A NOVEL METHOD FOR MAKING MICROCRYSTALLINE ALPHA ALUMINA BASED HIGH PERFORMANCE ABRASIVE GRAINS

FIELD OF THE INVENTION

The present invention is related to a novel process of making microcrystalline alpha alumina based abrasive grain. Particularly, described herein are new types of gel dispersion and drying systems and subsequent processes to produce microcrystalline alumina abrasive with improved product performance. The instant invention simplifies the manufacturing process, reduces the cycle time and improves the process efficiency.

BACKGROUND OF THE INVENTION

The most commonly used alumina based abrasives are fused alumina abrasives like brown fused alumina, white fused alumina, pink fused alumina etc. The well known processes for manufacturing these high alumina abrasive grains are by fusion of alumina or by the fusion of calcined bauxite in presence of charfines / coke and cast iron borings. Here alumina source is heated up to its melting point and followed by subsequent Cooling by water and air. The fused mass is crushed and graded to get the required abrasive grains or particles. Both suffer from a problem that the crystal size of the grains so produced will normally be large, in the order of a couple of hundred microns.

Few decades back, a new variety of alumina based abrasive was introduced in the abrasive market. This variety is well known as 'sol-gel derived alumina abrasive grain' which is manufactured through chemical-ceramic technology where crystals growth are tailored and controlled to get a desired sub-micron crystal in the matrix. The process starts with dispersing a special grade of alumina like boehmite, pseudo-boehmite or similar type of alumina sources in suitable dispersing medium like water, alcohol etc. with sufficient amount of acid to peptize the dispersion. Small percentage of additives and seeds are also added during the dispersion to control the phase, size, and hardness of the final grain. This peptized dispersion is called 'sol' which is allowed it to transform into 'gel'. This gel is dried in a tray to get small flakes of dried gel, and crushed into smaller particles, and these particles are calcined to remove chemically bonded water and volatile matters. Then sintering is done in advanced microwave sintering or conventional fuel burnt or electrically heated rotary furnace below its melting temperature.

The main advantage of these grains over the earlier version of fused grain is in their crystal size. Here it is possible to achieve crystal sizes in the order of 1 micrometer and less. The advantage of such low crystal size is that along with its use in bonded or coated class of abrasive products during the material removal the grit also loses its crystals. The amount of loss in fused alumina grains will be higher in the case of those from sintered product as in the fused the crystal size is larger. In the process of abrasion, a fresh cutting edge is opened up only after the removal of the crystals of abrasive material. As the crystal sizes increases the fresh cutting edge per unit weight of abrasive grain decreases.

When we compare conventional fused alumina against the sol gel derived grains the number of cutting edges opened up for the same weight loss of grains would be far more than that in the case of fused alumina. This is obvious as sol gel has crystal sizes in the order of 1 micron and less where as its counterpart from fused grade has sizes between 100 to 1500 microns. This improves the effective use of the sol gel derived material as an abrasive product. Also since the removal is in minute quantities there is better uniformity in wheel loss, the cutting is cooler and less dressing is required. A variety of them were invented including those processed with seeding & those using microwave sintering.

But these processing steps are very laborious and cycle time for producing this type of abrasive is excessively high. Hence, it is apparent that conventional methods have certain technical drawbacks that prevent implementing them. Therefore, a need was felt to provide novel nanocomposites which will materially alleviate the difficulties associated with the known methods mentioned above. In view of the aforesaid problems, the inventors of the present invention have endeavored to make the process simpler and faster. The new process of manufacturing as explicated in the present invention reduces the cycle time, enhances the productivity phenomenally apart from the product improvements.

OBJECT OF THE INVENTION

In accordance with the purposes of the present invention, as embodied and broadly described herein, a novel process to produce microcrystalline alumina based abrasive grain through chemical-ceramic technology for grinding applications which meets simultaneously the following requirements:

Provides for fast and consistent dispersion reducing the requirements of dispersing
media with effective reduction in particle size.

Efficient and fast instantaneous drying methods that helps freeze ion migration, obtain
a uniform and consistent product with a high production rate and low production cost, Calcination methods that afford continuous operation, maintenance of constant NOx
emission levels and simple process operations.

According to a preferred embodiment of the invention high quality dispersion is achieved by varying the type of disperser, the solid content to water ratio and other parameters such as solid content and pH of the dispersion. Advantageously, dispersion is afforded by means of ultrasonication employing an ultrasonic disperser providing sufficient mechanical energy for efficient boehmite dispersion.

According to another advantageous aspect fast instantaneous drying methods, preferably spray drying are adopted wherein gel droplets get dried instantaneously as it passes through hot air to form fine powders of less than 150 microns.

According to yet another aspect, there is provided a method of agglomeration by adding water or gel prior to extrusion such that the moisture content is atleast about 35%. Performed thereafter is compaction of obtained dry powder at around 100 to 200 bar pressure and subsequently the extruded mass is dried in a belt drier below about 130°C to obtained extruded particles of desired morphology and particle size.

The dried mass is further processed by calcination in the temperature range of 450°C to 850°C and sintered at a temperature effective to transform transitional alumina(s) to alpha alumina. Subsequent to calcination it is preferred to subject the calcined mass to selective
catalytic reduction (SCR) to reduce emission of NOx. The sintering temperature is preferably about 1200˚ C to 1550˚ c.

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 are not restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

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

Figure l.1 : Ultrasonic dispersion of boehmite a) Setup b) Dispersed Boehmite.

Figure 1.2: Ultrasonic dispersion of boehmite a) Normal impeller type mixer b) ultrasonic
disperser.

Figure 2: Particle size distribution after ultrasonic dispersion for different duration.

Figure 3: Cobalt Concentration in bottom layer and top layer of the dry gel.

Figure 4: Micrograph of spray dried alumina gel.

Figure 5: Micrograph of spray dried alumina gel after sintering.

Figure 6: (a) Screw (Auger) Extruder: (b) Dye, (c) extruded mass and (d) extruder screw.

Figure 7: Stereo-zoom microscopic images of dry gel crushed at different moisture level: (a) 3-4% moisture (b) 8-9% moisture (c) 12-13% moisture and (d) 16-17% moisture.

Figure 8: Effect of reconditioning and aging on dry gel in normal atmospheric condition.

Figure 9: Humidification system.

Figure 10: NOx conversion efficiency of new catalyst.

Figure 11: Micrograph of microcrystalline alpha alumina abrasive grain at different
magnification: (a) &. (b) Normal grain; (c) & (d) extruded grain.

Figure 12: New process flow for production of microcrystalline abrasive based on alpha alumina.

DETAILED DESCRIPTION OF THE INVENTION:

In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, 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 belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein.

As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.

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

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.

When the term "about" is used in describing a value or an endpoint of a range, the disclosure should be understood to include both the specific value or end-point referred to.

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.

"Boehmite" refers to alpha alumina monohydrate and boehmite commonly referred to in the art as "pseudo" boehmite (Al2O3.XH2O, wherein x=l to 2).

"Dispersion" refers to a solid-in-liquid two-phase system wherein one phase comprises finely divided particles (in the colloidal size range) distributed throughout the liquid. The process starts with dispersing boehmite (pseudo boehmite) in liquid medium like water, alcohol etc. more preferably in water for easy handling and abundant availability. Preferably 50-90°C hot water is used for this process.

Dispersion can be done in conventional mixers like stator rotor type, high shear mixer, saw tooth type mixed, etc. Quality of dispersion depends on the solid content to water ratio and the type of disperser used. Solid to water ratio for this kind of application could be 1:3 to 1:8. Viscosity of this mixture (gel) is highly dependent on the solid content and pH of the gel. Higher solid content and lower pH result in high viscosity which makes subsequent processing difficult.

Different additives are added to the dispersion which controls the final properties of the grain made out of this process. Generally additives are added as their salt solutions. Metal nitrates like lanthanum nitrates, Neodymium nitrate, cerium nitrates, yttrium nitrates, magnesium nitrates, cobalt nitrates, nickel nitrates, manganese nitrates etc can be used. Any of these additives or combination of them can be used. The preferable amount of additives addition is upto 10%.

The process of dispersing solute (Boehmite) in solvent (water) into a colloidal sate is called peptization. Peptization process involves breaking the larger agglomerate in finer particle especially in colloidal range (10-6-10-9 meter) with the help of mechanical agitation and chemical attack. Mechanical agitation, which is generally done by various type of mixers, causes breaking of bigger agglomerates of particle into finer size for example 150-200 micron particle agglomerates converted to 10-20 micron agglomerates.
Finally the chemical attack splits this particle into finer size i.e., 10-15 nanometer which is in the colloidal range employing a peptizing agent, also referred to as a dispersant, typically utilized to provide the boehmite dispersion.

High speed disperser or ultrasonic disperser provides the sufficient mechanical energy to split the bigger agglomerates into intermediate sizes. Further splitting of agglomerates is not possible by mechanical agitation. To get the further reduction of agglomerates size chemical agents (peptizing agent) are used which breaks intermediate agglomerates into colloidal range fine size particle. The Chemical agent (mainly mineral acid) used for peptization acts as catalyst in complex way. It alters the pH of the system, change the surface charge of these tiny particles, change the rheology of the system etc. The stability of the colloidal solution is greatly influenced by this chemical agent. And the stable colloidal solution is called 'Sol'. Gelation behavior of the sol is also influence by the chelating agent.

This peptizing agent could be done using different types of acids. Suitable peptizing agents include, but are not limited to monoprotic acids like Nitric acid. Hydrochloric acid, acetic acid, formic acid and the like. Nitric acid is a preferred peptizing agent.

Although not wanting to be bound by theory, it is believed that the nitric acid acts as peptizing agent breaking up the agglomerates into colloidal size material and acts as a source of hydrogen counter-ions to form the colloidal sized material.
pH of the dispersion greatly influence the rate of hydrolysis and condensation (polymerization) of the sol gel process. It is found that rapid hydrolysis occurred when pH is altered from neutral (pH) to acidic range. The rate and extent of hydrolysis is also greatly influenced by the strength and concentration of acid catalyst. Gelation time is determined by the rate condensation reaction which is highly dependent on the concentration of [H+] and [OH] species on the dispersed system.

Boehmite dispersion in aqueous medium provides better dispersion in acidic range. The gelation time as well as viscosity can also be controlled by adjusting the pH of the colloidal solution. Gelation time is moderate at medium pH 2-4 which is very much essential for subsequent processing (for spray drying). pH of the dispersion can be adjusted by adding require amount of monoprotic acid which is mentioned earlier.

Ultrasonic disperser provides several advantages over conventional dispersion. In conventional dispersion if air gets entrapped in the matrix of the gel it aids in having pore in the matrix of sintered product. This phenomenon is very common, when the viscosity of the gel is moderately high. In case of ultrasonic dispersion, vortex formation in the liquid is relatively low. Figure la and lb provides a schematic representation of ultrasonic dispersion of boehmite set-up, dispersed boehmite and the normal impeller type mixer and ultrasonic disperser respectively. Figure 2 provides the particle size distribution after ultrasonic dispersion for different durations.

Advantages are as follows:

1. It is very fast and consistent

2. Reduces the requirements of dispersing media in this case water. Material to water ratio can be reduced considerably. For a specific composition this could be lowered to 1:3 against 1: 4 in the conventional mixing. It is also seen that the process is capable of producing a product without air entrapment.

3. It reduces the particle size of the precursor material here boehmite which help the subsequent processing of microcrystalline alumina abrasive. Particle size reduction during the dispersion is effective for certain duration. Increase in duration beyond a certain period does not contribute further to the reduction in particle size.

Drying:

Drying of the gel is generally done in a hot oven, tray drier, spin flash drier, agitated thin film drier and spray drier. Out of these processes, tray drying is most popular and used for the drying of alumina gel. Here gel is filled in trays and heated in hot oven where 60 to 90°C temperature (more preferably 70 to 80°C) is maintained for as high as 90 hours.

Thickness of the gel in the tray plays a critical role. Higher thickness causes longer duration for dying whereas lower thickness is not sufficient for making the required sizes of grits. Preferred thickness of gel in tray could be 35 to 60mm, more preferably 40 to 50mm.

Temperature for drying plays a critical role, higher temperature beyond 100˚ C leads to boiling which causes porosity in the matrix of gel. This meso-porous structure of gel adversely affects density of the final product (sintered).

Mode of drying is also adversely affecting the properties of the dry gel which is also deteriorating the final properties of abrasive grain. Faster drying induces segregation of additives. Various additives (metal nitrates) added during dispersion start migrating along with the moisture. During drying, heat fiow from surface to bottom of the tray and moisture (water) travel from bottom to top of the tray and evaporates from the surface of the gel. During this drying operation metal ion, added during dispersion, starts migrating from bottom layer of the gel to top layer along with moisture. Colour of the dry gel varies across the thickness. If coloring agents like CoO, NiO, Cr203, V2O5 etc. are added to it, then this migration phenomenon is clear on heating the dry gel. Pink part of the dry gel turns dark shade whereas white part remains light shade. This case final product will be inconsistent in colour and a mere mixture of dark and light shade particle.

Drying methods other than tray drying are very fast for example spin flash drying, spray drying drum drying etc. The gel is dried into powder either by spraying as droplets or making a thin film. Here drying is very fast, within few minutes we can convert gel in very fine dry powder which reduce the drying cycle time drastically and increase productivity immensely.

Spray drier is most simple and fast for drying boehmite gel than other methods. Here gel is sprayed in a hot chamber where hot air flows into the chamber either co-currently or counter currently with the spraying direction. Gel droplets get dried instantaneously as it passes through hot air. Instantaneous drying time is typically few seconds (30-60 seconds) and varies with the operating condition of drier especially the flow and temperature of hot air. The inlet temperature of the hot air determines the productivity and drying efficiency. In this type of alumina gel drying, inlet temperature of hot air can be 150˚ C to 250°C more preferably 180 to 220''C and out let or material temperature is to be maintained below 130°C more preferably below 110°C.

If materials (dry powder) temperature reaches beyond 130°C, gel starts loose it chemically bound as a result recyclability of this fine powder in to gel can be lost. And dry powder will turn into bluish green colour in case of CoO colouring agent which indicates the material temperature exceeded 130°C.

Re-gelling test is also a convenient method for testing the reversibility and recyclability of the out powder (dry). This dry powder is added to solvent in same ratio as feed material to spray dried and dissolve it by continuous stirring. This stirring action could be done convention lab stirrer, ultrasonic stirrer, and magnetic stirrer. Then keep the gel in a beaker for 10 minutes to 30 minutes more preferably 15 to 25 minutes and check for residue at the bottom of the beaker. If all particles dissolve and remains in suspended condition then reversibility and recyclability is ensured.

Spray dried powders are very fine, particle size could be varied by controlling several parameters of spray drier like feed rate, hot air inlet etc. Typical size range would be 10 to 100 micron. Particles are spherical in nature and some particles are hollow sphere and porous. Figure 4 provides a micrograph of spray dried alumina gel and Figure 5 provides a micrograph of spray dried alumina gel after sintering.

The main advantage over conventional drying are 1) Freeze the ion migration 2) produce a uniform and consistent product 3) high production rate 4) low in process inventory 5) low processing lead time 6) lower specific energy consumption 7) lesser material handling 8) simplified process etc.

Spray dried powder is not directly suitable for abrasive grain manufacturing application just by sintering because of following reasons:

1. Size of the pray dried powder is too small for abrasive application (10 to 100 micron).

Abrasive application requires 50 to 1300 micron.

2. Particle shape of the spray dried powder is spherical. Abrasive application requires acicular shape with sharp edges.

3. Most of the particles of spray dried powder are hollow and porous.

Therefore, we need additional step to compact this powder to get the desired shape which suits the abrasive application.

Compaction:

Compaction of spray dried powder can be done by Dry roll compaction, tableting, hydraulic pressing, extrusion etc. Strength of compacted mass out of dry roll compaction is very poor and varies from center to periphery. Similar effect found in case if tablet making. Hydraulic pressing and extrusion are very effective and produce more dimensionally accurate compacted mass. But final process required acicular shape with sharp edges grain which means we need to crush the compacted grain again. Extrusion provides advantages over hydraulic pressing in terms of simple and continuous operation.
In an extruder feed material (plastic mass) is get compressed against a die by applying pressure and material takes shape of the die. There are two basic type of extruder; one is piston type and another augur (screw) type.

The piston extruder consists of a piston, a barrel and a die. Feed material for extrusion fed through the hopper and piston press the mass against the die. This is a batch process. Feeding of materials can be done after completing first cycle only.

The augur extruder consists of an auger (screw), a barrel and a die. Feed material charge through feed hoper continuously. Feed material get conveyed and compressed against the die by applying pressure. Extruded mass take the shape of the die and comes out as stream.

Compaction process is also influence the final property of sintered alumina abrasive over conventional process. Compaction of gel increases the green density of the extruded mass. As a result the diffusion distance for sintering is reduced and high density achieved at lower temperature / lower soaking time. Green density is dependent on compaction pressure. The compaction pressure is to be maintained 80 to 250 bar more preferably 100 to 200bar.

Granulation:

Spray dried powder is to be granulated before extrusion. The free moisture of this spray dried powder is 2 to 12%. Higher moisture is required for better compaction and green strength of the extruded mass. The preferable moisture content for this kind of spray dried alumina powder compaction requires 30 to 45%.

To add this additional percentages of moisture to the power, addition mixer is required where required amount of water can be sprayed. As a result small and uniform agglomerates will form and this could be the feed material for extrusion.

Different type of mixers can be used for this application like paddle mixer, intense mixer, plough mixer, granulator and the like.

If moisture content of this powder exceeds 45%, material get stuck to the extruder and also causes flow issue. Laminations are seen on the periphery of the extruded rods. It also create higher shear on the surface. This portion could be less compact. There could be difficulty in getting the desire sizes. Hence the percentage of water is critical and type of extruder is also to be selected.

There is slight reduction of water percentage in extruded material. This loss of moisture is due to the heat generated during extrusion. The moisture content of extruded mass varies between 18 to 25%.

Subsequently drying is required prior to crush and grade the material in desired size range. Extruded mass is passed through a suitable drying system like belt dryer, hot oven etc where temperature is maintained below 130°C more preferably below 110°C. This drier could be heated by electrical heating, radiant heating, microwave heating etc. Since water content is considerably low Microwave and radiant heating are effective. Especially microwave drying provides immense advantages over other type of drying.

Advantages of microwave drying are:

1. Faster drying. Drying time is 5 to 15 minutes more preferably 5 to 10 minutes.

2. Highly efficient. Heat transfer mechanism is reverse of conventional heating. Material absorb microwave and get heated. Therefore heat flow occurs from inside to outside of the mass.

3. Drying is very uniform and effective.

4. Ion migration is much lower than other type of drying.

Some amount of ion migration, as discussed earlier, is also occurred during microwave drying. Since water percentage of extruded mass (20-25%) is much lower than the gel (75-90%).

Crushing and grading:

Extruded mass passes through a suitable communition system like roll crusher, hammer mill, ball mill, pulverizer etc. where they are crushed into small particles. Then this crushed material passes through a set of sieves to get the desire size range. Crushing pattern plays a very important role to suit the final 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 of 1-1.4) are prefer for bonded abrasive.

Shape of the grain is highly influenced by the moisture content of feed material of the crusher i.e. dry extruded mass. Preferable moisture content is up to 6% more preferably 2 to 4%. Fracture characteristics of this dry gel vary with moisture content of dry gel. If moisture percentage is lower than 4%, concoidal fracture takes place which produce the desired sharp edges shown in figure 7 Edges of the grain became straight and sharp. Multilayer and saw tooth type fracture is totally disappeared. Photographs of various graded grit F60 is given below.

On the other hand, moisture content greater than 4% dry extruded gel fracture as blunt and spherical shape which does not suit the abrasive application. Materials crushed at higher moisture content also produce inferior microstructure on sintering.

Figure 7 depicts the stereo-zoom microscopic images of dry gel crushed at different moisture level: (a) 3-4% moisture (b) 8-9% moisture (c) 12-13% moisture and (d) 16-17% moisture. Crushed gel is graded in mechanical graders or vibro-screens. In the vibro-screens the material is agitated vertically as well as horizontal to get classify the material effectively. By adjusting the vibration amplitude horizontally and vertically, desired particle size distribution is obtained.

Reconditioning of gel (Humidification and aging):

It is discussed that the metal ions start migrating along with the drift of moisture flow during drying. This results uneven distribution of additives leads to non-uniform product and deteriorates the properties of grain on sintering. Even prolonged aging (6 to 1 Odays) is not sufficient to redistribution of metal ions in normal atmosphere. To overcome this inconsistency and make the product more uniform reconditioning of graded gel is required. As we have seen earlier, during drying moisture escapes only from the surface of the gel i.e., moisture flows from interior to the surface of the gel irrespective of heating mechanism, shape and thickness. Therefore, moisture acts as a carrier to transport the metal ions from bulk of the material to the surface and causes uneven distribution of additive. Same principle could be applied to redistribute the additives (metal ions) though out the gel matrix.

Graded gel is to be held in a humidified chamber for certain duration. Relative humidity can be maintained between 85 to 98% more preferably 90 to 95%. The duration of aging inside the humidified chamber could vary from the size of the particle. Coarser grits takes longer duration whereas finer grits needs shorter duration to get the homogeneous distribution of metal ion. Homogeneous distribution of metal ions can checked by colour as well as inductively coupled plasma method (ICP) method. If additives (metal ions) responsible for colour is present inside the dry gel, it produce uniform colour on optimum aging.

Various type of humidifier can be used for this application like conventional water atomizer, ultrasonic (water atomization done by piezoelectric transducer) humidifier etc. Piezoelectric type humidifier provides advantage over others. It splits water into very fine droplets which prevents condensation in localized area like comer of the chamber etc.
Here also moisture acts as a carrier. In humidified chamber, moisture content is much higher than the input material (graded gel) and this gradient helps to diffuse or absorb the moisture from atmosphere to the surface of the material. Subsequently, moisture start moving from surface to the interior of the materials (here is also moisture gradient acts as a driving force for this moisture movement). As discussed earlier, metals ions have tendency to move along with moisture, here moisture moves from surface to interior as well as metal ions follow the same path and get homogenized.

Figure-8 represents the effect of reconditioning and aging on dry gel in normal atmospheric condition.

Figure 9 provides a schematic representation of the humidification system.

Calcinations:

Calcination is done in conventional batch furnace or rotary furnace. The main purpose of calcination is to remove chemically bound water and volatile materials like NOx etc. Crushed gel is generally heated to a temperature between 450°C to 850°C and soaking time is provided around 30 minute to 120 minutes. 25 to 35 percentage of volatile is removed during this phase. Major additives are added in dispersion stage as their salt solution preferable in the form of nitrate solution to the mix. These nitrates splits up during heating and form various nitrogen oxide mainly nitrogen dioxide which is harmful to the environment. Therefore scrubbing subsequent to calcination is very important form commercial manufacturing point of view to convert this NOx into environment friendly gases (H2O and N2). Various types of scrubbing method are reported in many journals and patents. The most preferable scrubbing is the selective catalytic scrubbing (SCR) for this particular process. SCR generally converts NOx to N2 and water in presence of a source of ammonia. The source of ammonia can be ammonia gas (NH3), urea {(NH2)2CO} etc.

Commercially available SCR are generally work efficiently at high temperature most likely greater than 350°C, lot of research has been undergone to develop a suitable catalyst which works at low temperature (most preferably below 200°C). This newly developed Titania based SCR works very efficient below 200°C and its NOx conversion efficiency is much better than commercially available SCR.

Rotary calciner provides several advantages over batch type calciner.

1. Continuous operation

2. NOx emission level is constant as feed rate is constant which helps in scrubbing. Constant amount of ammonia is required to scrub the emitted NOx. In case of batch type, NOx emission reaches its peak at 300 to 500''C more preferably 350 to 450°C. Due to variation of NOx emission, ammonia purging to the scrubber is to be varied for efficient conversion into environment friendly H2O and N2 which make process a bit complicated and need continuous attention.

3. Process is very ' simple and consistent as constant feed rate set the other process parameter.

Figure-10 graphically represents the NOx conversion efficiency of new catalyst.
Sintering:

Sintering of the calcined grains can be done by advanced microwave sintering technique or conventional sintering techniques like muffle furnace sintering or rotary furnace sintering. The sintering temperature and residence/soaking time of the abrasive grain are highly depends on the type sintering process. For example muffle furnace or batch type furnace heating temperature will be in between 1300°C to 1550°C and soaking time/ holding time will vary in between 10 minutes to 120 minutes. For rotary furnace, required heating temperature will be in between 1350°C to 1500°C with residence time from 15 minutes to 60 minutes. On the other hand, microwave sintering is very fast and efficient which requires 5 to 30 minutes residence time in the temperature range of 1300°C to 1500°C.

The inlet temperature of rotary microwave furnace or conventional rotary furnace is around 350°C to 450°C, hot zone temperature is around 1350 to 1450°C and discharge point temperature is around 150°C to 300°C.

Sintering is the process of densification. Actually at this elevated temperature transition alumina like (gamma, kappa, theta, chi, delta etc) transform to alpha form which is the most stable phase of alumina.

Transformation of boehmite to other form of alumina occurs with the increment of temperature. The approximate transformation temperatures under microwave sintering are given below Around 450°C, first phase transformation occurs, boehmite or pseudo boehmite converted to γ alumina which is a defect spinel structure. Next it transforms to 5, 9 alumina polymorph at 800°C and 1000°C respectively. Finally at 1200°C alumina transform into alpha alumina which is hexagonal closed pack corundum structure. And alpha form is the most stable polymorph of alumina.

Reaction products of alumina and other metal oxides are formed depending upon the additives. For example MgO, NiO, CoO, MnO etc form spinel, whereas combination of yttrium oxide forms a garnet structure in the matrix. These phases are having particular role in determining final property of the grain. Generally spinel and perovskite 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.

Seeding:

Transformation of transition alumina to alpha alumina is highly influenced by the addition of seed. It provides the heterogeneous nucleation site thus reduces the requirement of activation energy for nucleation. Alpha alumina seeding reduce the transition temperature at least by 100°C. Particle size and distribution of seed has great impact on the final morphology of crystal/grain size of alpha alumina abrasive. Finer and narrow distribution of alpha alumina seed (nano alpha alumina) addition reduces the average crystal size and produces homogeneous microstructure of alumina abrasive.

TABLE-5: Property comparison of normal and extruded gain

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, in whole or in part, 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 will be understood that many variations can be made in the procedures herein described while still remaining within the bounds of the invention. It is the intention of the inventors that such variations are included within the scope of the invention.

EXAMPLE 1:

A dispersion was made by dispersing 500 grams of boehmite in 3.5 liters hot water at 80°C in stator-rotor type dispersion unit. Dispersion was peptized by slowly adding dilute nitric acid and pH was brought down to 2.5. To the above dispersion, 20 grams of magnesium nitrate, 9 grams of lanthanum nitrate, 3 grams of cobalt nitrate followed by 5ml polyethylene glycol were added. Finally 7 gms of alpha alumina seed was added to the mix and allow to another 30 minutes for homogeneous dispersion. Gel was dried in spray drier at 90°C material temperature. Then this powder was granulated in Foreberg mixer and extruded in rods of 5 mm diameter. Subsequently extruded rod is dried and then crushed &. graded into required grit size. Grit F60 is taken for further processing. Humidification of grit 60 is done in humidification chamber for 3 days where relative humidity maintained at 95%. This Graded gel is calcined at 650°C in rotary calciner and followed by sintering in rotary kiln (microwave /conventional) at 1450°C for 40 minutes residence time.

Results:

Product showed an average crystal size of 0.7 micron and also few nano dimensional spinel crystals distributed in the matrix. True density of the sample was 3.9352 g/cc, and micro Vickers hardness of 23.38 GPa.

EXAMPLE 2:

500 grams of boehmite is dispersed in 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 liter, hot water of 80°C using ultrasonic lab dispersion set up. Dispersion was peptized slowly by adding dilute nitric acid and pH of the dispersion was brought down to 2.5. 20 grams of magnesium nitrate and 9grams of lanthanum nitrate 3gram of cobalt nitrate added to the dispersion. 5ml polyethylene glycol and 7gram alpha alumina seed were also added to the mix and allow 30 minutes for homogeneous dispersion. Subsequent processing is as per example 1.

Results:

Product showed an average crystal size 0.5 micron

EXAMPLE 3:

A dispersion was made by dispersing 500 grams of boehmite in 3.5 liters hot water at 80°C in stator-rotor type dispersion unit. Dispersion was peptized by slowly adding dilute nitric acid and pH was brought down to 2.5. To the above dispersion, 20 grams of magnesium nitrate, 9 grams of lanthanum nitrate, 3 grams of cobalt nitrate followed by 5ml polyethylene glycol were added. Finally 7 gms of alpha alumina seed was added to the mix and allow to another 30 minutes for homogeneous dispersion. Gel was dried in spray drier at 90°C material temperature. The powder was then granulated in paddle mixer by adding 30 to 45 percentage of moisture and extruded in rods of 5 mm diameter at different pressure varied from 160 bar to 200 bar. Subsequently extruded rod is dried and then crushed & graded into required grit size. Grit F60 is taken for further processing. Humidification of grit 60 is done in humidification chamber for 3 days where relative humidity maintained at 95%. This Graded gel is calcined at 650°C in rotary calciner and followed by sintering in rotary kiln (microwave /conventional) at 1450°C for 40 minutes residence time.

EXAMPLE 4

A dispersion was made by dispersing 500 grams of boehmite in 3.5 liters hot water at 80°C in stator-rotor type dispersion unit. Dispersion was peptized by slowly adding dilute nitric acid and pH was brought down to 2.5. To the above dispersion, 20 grams of magnesium nitrate, 9 grams of lanthanum nitrate, 3 grams of cobalt nitrate followed by 5ml polyethylene glycol were added. Finally 7 gms of alpha alumina seed was added to the mix and allow to another 30 minutes for homogeneous dispersion. Gel was dried in spray drier at 85°C to 115°C material temperature whereas inlet temperature varied between 150°C to 280°C. Then this powder was granulated in Foreberg mixer and extruded in rods of 5 mm diameter.

Subsequently extruded rod is dried and then crushed & graded into required grit size. Grit F60 is taken for further processing. Humidification of grit 60 is done in humidification chamber for 3 days where relative humidity maintained at 95%. This Graded gel is calcined at 650''C in rotary calciner and followed by sintering in rotary kiln (microwave /conventional) at 1450°C for 40 minutes residence time.

EXAMPLE 5

A dispersion was made by dispersing 500 grams of boehmite in 3.5 liters hot water at 80°C in stator-rotor type dispersion unit. Dispersion was peptized by slowly adding dilute nitric acid and pH was brought down to 2.5. To the above dispersion, 20 grams of magnesium nitrate, 9 grams of lanthanum nitrate, 3 grams of cobalt nitrate followed by 5ml polyethylene glycol were added. Finally 7 gms of alpha alumina seed was added to the mix and allow to another 30 minutes for homogeneous dispersion. Gel was dried in spray drier at 90°C material temperature. Then this powder was granulated in paddle mixer varying moisture percentage from and extruded in rods of 5 mm diameter. Subsequently extruded rod is dried and the moisture content of the dried product varies from 3% to 17%. Different moisture content dried gel was then crushed & graded into required grit size. Grit F60 is taken for further processing. Humidification of grit 60 is done in humidification chamber for 3 days where relative humidity maintained at 95%. This Graded gel is calcined at 650oC in rotary calciner and followed by sintering in rotary kiln (microwave /conventional) at 1450°C for 40 minutes residence time.

Figure-11 represents a micrograph of microcrystalline alpha alumina abrasive grain at different magnification: (a) & (b) Normal grain; (c) & (d) extruded grain.

Figure-12 represents the novel process flow for production of microcrystalline abrasive based on alpha alumina.

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 microcrystalline abrasive based on alpha alumina abrasive grain comprising the steps of

a. mechanically dispersing aluminum mono hydroxide (boehmite) in an aqueous medium to form a Sol;

b. converting the Sol to a Gel by means of a peptizing agent;

c. drying the Gel to form a powder with a particle size in the range of 10 to 100 microns;

d. Compacting the dried powder at a pressure of from 80 to 250 bar to form compacted mass;

e. Increasing the moisture content of the mass in the range of 30 to 45%;

f Granulation of the compacted mass prior to extrusion;

g. Drying the extruded mass at a temperature maintained below 130°C;

h. Subjecting the dried extruded mass with a moisture content of less that 6% to crushing and grading to obtain a graded gel with desired morphology;

i. Reconditioning the graded gel to obtain a uniform crushed graded gel;

j. Calcining the crushed graded gel at a temperature to effect the removal
of chemically bound water;

k. Scrubbing the calcined grains to convert the NOx into environmental friendly gases; 1. Sintering the calcined grains to obtain alpha form of alumina.

2. The process as claimed in claim 1, wherein the boehmite and water are in a ratio of 1:3 to 1:8.

3. The process as claimed in claim 1, wherein the disperser is selected from a conventional disperser, ultrasonic disperser and both.

4. The process as claimed in claim 1, wherein the aqueous medium is selected from water and alcohol.

5. The process as claimed in claim 4, wherein the aqueous medium is water.

6. The process as claimed in claim 1, wherein the dispersion is carried out at a temperature in the range of from 50˚ C to 90°C.

7. The process as claimed in claim 1, wherein additives are added to the dispersion in the form of their salt solutions.

8. The process as claimed in claim 1, wherein the dispersion is subject to peptization by means of mechanical agitation and addition of peptizing agents to produce a stable colloidal solution (Sol).

9. The process as claimed in claim 1, wherein the pH of the dispersion is maintained in a range of from 2 to 4 by the addition of the peptizing agent.

10. The process as claimed in claim 7, wherein the additives are selected from at least one precursor of oxide of magnesium, manganese, nickel, cobalt, lanthanum, cerium, neodymium, yttrium and titanium.

11. The process as claimed in claim 10, wherein the additives are in water soluble form like nitrate, chloride.

12. The process as claimed in claim 7, wherein the additives are added in an amount of up to about 10 wt%.

13. The process as claimed in claim 9, wherein the peptizing agent is a mineral acid.

14. The process as claimed in claim 13, wherein the mineral acid is a monoprotic acid.

15. The process as claimed in claim 14, wherein the monoprotic acid is selected from nitric acid, hydrochloric acid and acetic acid.

16. The process as claimed in claim 9, wherein the pH is maintained between 2.5 to 4 by adding dilute monoprotic acid such as HNO3 to the water.

17. The process as claimed in claim 1, wherein the drying of the gel is by means of spray drying, thin film agitated drying, drum drying or spin flash drying to maintain the moisture content of the dried powder between 5-10 wt%.

18. The process as claimed in claim 1, wherein the increase in the moisture content in step e) is achieved by means of agglomeration.

19. The process as claimed in claim 1, wherein agglomeration is achieved by a mixer such as paddle mixer, intense mixer, plough mixer and granulator.

20. The process as claimed in claim 1, wherein the extruded mass is dried in belt drier which in turn is heated by any one of the heating systems like electrical heating, radiant heating and microwave heating.

21. The process as claimed in claim 1, wherein the moisture content of the extruded mass is from 18-25%.

22. The process as claimed in claim 1, wherein the grading is carried out by mechanical graders or vibro-screens.

23. The process as claimed in claim 22, wherein the material is agitated vertically and horizontally to improve the screening efficiency.

24. The process as claimed in claim 1, wherein the reconditioning of the graded gel is carried out in a humified chamber with the relative humidity maintained between 85 to 98%.

25. The process as claimed in claim 24, wherein the humidifier is selected from conventional water atomizer, ultrasonic humidifier and Piezoelectric type humidifier.

26. The process as claimed in claim 1, wherein the calcinations is carried out in a rotary calciner at a temperature of 450°C to 850 °C.

27. The process as claimed in claim 1, wherein the crushed graded gel after calcinations is optionally subjected to soaking for a period of 30 to 120 minutes to remove 25 to 35% of volatile materials.

28. The process as claimed in claim 1, wherein the sintering of the calcined grains is carried out at a temperature of 1300 to 1500 °C with a residence time of from 5 to 60 minutes.

29. The process as claimed in claim 1, wherein the sintering is carried out by way of microwave sintering technique, muffle furnace sintering or rotary furnace sintering.

30. The process as claimed in claim 1, wherein transformation into alpha alumina occurs at a temperature of 1200°C.

31. The process as claimed in claim 1, wherein the transformation into alpha alumina is influenced by addition of seed which reduces the transformation temperature by at least 100˚ C.

32. A microcrystalline alpha alumina based abrasive grain obtained by the process as claimed in any one of the preceding claims.