Title of Invention

PROCESS FOR THE PREPARATION OF CERIA NANO POWDER SUITABLE FOR OPTICAL POLISHING

Abstract

Ceria powder is used for abrasive and polishing purposes but it leaves scratches on optical glasses. Optical polishing grade ceria powder is conventionally made by milling it to fine powder. The present process of the invention synthesizes directly the ceria nano powder for optical polishing. The process; makes use of a reactor for gel-combustion reactions, which provides safe working of the process on kilogram scale.

Full Text

FORM 2 THE PATENTS ACT 1970 (39 of 1970) COMPLETE SPECIFICATION (See Section 10; Rule 13) TITLE rPROCESS FOR THE PREPARATION OF CERIA NANO POWDER SUITABLE FOR OPTICAL POLISHING 2. (a) Secretary, Department of Atomic Energy, Government of India, (b) Anushakti Bhavan, Chhatrapati Shivaji Maharaj Marg, Mumbai 400 001, Maharashtra, India, (c) an Indian company. The following specification particularly describes the nature of the invention and the manner in which it is to be performed. GRANTED 5-4-2004 ORIGINAL 708/MUM/2003 10/07/2003 Field of invention The present invention relates to a batch process for large scale manufacture of cetia powder suitable for optical polishing on a kilogram scale. This invention particularly relates to a process for the preparation of ceria powder suitable for optical polishing from cerium nitrate by gel combustion reaction. Background and prior art Ceria powder suitable for optical polishing is generally manufactured by a process as taught in US 6238450. In this method commercially available ceria powder is milled at a pH of 9 to 12.5 to get a BET surface area of at least 15 m2 /g. This is used to form slurry with surfactant for polishing purposes. JP 2002 201023 teaches preparation of ceria nano powder by reacting acidic solution of cerium with alkali so that cerium hydroxide is precipitated and then it is calcined to give nano size ceria. However, ceria made by this process is not useful as an optical polishing agent. US 6043155 teaches a polishing slurry useful in optical and chemical mechanical planarisation (CMP) applications which is made by subjecting a commercial ceria slurry by a mechanico -chemical treatment at a pH of 9 - 11 using media that are low purity alpha alumina or zirconia. US 5728362 teaches a process for preparing mixed metal oxide powder, in which aerosol-droplets are generated from the tuned solution of the reactants in certain proportion, and sprayed into a heated gas jet. As seen above, ceria powder suitable for optical polishing is not known to be manufactured directly by gel combustion reaction. This is due to the vigor of such reactions with flame and gas evolution when the reactions are carried out in large scale. For such reasons, the commercially available ceria powder, not suitable for polishing applications, is conventionally treated as discussed above in order to make it suitable for optical polishing. Optionally ceria powder is prepared by alternative lengthy routes, Objects of the present invention The principle object of the present invention is to provide, a process.for manufacturing ceria nano powder suitable for optical polishing application on kilogram/industrial scale safely overcoming the vigour and explosive nature of the reaction. Another object of the present invention is to provide a process for manufacturing ceria nano powder suitable for optical polishing application on kilogram/industrial scale at high yield. In the co pending application No. 707/mum/2003 which is incorporated herein by reference, the applicants have described and claimed a novel chemical batch reactor to carry out safely any gel combustion reaction, between at least one oxidising reactant and at least one reducing reactant, which on scale-up may be explosive in nature. This invention provides a process for preparation of ceria powder suitable for optical polishing, on a kilogram scale, by carrying out safely in this chemical reactor the gel combustion reaction between cerium nitrate and citric acid in aqueous medium under controlled conditions. Summary of the invention Accordingly the present invention provides a batch process for large scale manufacture of ceria nano powder suitable for optical polishing by gel combustion reaction, the batch process comprising: a) providing a reactor comprising: (i) a reaction vessel having an outlet for the fumes generated during the reaction; (ii) means for agitating the reaction mixture comprising at least two impellers wherein at least one of said impellers adapted to dip into the reactant mix and at least one of said impellers adapted to be situated in the space above the reactant mix level at the time of start of the reaction; (iii) means for heating the reaction vessel; (iv) means for controlling the overall temperature of the reactor; and (v) means for controlling the speed of the agitator. b) mixing aqueous solutions of cerium nitrate and citric acid or other aliphatic carboxylic acids or their salts such as acetate in said reaction vessel; c) heating the said mixture under controlled agitation to a temperature between 75°C and 80°C to form a gel; d) maintaining the overall temperature of the reaction mass below ?00°C; e) controlling the speed of said agitator as the gel combustion reaction progresses and the gel viscosity increases such that the heat dissipation is uniform and the fumes evolving along with the formation of ceria nano powder suitable for optical polishing is expelled without driving the ceria out through said outlet for fumes; f) cooling the reaction mass of step (e). Detail description of the invention The reaction is carried out in a batch reactor disclosed in our copending patent application for gel-combustion reactions of aqueous solutions of metal nitrate(s) and an organic reducing agent(s). This reactor is particularly useful for carrying out safely these vigorous reactions on kilogram scale. The batch reactor comprises a reaction vessel kept in a fume hood having an exhaust fan. There is an agitator with two impellers inside the reaction vessel such that one of the impellers remains dipped in the reacting liquids and the other remains above the liquid level. A speed regulator controls the speed of the agitator. A sufficiently large vent is provided in the openable lid of the reactor for letting off the gases generated during the reaction into the fume hood before they build up any pressure in the reaction vessel. The exhaust fan of the fume hood takes off the gases. There is provision for controlled heating of the reaction vessel from outside from bottom and sides of the vessel when the reactor is charged with the aqueous solutions of the reactants, in this case cerium nitrate and citric acid. The pH of the mixture is 1 to 3. The progress of the reaction inside the vessel is closely observed through either of the two ports with sight glasses which are at an angle to each other. One port is preferably the light port and the other the view port. Initially the temperature is adjusted so as to trigger the gel combustion reaction. Thereafter, as the combustion reaction proceeds the temperature of the liquid inside the reaction vessel is closely monitored. The over all temperature of the reactor is controlled to maintain it below 200 °C even while local temperature at the flame points shoots up to 450 - 550°C. The stirrer speed is controlled to meet the rising viscosity of the liquid and the rising load of the fumes to be taken off, without letting off the ceria nano powder suitable for optical polishing formed, along with the fumes. After the batch reaction is over, the reactor temperature falls down, the external heating is stopped and the reactor is cooled to room temperature. The reactor is opened from the top and the ceria nano powder is collected. It can also be drained from the bottom opening of the reactor. The ceria nano powder made by this process was found to be of optical polishing grade and does not leave any scratches on an optical glass/quartz etc. on polishing with it. The surface finish achieved by this polishing is.about 0.5 nano meter. The invention will now be described by way of an example. The example is by way of illustration only and in no way restricts the scope of the invention. Example: Preparation of Ceria powder suitable for optical polishing AR grade materials from E. Merck were used except Cerium nitrate which was of Indian Rare Earths make. Aqueous solutions of Cerium nitrate (0.48 g / ml) [1300ml] and citric acid (255 g solid) in 1: 0.83 molar ratio were mixed and heated in the reactor for gel-combustion reactions (see co-pending patent application) to 80 °C and it formed a gel. The gel was heated by external heaters. Initially it was heated to a temperature just sufficient to trigger the reaction. Heating was continued and the temperature of the bulk mass of the reaction was controlled between 180 and 200°C. The temperature of the ceria powder formed was not allowed to rise beyond 200 °C for more than a few seconds, even though the local temperature shot up to 450 to 550°C. As the reaction proceeded the gel became viscous and stirrer power was adjusted so that agitator speed of 50 RPM is achieved. As the ceria powder formed flew in the reactor, care was taken to control the speed of the agitator so as to prevent the product from venting out along with gases generated. At the end of the batch reaction which ran for 4 hours, the reactor was cooled down to room temperature and the product, 250 g of ceria powder, was drawn from the top opening of the reactor. The yield of the ceria powder was more than 90 % wt of the theoretical value. The particle size of the ceria powder produced was found to be 13 nm when examined by X-ray diffraction and calculated by X-ray diffraction using Scherrer formula. (See Elements of X-ray diffraction, Addison Wesley, Culity BD 1978) and also by TEM (Transmission electron microscopy). The ceria powder produced by this process was tried on optical glass and quartz for polishing. It was found to give a good polish without producing any scratch on the surfaces. It was found that ceria powder produced by using a sustained temperature 600°C or above gave rise to scratches on quartz surface during polishing, when seen under microscope at a magnification of 16x, whereas the powder produced by the process of present invention did not show any scratches under similar conditions. We Claim: 1. A batch process for large scale manufacture of ceria nano powder suitable for optical polishing by gel combustion reaction, the batch processcomprising: (a) providing a reactor comprising: (i) a reaction vessel having an outlet for the fumes generated during the reaction; (ii) means for agitating the reaction mixture comprising at least two impellers wherein at least one of said impellers adapted to dip into the reactant mix and at least one of said impellers adapted to be situated in the space above the reactant mix level, at the time of start of the reaction; (iii) means for heating the reaction vessel; (iv) means for controlling the overall temperature of the reactor; and (v) means for controlling the speed of the agitator. (b) mixing aqueous solutions of cerium nitrate and citric acid or other aliphatic carboxylic acids or their salts such as acetate in the molar ratio of 1:0.75 to 1:0.85 in said reaction vessel, the mixture having ailpH of between 1 and 3; (c) heating the said mixture under controlled agitation to a temperature between 75°C and 80°C to form a gel; (d) maintaining the overall temperature of the reaction mass below 200°C; (e) controlling the speed of said agitator as the gel combustion reaction progresses and the gel viscosity increases such that the heat dissipation is uniform and the fumes evolving along with the formation of ceria nano powder suitable for optical polishing is expelled without driving the ceria out through said outlet for fumes; (f) cooling the reaction mass of step (e). 2. A batch process according to claim 1, wherein said heating of reaction mixture in step (iii) is carried out under controlled stirring at rpm of 40 to 60. 3. A batch process according to claim 1, wherein in step (f) the reaction mass is cooled to ambient temperature. 4. A batch process according to claim 1, wherein said reactor is kept in a fume hood having an exhaust fan. 5. A batch process according to claim 1, wherein said means for heating the reaction vessel comprises a side heater and a bottom heater. 6. A batch process according to any of the preceding claims, wherein in step (d) the temperature is maintained by adjusting the speed of said agitator, exhaust fan, and bottom and side heater controls. 7. A process for manufacture of ceria nano powder suitable for optical polishing by gel combustion reaction substantially as herein described in the text and example. Dated this 7th day of July 2003 SIDDHARTHA NAG Of S. MAJUMDAR & CO. Applicants' Agent

Documents:

708-mum-2003-abstract(granted)-(05-04-2004).pdf

708-mum-2003-abstract-(5-4-2004).doc

708-mum-2003-cancelled pages(05-04-2004).pdf

708-mum-2003-claims(granted)-(05-04-2004).pdf

708-mum-2003-claims(granted)-(5-4-2004).doc

708-mum-2003-correspondence(29-11-2006).pdf

708-mum-2003-correspondence(ipo)-(07-01-2004).pdf

708-mum-2003-form 1(10-07-2003).pdf

708-mum-2003-form 19(28-07-2003).pdf

708-mum-2003-form 2 (granted)-(5-4-2004).doc

708-mum-2003-form 2(granted)-(05-04-2004).pdf

708-mum-2003-form 3(07-07-2003).pdf

708-mum-2003-power of attorney(08-09-2003).pdf