring down the pH of slurry in the range of 4 to 7. Little amount of ammonium hydroxide can be added to increase the pH and maintained in between 8 to 9. Then the slurry is heated to 70-90°C for 30 to 90 minutes (more preferably for 50-60 minutes).

Treated slurry is then filled into an autoclave. Autoclave can be made of stainless steel, stainless steel with a teflon lined inside etc. The autoclave is heated up slowly from temperature to working temperature ranging from 140 to 250°C (more preferably 160 to 180°C) with anautogenous pressure of 3 bar to 40 bar. The soaking time can vary from 30 minutes to 12 hours (more preferably 4 to 8 hours). Aluminium hydroxycarbonate is converted in crystalline boehmite/ pseudo boehmite after the autoclaving. Magnetic stirrer is fitted with the autoclave to avoid settling in the bottom of the autoclave. Spiral cooling tube is attached to cool down autoclave after soaking. Online sample collection provision is also attached in order to take intermediate samples and confirm the required properties of boehmite.
Boehmite slurry is then place into a high speed disperser and required quantity of hot water added to the disperser. The ratio of boehmite slurry and water is maintained around 1:4 to 1:8. The temperature of this dispersion system is controlled between 70°C to 90°C. After achieving homogeneous dispersion, required quantity of monoprotic acid is added to peptize the slurry. Peptization is the process of breaking agglomerate particles to finer particle especially in the colloidal range (10"6 to 10"9m) with the help of mechanical agitation and chemical attack. Suitable peptizing agents are generally soluble ionic compounds which are believed to cause the surface of a particle or colloid to be uniformly charged in a liquid medium, for instance water. Examples of such acids include monoprotic acids and acid compounds such as nitric acid, acetic acid, hydrochloric acid with nitric acid being the most preferred. High speed disperser generates the sufficient mechanical force to split the particle into finer size and further finer particles are generated by chemical attack of monoprotic acid. The pH of the system is maintained around 2 to 4. Dispersion can be done in conventional mixers like stator rotor type, high shear mixer, saw tooth type mixed, etc.or in advanced ultrasonic dispersers.

The stable colloidal solution is called 'sol' and then this sol gets converted into 'gel' by forming three dimension networks of solute and solvent with in it.

Small amount of additives are added to the sol during dispersion. They generally control the microstructure and help to obtain desired properties. Additives are various metal oxides like MgO, MnC>2, Y203, La203, Nd2C>3, Ce203, Zr02 etc. They are added to the dispersion upto 10 wt.% in the form of their salt solution. The additives can be added individually or a combination of them.

Seeds are also added to the dispersion with helps to obtain alpha phase of alumina in low temperature in the range of 1150°C to 1200°C. Seed can be added upto 10 wt. % (more preferable 2 to 6 wt.%). Seed particle are iso structural with the final product phase, thus provide low energy sites for heterogeneous nucleation. The preferable concentration of seed particle reported in literature is around 5xl013 seeds/cm3 to get an optimum heterogeneous nucleation effects. The most popular seed for nucleation alpha alumina is ferric hydroxide, alpha alumina etc. they can be added individually or a combination of them.

Gel can be dried in different types of driers like hot oven, tray drier, agitated thin film drier, rotary drum drier, spray drier etc. Most commonly used drier for gel drying is tray drier, where drying temperature could be maintained in between 60 to 90°C and the drying cycle is very high. In case of spray drying, drying is very fast and instantaneous. Spray drier could be a co-current or counter current type. Inlet and outlet temperature of the spray drier determines the product (dry gel) quality as well as drying efficiency. Inlet temperature could be maintained in the range of 220 to 280°C (more preferably 240 to 260°C) and the out let temperature in the range of 80 to 140°C (more preferably 100 to 120°C).

Compaction of dry gel can be done in roll compactor, tableting press, hydraulic press, extruder etc. strength of the compacted mass for roll compactor and tableting press is very weak and also non uniform. Hydraulic press and extruder provide dimensional accurate and uniformly compacted mass. Extruder provides distinct advantage over hydraulic press due to its continuous operation. Extruder can be piston type or augur type. In case of augur extrusion axial pressure is to be maintained at 180 to 200 bar.

Compaction process is also greatly influence the final properties of sintered grain. Compaction increased the green density of the extruded i.e., reduces the diffusion distance of mass transfer during sintering. As a results higher density of the sintered product can be obtain at lower temperature and/or lower soaking time.

Extruded mass is then dried at a temperature ranging from 80 to 120°C. Tray drier, belt drier etc. can be used for this application. The mode of heating of these drier could be electrical, microwave or infrared. Infrared as well as microwave drying proves faster and efficient drying than the tray drying. The final moisture content of the dried mass could be in the range of 2 to 6%.

This dry extruded gel is passed through a suitable comminution system like roll crusher, hammer mill, ball mill, pulverizer etc., where they break into small pieces. Crushed gel is then passed through a set of sieves to classify them into desired size range. Shape of the grain is very important for abrasive applications. For example acicular grains with sharp edge (preferred aspect ratio in the range of 1.6-2.0) are preferred for coated abrasive whereas blocky with sharp edges (Preferred aspect ratio in the range ofl-1.4) are prefer for bonded abrasive.

Desired aspect ratio of the grain can be achieved by suitable selection of communition system. For example, ball milling will generate more of blocky particle whereas roll crusher will produce sharper grain. Crushed and graded gel is calcined in batch furnace or in rotary furnace where. Calcination temperature is maintained around 450 to 750°C for period of 30 to 150 minutes. The objective of calcination is to remove volatile materials especially chemically bound water, NOx (in the case of metal nitrates added as additives). NOx can be reduced to H2O and N2 using NH3 in presence of catalyst. The source of NH3 could be ammonia gas, urea etc. total volatile content can vary from 20 to 35 wt%.

Calcined gel is then sintered in high temperature batch furnace as well as rotary furnace. The temperature for sintering is to be maintained round 1300°C to 1600°C (more preferable 1400°C to 1550°C) with as soaking/residence time 30 to 150 minutes (more preferably 60 to 120 minutes). The source of heating fuel fired, electrically heated and microwave heated. Rotary furnace provides distinct advantages over batch furnace being a continuous system and produce uniform products.

Sintering process is very crucial and the degree of sintering determines the grain properties. By altering sintering parameters (temperature/residence time) microstructure as well as properties of the grain can be tailored. The microstructure of sintered grain will contains well grown submicron crystals of alpha alumina and homogeneously distributed reaction products of alumina and additives. For example, the reaction product of MgO, NiO, CoO, MnC»2 etc. and alumina form spinel; yttrium oxide and alumina form garnets; La2C>3, Nd2(>}, Ce203 etc. and alumina produces magnetoplumbite structure (rare earth hexaaluminate). Each reaction products are having specific role on the properties of the grain. For example, spinel structure compounds are imparting toughness to the final grain on the other hand garnet structure provides the desired hardness to the grain. These special phases are uniformly distributed in alpha alumina matrix and provide the matrix advantage system.

The abrasive particles according to the present invention are useful, for example, in loose form or used incorporated into abrasive particles. Abrasive particles according to the present invention comprise binder and a plurality of abrasive particles, wherein at least a portion or all of the abrasive particles are the abrasive particles according to the present invention. Exemplary abrasive products include coated abrasive articles, bonded abrasive articles (eg., wheels), non-woven abrasive articles and the like.

Another embodiment of the invention pertains to a method of abrading a surface, said method comprising

Providing an abrasive article comprising a binder and a plurality of abrasive particles, wherein at least a portion of the abrasive particles are in accordance with the present invention; Contacting at least one of the abrasive particles in accordance with the present invention with a surface of a workpiece;

Abrading at least a portion of the contacted surface of the workpiece with the contacted abrasive particle according to the present invention.

Examples:

Example 1:
1 kg of sodium aluminate powder was dissolved in 6 kg demineralized water in room temperature. Slow stirring was provided to make a homogeneous solution. This sodium aluminate solution is the filled into reactor and CO2 gas purge in to the rector at 2 bar pressure. Solution was kept under stirring and allowed for complete precipitation of aluminium hydroxycarbonate. The precipitate was washed using 80°C hot water in continuous vacuum belt filter for 6 times to remove the sodium hydroxide from the precipitates. The final pH of the precipitates after washing was 7.5. Washed precipitates ware mixed with demineralized water (concentration 30 wt.%) and dilute HNO3 acid added to set the pH at 4.5. This slurry was then kept in hot plate (80°C) for 1 hour.

The treated slurry is then poured into a stainless steel autoclave and heated to 180°C for 6 hours. The slurry kept under stirring (25 rpm) for homogenous transformation. Crystalline boehmite slurry was collected and diluted by adding demineralized water(concentration 15wt.%). 1 litre of boehmite slurry in then placed into a high speed disperser and peptized using dilute nitric acid. lOg of magnesium nitrate and 5g of lanthanum nitrate were dissolved in demineralized water and added to the dispersion. Dispersion was continued for 30 minutes and the temperature of the dispersion was maintained at 85°C. During this process slurry converted into 'sol' and this sol is spray dried. Dried materials is then extruder into small cylinder by adding 20wt.% moisture. Extruded mass was then crushed into small pieces and graded to get grit 36. Graded gel was calcined and sintered at 650°C for lh and 1400°C for 30 minutes.

Properties of sintered grain: True density 3.97g/cc, Micro-hardness 2573 HV and Fracture toughness 4.1 MPa.m1/2.

Example 2:

1 kg of sodium aluminate powder was dissolved in 6 kg demineralized water in room temperature. Slow stirring was provided to make a homogeneous solution. This sodium aluminate solution is the filled into reactor and CO2 gas purge in to the rector at 2 bar pressure. Solution was kept under stirring and allowed for complete precipitation of aluminium hydroxycarbonate. The precipitate was washed using 80°C hot water in continuous vacuum belt filter for 6 times to remove the sodium hydroxide from the precipitates. The final pH of the precipitates after washing was 7.2. Washed precipitates ware mixed with demineralized water (concentration 30 wt.%) and dilute HNO3 acid added to set the pH at 5.2. This slurry was then kept in hot plate (80°C) for 1 hour.

The treated slurry is then poured into a stainless steel autoclave and heated to 200°C for 4 hours. The slurry kept under stirring (25 rpm) for homogenous transformation. Crystalline boehmite slurry was collected and diluted by adding demineralized water (concentration 15wt.%). 1 litre of boehmite slurry in then placed into a high speed disperser and peptized using dilute nitric acid. lOg of magnesium nitrate and 5g of lanthanum nitrate were dissolved in demineralized water and added to the dispersion. Dispersion was continued for 30 minutes and the temperature of the dispersion was maintained at 85°C. During this process slurry converted into 'sol' and this sol is spray dried. Dried materials is then extruder into small cylinder by adding 20wt.% moisture. Extruded mass was then crushed into small pieces and graded to get grit 36. Graded gel was calcined and sintered at 650°C for lh and 1400°C for 30 minutes.

Properties of sintered grain: True density 3.95g/cc, Micro-hardness 2512 HV and Fracture toughness 4.5 MPa.m1/2.

Example 3:

1 kg of sodium aluminate powder was dissolved in 6 kg demineralized water in room temperature. Slow stirring was provided to make a homogeneous solution. This sodium aluminate solution is the filled into reactor and CO2 gas purge in to the rector at 2 bar pressure. Solution was kept under stirring and allowed for complete precipitation of aluminium hydroxycarbonate. The precipitate was washed using 80°C hot water in continuous vacuum belt filter for 6 times to remove the sodium hydroxide from the precipitates. The final pH of the precipitates after washing was 7.0. Washed precipitates ware mixed with demineralized water (concentration 30 wt.%) and dilute NH4OH added to set the pH at 8.0. This slurry was then kept in hot plate (80°C) for 1 hour.

The treated slurry is then poured into a stainless steel autoclave and heated to 180°C for 4 hours. The slurry kept under stirring (25 rpm) for homogenous transformation. Crystalline boehmite slurry was collected and diluted by adding demineralized water (concentration 15wt.%). 1 litre of boehmite slurry in then placed into a high speed disperser and peptized using dilute nitric acid. lOg of magnesium nitrate and 5g of lanthanum nitrate were dissolved in demineralized water and added to the dispersion. Dispersion was continued for 30 minutes and the temperature of the dispersion was maintained at 85°C. During this process slurry converted into 'sol' and this sol is spray dried. Dried materials is then extruder into small cylinder by adding 20wt.% moisture. Extruded mass was then crushed into small pieces and graded to get grit 36. Graded gel was calcined and sintered at 650°C for lh and 1400°C for 30 minutes.

Properties of sintered grain: True density 3.96g/cc, Micro-hardness 2537 HV and Fracture toughness 4.21 MPa.m1/2.

Example 4:

1 kg of sodium aluminate powder was dissolved in 6 kg demineralized water in room temperature. Slow stirring was provided to make a homogeneous solution. This sodium aluminate solution is the filled into reactor and CO2 gas purge in to the rector at 2 bar pressure. Solution was kept under stirring and allowed for complete precipitation of aluminium hydroxycarbonate. The precipitate was washed using 80°C hot water in continuous vacuum belt filter for 6 times to remove the sodium hydroxide from the precipitates. The final pH of the precipitates after washing was 7.5. Washed precipitates ware mixed with demineralized water (concentration 30 wt.%) and dilute HNO3 acid added to set the pH at 4.5. This slurry was then kept in hot plate (80°C) for 1 hour.

The treated slurry is then poured into a stainless steel autoclave and heated to 170°C for 4 hours. The slurry kept under stirring (25 rpm) for homogenous transformation. Crystalline boehmite slurry was collected and diluted by adding demineralized water (concentration 15 wt.%). 1 litre of boehmite slurry in then placed into a high speed disperser and peptized using dilute nitric acid. 15g of magnesium nitrate and 7.5g of lanthanum nitrate were dissolved in demineralized water and added to the dispersion. Dispersion was continued for 30 minutes and the temperature of the dispersion was maintained at 85°C. During this process slurry converted into 'sol' and this sol is spray dried. Dried materials is then extruder into small cylinder by adding 20wt.% moisture. Extruded mass was then crushed into small pieces and graded to get grit 36. Graded gel was calcined and sintered at 650°C for lh and 1400°C for 30 minutes.

Properties of sintered grain: True density 3.96 g/cc, Micro-hardness 2555 HV and Fracture toughness 4.3 MPa.m .

A person skilled in the art will be able to practice the present invention in view of the description presented in this document, which is to be taken as a whole. Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, one of skill in the art will appreciate that certain changes and modifications may be practiced within the scope of the appended claims. While the invention has been disclosed in its preferred form, the specific embodiments and examples thereof as disclosed and illustrated herein are not to be considered in a limiting sense. It should be readily apparent to those skilled in the art in view of the present description that the invention can be modified in numerous ways. The inventor regards the subject matter of the invention to include all combinations and sub-combinations of the various elements, features, functions and/or properties disclosed herein.

WE CLAIM:

1. A process for manufacturing sintered alumina abrasives from sodium alginate comprising the steps of:

(a) Preparing sodium aluminate solution by dissolving sodium aluminate in water wherein the sodium aluminate and water ratio is in the range of 1:2 to 1:10;

(b) Precipitating aluminium hydroxycarbonate by purging C02 at 0.5 to 5 bar pressure;

(c) Washing the precipitate with water of temperature of at least 70°C in a vacuum belt for atleast 4 times;

(d) Autoclaving the slurry at temperature of 140°C to 200°C for 30 minutes to 12 hours

(e) Preparing the boehmite slurry by dispersing boehmite in demineralised water;

(f) Peptizing the boehmite slurry in the presence of a peptizing agent and optional additives;

(g) Converting the dispersion to particulate alpha alumina-based abrasive particle precursor material;

(h) Drying, compacting and grading the mixture;

(i) Calcining and sintering the graded gel to form sintered alumina abrasives.

2. The process as claimed in claim 1, wherein the ratio of washed precipitates and water is maintained between 3:1 to 1:5 to obtain the slurry.

3. The process as claimed in claim 1, wherein the pH of the treated slurry of aluminium hydroxycarbonate step (c) is maintained between 4 to 6 in respect of an acid treatment or 8 in respect of ammonium hydroxide treatment.

4. The process as claimed in claim 1, wherein the autoclaving is done at an autogenous pressure of 3 bar to 40 bar.

5. The process as claimed in claim 1, wherein the boehmite and water ratio is maintained at 1:4 to 1:8 in step (e) to make the boehmite slurry and the system is heated upto 70°C to 90°C.

6. The process as claimed in claim 1, wherein the dispersion of step (e) is done in either a conventional disperser, ultrasonic disperser or a combination of both.

7. The process as claimed in claim 1, wherein the peptizing agent is a monoprotic acid.

8. The process as claimed in claim 1, wherein the peptization of step (f) is undertaken at a pH of 2 to 4.

9. The process as claimed in claim 1, wherein the additives are selected from the group comprising MgO, MnC>2, Y2O3, La2C>3, Nd203, Ce2C>3, ZrC>2.

10. The process as claimed in claim 9, wherein the metal oxide is added to the dispersion in the form of their salt solution.

11. The process as claimed in claim 9, wherein the amount of additives added are upto 10 wt%.

12. The process as claimed in claim 1, wherein the seed particles are added.

13. The process as claimed in claim 12, wherein the seed is added upto 10 wt%.
14. The process as claimed in claim 1, wherein the drying of the gel is undertaken in a spray drier where the inlet and outlet temperature are maintained at about 220 to 280°C and 80 to 120°C.

15. The process as claimed in claim 1, wherein the compaction is done in piston as well as augur extruder at pressure of 180 to 200 bar.

16. The process as claimed in claim 1, wherein the calcination is performed in a batch furnace as well as rotary furnace where the temperature is maintained around 450°C to 750°C with soaking time of 30 minutes to 150 minutes.

17. The process as claimed in claim 1, wherein the sintering is performed in a batch furnace as well as rotary furnace at a temperature of about 1300°C to 1600°C with a soaking/residence time of 30 to 150 minutes.

18. The process as claimed in claim 17, wherein the mode of heating is electrical heating, microwave heating or a combination of both.

19. An abrasive grain obtained from the process as claimed in the preceding claims, wherein the said abrasive grain has a density greater than 3.95g/cc, microhardness greater than 2500 HV and a moderate toughness of about 4 Mpa.m1/2.