Title of Invention	AN IMPROVED PROCESS FOR THE PREPARATION OF ALUMINA ABRASIVE GRAINS
Abstract	(57) Abstract: The invention relates to an improved process for the preparation of alumina grains with 98% theoretical density high hardness and low crystal sizes in titer range of 0.5 microns through microwave sintering. These grains have high abrasion properties and could be used for the manufacture of both bonded and Batted abrasive products.

Full Text

This invention relates to an improved process for the preparation of alumina abrasive grains. The abrasive prepared by the process is used for both coated and bonded alumina abrasives The conventional process for the production of alumina abrasive grains is through fusion using arc furnace. The calcined alumina is melted at temperatures above 2000 deg C or calcined bauxite is fused at temperatures above 2000 deg C in presence of carbon to reduce its impurities such as titania, iron oxide, silica, etc. In both the above cases the molten material is cooled, crushed, magnetite and graded to obtain the various grit sizes required for abrasive grain preparation. Here as this process requires melting of the raw material, the process becomes highly energy intensive and also the crystal size of the resulting abrasive grain would be more than 150 microns in size. Subsequently another process was developed through sol gel route yielding abrasive grains with small crystal sizes of about 0.5 microns consuming low energy. In this route the major steps so far employed include dispersing the hoehmite in water to form alumina in sol state, addition of seeds like alpha alumina either by milling using alumina grinding media or direct addition of microns alpha alumina particles. To this is added additives such as magnesia, titania, yttria, etc. and dried, crushed, calcined and sintered using conventional sintering furnace. The conventional sintering furnace could be an electric or oil or gas fired batch kiln /a rotary kiln. The heating rate would be about 20 deg / Minute as the system adopts external heating of the material The main objective of the posit invention is to provide an improved process for the preparation of alumina abrasive grains employing microwave processing. Another objective of the present invention is to provide an improved process for the preparation of alumina abrasive grains having low crystal sizes, high hardness and toughness and density overcoming the drawbacks of the hitherto known processes. Yet another objective of the present invention is to produce an improved process for the preparation of alumina abrasive grains having Micro Vickers hardness above 2100 kg/ sq. mm, B8% thooretical density with crystalline uniformity and average crystal size less than 0.5 microns. The invention is based on our finding that it is possible to achieve very low crystal sizes with high hardness , toughness and density using the combination of sol gel and microwave processing. It is observed during the course of our continued research that as temperature increases above a point, the dielectric loss begins tx» increase rapidly and the material begins to absorb microwave more efficiently. This also raises the temperature and causes the dielectric loss to rise even faster. The net result is an exponential increase in temperature. Hence we get high heating rates as high as 300 deg / minute. More over as microwave is employed, the material is heated internally with in the material instead of external heating as in the other cases. Both batch and continuous processing can be employed. The raw material used in the process of the present invention is bohemia dispersed in water. The concentration may range from 15 to 30 weight percentage. The pH may be adjusted between 2 to 4 by addition of an acid such as nitric acid. The additives such as iron, silicon, titanium, magnesium, yttrium, neodymium, lanthanum etc. maybe added as their hydroxides, oxides or nitrates. The amount of additives may vary from 0.1 to 7 weight percentage when compared as their oxides. Seeding was done using sub-micron alpha alumina seeds preferably in an amount of 0.1 to 2.0% by weight. Drying control chemical additive such as polyethylene glycol may be added in the range of 0.1 to 2.0% to aid in pore free drying. The gel prepared was dried at low temperatures preferably in the range of 40 to 80 deg C. The dried gel is crushed to the required size giving allowance for the shrinkage during sintering. The shape of the grains were modified to suit the end use by selection of a suitable crushing system. The graded gel was calcined In the temperature range of 300 to 900 deg C. This may be done preferably for a period 30 minutes to drive away the volatiles. The calcined grains are then sintered using microwave at temperatures upto 1500 deg C. This comprises exposing the grains to microwave by placing them inside the microwave oven and taking them to temperatures between 1300 and 1500 deg C for a residence time varying between 0 to 60 minutes. Accordingly the present invention provides an improved process for the preparation of alumina abrasives using a combination of sol gel and microwave processes which comprises i) dispersing boehmite ( mono hydroxy aluminium oxide, AlOOH) in water at a temperature in the range of 30 to BO deg C, U) adjusting the pH of the dispersed system in the range of 2 to 4, Hi) adding the additives to the system to improve hardness, toughness, sintering temperature, microstructure, etc., iv) seeding the resulting system using micronised alpha alumina, v) drying at a temperature in the range of 40 to 80 deg C, vi) crushing the dried gel to the desired size and shape, vii) claiming the crashed gel in temperature range of 300 to 900 deg C for a period ranging form 30 to 120 minutes, viii) sintering the calcined grains by using microwave at temperature below 1500 deg C for a period ranging from 0 to SO minutes. The microwave oven employed may be equipped with a power control and a timer and produced microwave energy of 2.45 GHz frequency and power out put of 900 W. The calcined grain is placed in the closed insulating chamber, called the microwave packet The insulating material is aluminium silicate based material. An inner chamber of porous Zirconia cylinder is also used, the system ensures heat preservation and high temperature generation. The sheathed thermocouple is introduced for temperature measurement, and placed just near the sample. As a precaution the body of the oven is provided with water cooled tubes. This microwave oven used for batch processing of alumina abrasive grains. For a continuous set up the material is filled in a sintered alumina / or fused quartz tube in the microwave field. The material for sintering is continuously fed from top and sintered alumina grains are drained at the bottom of the tube. The invention is described in detail in the examples given below which are provided by way of illustration only and should not be construed to limit the scope of this present invention. Example 1 400g of boehmite was dispersed in 2.4 liters of hot water at 80 deg C , pH brought down to 2.5 . To this 3.46 gms of ferric oxide was added in the form of its hydroxide followed by addition of 0.62 gms of titania as its hydroxide , 0.78 gms of precipitated silica and 0.51 gms of calico. 2.0 gms Poly ethylene glycol was added. Seeding was done using 4.5 gms of submicron alpha alumina seeds. This was followed by the addition of 30.8 gms of magnesium nitrate as a solution and the dispersion was continued for an additional 20 minutes more. This was dried at 80 deg C for 24 hours and crushed, graded and calcined at 600 deg C for 30 minutes. Calcined gel was then sintered in microwave oven at a temperature of 1400 degree C for 45 minutes. The product showed an average crystal size of 0.5 microns, specific gravity of 3.91 g/cc, micro Vickers hardness of 2205 kg/sq mm. with an abrasion index of 85. Example 2 400 gms of boehmite was dispersed in 3 liters of water, pH brought to 2 followed by the addition of 1.5 gms of titania and seeded using 3.0 gms of submicronised alpha alumina seeds followed by addition of 46.2 gms of magnesium nitrate as a solution in water. After dispersing again for half an hour more the gel was dried at 70 deg C for 30 hours. The dried gel was crushed, graded and calcined at 500 deg C for an hour. These grains were then sintered in microwave at a temperature of 1450 deg C for 15 minutes. The product yielded showed an average crystal size of 0.6 microns, specific gravity 3.84, micro Vickers hardness of 1883 kg/sq mm and abrasion index 80. example 3. 400 g of boehmite was dispersed in 2 liters and brought down to 3. Both yttria and lanthanum oxide were taken in equal quantities of 2.2S gms and dissolved in dilute nitric acid and added to the system. 2 gms of polyethylene glycol was added followed by 3 gms of submicron sized alpha alumina seeds. The gel was then dried at 80 deg C, crushed and graded. The graded material was then calcined at 600 deg C for 30 minutes followed by microwave sintering at 1450 deg C for 30 minutes. The abrasive grains had an average crystal she of 0.5, sperm gravity 3.84, micro dickers hardness of 1863.25 kiss mm and an abrasion index of 95. Example 4 400 gms of boehmite was dispersed hi 2 liters of water at 80 deg C, the pH was brought down to 3 by the addition of nitric acid and while being dispersed the addithfos 2.25 gms of yttria, 2.25gms of lanthanum oxide, 2.25 gms of neodymium oxide were added. 2 gms of Polyethylene glycol was added followed by alpha alumina seeds of 3 gms. The gel was then dried at 70 degree C. The dried gel was crushed, graded and calcined at 600 deg C for an hour. Tate calcined gel was then sintered at 1400 deg C torn 46 minutes. The sintered grains where of average crystal size 0.4 microns, specific gravity 3.85 g/cc, Micro VIckers Hardness 1897.3 kg/sq mm and abrasion index of 95. Example 5 400 gms of boehmite was dispersed In 1.8 liters of water and the pH was brought down to 2.5. To the above so! was added 2.25 gms of yttria, 2.25 gms of lanthanum oxide, 2.25 gms of neodymium oxide, 1.15 gms of cobalt nitrate and 3 gms of aluminium nitrate followed by 1.5 gms of polyethylene glycol. The dispersion was continued and 4.5 gms of alpha alumina seeds were added and the dispersion was stopped after another 20 minutes. This was then dried at a temperature of 80 degree C for 20 hours and the dried gel was crushed, graded and calcined at 500 degrees for 1 hour. This was then microwave sintered at 1450 deg C for 30 minutes. These showed an average Causal size of 0.5 micron, specific gravity 3.94, Micro Vickers Hardness 2387.66 kg/sq mm and abrasion index of 100. Example 6 100 gms of boehmite was dispersed in 1.8 liters of water pH was brought iown to 2.5 by adding nitric acid. 4.5 gms of yttria, 2.25 gms of lanthanum ixide and 1.15 gms of cobalt nitrate was then added followed by 3 gms of }lpha alumina seeds and 1.5 gms of polyethylene glycol. This after dispersing or 20 minutes was dried at 80 deg C. The gel so dried was crushed and raided. This was then calcined at 500 degree C for an hour and microwave entered at 1400 deg C for 30 minutes. he abrasive grains so produced showed an average crystal size of 0.4 microns, specific gravity 3.86, Micro Vickers hardness 1863 and an abrasion index of 88. We Claim 1. An improved process for the preparation of alumina abrasive grain Which comprises i) dispersing boehmite (mono hydroxy aluminium oxide) in water at temperature in the range of 40 to 80°C ii) Adjusting the pH of the dispersed solution in the range of 2 to 4 by conventional means iii) adding additives such as iron, titanium, silicon, magnesium, calcium, yttrium, neodymium, lanthanum, etc as their oxides, nitrates, hydroxides, etc, in quantities such as 0.1 to 7% of the total weight of material as oxide to improve the hardness, toughness, sintering temperature etc., iv) Seeding the solution by adding submicron sized alpha alumina v) Drying the resulting gel at a temperature in the range of 60 to 80 °C, vi) Crushing the dried gel to the desired shapes and sizes, vii) claiming the crushed gel at a temperature in the range of 300 to 900 °C for a period in the range of 30 to 120 minutes and viii) Sintering the claimed grains by using microwave energy as herein described at a temperature between 1200 and 1500 °C for a period of 5 to 60 minutes. 2. An improved process as claimed in claim 1 wherein the pH is maintained between 2 to 4 by adding an acid like nitric acid. 3. An improved process as claimed in claim 1 wherein the amount of seed used ranges from 0.1 to 2% by weight. 4. An improved process as claimed in claim 1, step (v) wherein drying control chemical additive polyethylene glycol is used for improving uniformity in drying. 5. An improved process as claimed in claim 4 wherein the amount of drying control chemical additive used range from 0.1 to 2%. 6. An improved process as claimed in claims 1-5 where in the gel is dried at a temperature in the range of 60 to 80 deg C. 7. An improved process as claimed in claim 1, in step (vii) wherein the calcinations is carried out at 600 deg C for atleast half an hour. 8. An improved process as claimed in claim 1, in step (viii) wherein the sintering is done at temperatures between 1200 °C to 1500 °C by applying microwaves having energy of 2.45 GHz frequency and power out put of 900 watts. 12. An improved process for the preparation of alumina abrasive grains substantially as herein described with reference to the examples.

Documents:

0409-mas-1996 abstract.pdf

0409-mas-1996 claims.pdf

0409-mas-1996 correspondence-others.pdf

0409-mas-1996 correspondence-po.tif

0409-mas-1996 description(complete).pdf

0409-mas-1996 form-1.pdf

0409-mas-1996 others.pdf