Title of Invention

A PROCESS FOR THE PRODUCTION OF DENSE MAGNESIUM ALUMINATE SPINEL GRAINS

Abstract

A process for the production of dense stoichiometric Meg l204 grains having bulk density of >3.30 g/cc, apparent porosity of <2.2% and water absorption of <1.0%, from aluminum tri-hydroxide produced by the Bayer process and seawater magnesia, using halides of Ileac group elements as sintering aids is disclosed in this process. A method disclosed in the process is involved two stages firing, in which initially a mixture containing requisite quantities of magnesia and alumna raw materials claimed at <1600 K and subsequently sintering at <1873 K by adding AICI3 as a new sintering additive. MgAl204 dense grains produced by this method are useful as refractory additives in burning and transition zones of cement rotary kilns and constables for steel ladles. The new sintering additive AICI3 pyrohydrolyze at temperatures >873 K in the air and converts into one of the constituents of the spinal body without contaminating original component. Sintering aid used in this method also helps in removing the Na20 present as impurity, by converting it into a volatile compound at temperature >1773 K.

Full Text

The present invention relates to a process for the production of dense magnesium aluminates spinal grains. The process of the present invention is capable of sintering magnesium aluminates spinel (MAS) to higher density at relatively low temperatures. The present invention, particularly relates to an improved process by which the MAS can be sintered at temperature of about 1823 K to a density of 3.30 g/cc. The process of the present invention for producing dense MAS with higher purity is much more effective as compared to that of the conventional one. BACKGROUND OF THE INVENTION Magnesium Acuminate Spinel offers unique combination of desirable properties like high melting point, high strength at elevated temperature, high chemical inertness, low thermal expansion coefficient, high thermal shock resistance to make it an excellent material for refractory application. In addition, the spinel exhibits first deformation at 2 kg/cm at 2273 K, does not react with silica until 2008 K, CIO or Mao until 2273 K, a-A Os until 2198 K, and can be used for all metals except alkaline earth. The spinel refractory is mainly used in lining of steel ladles, cement rotary kilns and glass furnaces. Although, development of spinel refractoriness was initiated about 100 years ago but due to its prohibitive cost of production its use in refractory was very limited. However, nowadays Mg-AI-spinel is being extensively used in refractory industries because of its production cost has been brought down substantially by incorporating improved process routes. Although MAS is an outstanding refractory material, its manufacturing process is not straightforward. It is because, during formation of Mega OA spinel from its constituents like Mao and AI2O3, there is an appreciable (about 8%) volume expansion. Due to this expansion, it is very difficult to get a dense MgAl204 spinel body by using single stage firing process. Among the few currently commercially exploited processes, double stage firing process is the most popular for producing high purity MAS. In this process, in order to reduce or eliminate the volume expansion, the MgAl204 spinel is synthesized by prior calcination of its raw materials at around 1673 K to get appreciable spinel phase and then sintered into dense magnesium aluminate spinel body. However, these high calcination temperatures hinder the formation of dense high-purity spinel bodies. As reported in literature, mineralizes such as, V2O5, Y2O3, Niacin, and MgCb lower this calcination temperature to some extent. In most cases, the spinel so obtained has a low reactivity and high sintering temperatures (>1973 K) are required to form a dense product. Becker and Lindsay (Ceramic Bulletin, 46 [11] (1967) 1094-1097) have shown that the addition of aluminum fluoride even in small amounts (i.e., 1-3 wt. %) to un-calcined raw mix could remarkably enhance the conversion of spinel by claiming it at 1573 K and thus increase the sinter ability leading to higher bulk density. But this process require extensive grinding as well as sintering temperature as high as 1923K. However, AIF3 is not effective at all to increase the sintered density when it is added as a sintering aid before sintering. The US patent publication No. 3, 950, 504 assigned to North American Refractoriness Co., U.S.A., discloses a method, in which a high purity magnesium aluminate spinel is prepared by blending alumna in the form of electrostatic precipitator (ESP) fines with a finely divided source of magnesia in such proportion as to provide a magnesia : alumina weight ratio of about 0.4 to 0.8 and heating said blend to a temperature of about 1873 K to 2373 K for a period of time sufficient for substantially complete reaction of said blend to produce spinel having a bulk density of at least about 90 percent of theoretical density. Additionally, in this process fluoride containing salts such as AIF3 and crinoline (Massifs) have been added in the range of 1 to 3 weight percent in order to remove Na20 in the form of sodium containing volatile compounds during sintering. However, this process produces a low-density product at substantially higher sintering temperatures and the ESP fines used as alumina source is not abundant as that of Bayer processed aluminas to produce spinets to meet the market demand. Further, the reduction of sintering temperature by using various sintering aids like Ti02, B2O3, LiF, ZnF2, BaF2, CaF2 (Interterm, 42 [2] (1993) 89 - 92) and Fe203, CaO, (Interceram, 41 [1] (1992) 22 - 26) to facilitate liquid phase sintering have also been reported. However, use of these additives certainly contaminates the product and restricts its application to low temperatures only. In all the above said known process, the additives have been incorporated either to raw-mixture to improve degree of the signalizations on calcination at relatively low temperatures or as an additive prior to sintering and thus reduce the sintering temperature. But, these techniques have their own limitations like sensitivity to specific raw materials, requirement of fine grinding, higher sintering temperature of 1923K, tendency to contaminate the product, etc. Taking into consideration the unique combination of desirable properties like high melting point, high strength at elevated temperatures, high chemical inertness, low thermal expansion co-efficient, high thermal shock resistance, magnesium acuminates spinel offers, thereby making it an excellent material for refractory applications, we took the initiative to develop an improved process for the preparation of MAS without any contamination and employing lower temperature Accordingly the main objective of the present invention is to provide a process for the production of dense magnesium aiuminate spinel grains which does not have any contamination. Another objective of the present invention is to provide an improved process for the production of dense magnesium aiuminate spinel grains at about the temperature of 1823 K. Still another objective of the present invention is to provide an improved process for the production of dense magnesium aiuminate spinel grains with densities not less than 3.30 g/cm^, apparent porosity not more than 2.2% and water absorption not more than 1.0% by sintering at about 1823K. The present invention has been developed on the basis of our surprising finding that the presence of aluminum chloride as an additive (as mineralizer) economizes the process by reducing sintering temperature. Like AIF3, although AICI3 enhances the y (gamma) to a (alpha) transition in AI2O3 facilitates the formation of spinel at relatively lower temperatures. However, the effect of AIF3 on spinelization is much stronger than AICI3. But, employment of AICI3 as an sintering additive helps in sintering of MAS leading to the formation of dense mass even at a temperature as low as 1823K. Further, compared to the other sintering aids like Ti02, Fe203, B2O3, CaF2, CaO, etc., AICI3 does not produce any impurity on the resulting MAS as it decomposes to Al203only. Though AIF3 and AICI3 fall under the same category, our finding that the use of AICI3 as an additive for the preparation of dense MAS is surprising and unique in the sense that the chemistry of their involvement in the process is quite different. It would be seen that AIF3 was used in the process to increase spinelization and is added to the reaction mixture before calcination of the mixture. Although AIF3 is able to enhance the spinelization at lower temperatures, the spinelized powder does not sinter below 1873 K even after extensive grinding. On the other hand, the present invention proposes the use of AICI3 as an additive for enhancing sintering and for this purpose it is added to the reaction mixture just before sintering. The use of AICI3 as a sintering agent results not only in lowering the sintering temperature but also results in a final product, which is not contaminated. This is an unexpected and unique finding and imports novelty to the process. The invention also envisages the use of AICI3 as an agent for spinelization. Accordingly, the present invention provides a process for the production of magnesium aluminate spinel grains (MAS), which comprises: i. Mixing and co-grinding of magnesia yielding source and alumina yielding source for a period of less than 6 hours i. Making of a dough of the above said admixed employing a binder solution ii. Extruding the resultant dough by the process of extrusion V. Drying resultant extrudate(s) in an electric furnace at a temperature of around 423 K V. Coalmining the dried extrudate(s) in an electrical furnace or kiln at a temperature around 1573 K vi. Grinding the calcined extrudate(s) to particle size of less than 10-micron (D100) in a ball mill vii. Intimately mixing the spinel powder obtained in step (vi), with aluminum chloride viii. Granulating the ground calcined and AICI3 coated powder in the range of 0.15 mm to 1.5 mm ix. Pressing or briquetting granules obtained under a pressure in the range of 1800 to 2000 kg/cm^ and X. Sintering the pressed pellets or briquettes at a temperature in the range of 1823 to 1873 K The magnesia yielding material such as caustic MgO and alumina yielding material such as aluminum tri-hydroxide are admixed in such a proportion such that the weight ratio of MgO and AI2O3 in the fired body is 27.0 - 30.0 :: 73.0 - 70.0. The sources of alumina and magnesia are having purity greater than 98 % on loss free basis. This admixture is milled in a conventional mill like, planetary mixer, ball mill, rod mill, etc., for a period of less than 6 hours. The mixture is then converted to dough with an aqueous solution containing polymeric binder like dextrin solution, polyvinyl alcohol solution, wax emulsion, etc. The binder percentage may be maintained in the range of 3 to 5%. The dough is then subjected to extrusion into rodlets preferably of dimension of 25-mm diameter and 150 - 200 mm lengths. The extrudate(s) may be dried in an oven at around 423 K so that moisture content is less than 0.5%. The dried extrudate(s) are calcined at a temperature in the range of 1373 - 1673 K to obtain 60 - 80% spinel phase. The calcined material is ground in ball mill or any other conventional mill for sufficient time to reduce the particle sizes less than 10 mm. To the ground powder of spinel thus obtained, AICI3 in different amounts ranging from 0.01 to 0.03 mole % is mixed to polymeric binder like polyvinyl alcohol, dextrin, etc., and was added to form a dough in a conventional dough making machine prior to granulation through granulator to a definite size fraction, which may lie between -8 to +325 BSS mesh. The dried granules containing less than 2% moisture are pressed in the form of pellets having volume within 15 - 50 cm^ under the pressure of 1 - 3 tons/cm^. The pressed pellets having green density of >2.0 g/cm^ are fired in muffle, tunnel or down draft at a temperature in the range of 1823 Kto 1873 K and soaked for 0.5 to 1.5 hours. The sintered pellets having density of 3.30 g/cc, apparent porosity 2.2% and water absorption The following Examples are provided merely to illustrate the invention and therefore should not to be construed as limiting the scope of the invention. In the Examples, unless indicated otherwise, all temperatures are in Kelvin (K), all decants are in mole percent or mole ratios and all densities are in grams per cubic centimeter (g/cm ), apparent porosity is in (%) and water absorption is also in (%). EXAMPLE 1 4.66 kg of aluminum tri-hydroxide and 1.33 kg of caustic MgO were mixed and co-ground in a steel ball mill using steel media for a period of around 6 hours. The chemical compositions of above alumina and magnesia raw materials are given in Table I. Table I. Chemical Composition of Raw Materials (wt %) Material AI(0H)3 Caustic MgO AI2O3 (%) 66 - MgO (%) - 82.86 NBZO (%) 0.2-0.3 0.093 CaO (%) 0.02-0.03 0.908 SiOz (%) 0.009 0.97 FezOa (%) 0.007 0.106 LOI (RT-1273 K) 34.5 15.5 The ground mixture was formed into dough with 900 ml of dextrin solution (binder percentage as 4.0%). The dough was then subjected to extrusion in the form of rods having dimensions 25 mm of diameter and 150 mm of length followed by oven drying at 423 K to bring down the moisture content to less than 0.5 %. The dried extrudate(s) were calcined in an electrically operated furnace at 1573 K to obtain around 70% of spinel phase. The calcined material was then ground in a ball mill to reduce the particle size (D100) to less than 10 microns. The ground powder was then mixed with 900 ml of PVA solution (binder concentration 6%) to form dough in a conventional dough-making machine prior to granulation through granulator to a definite size fraction, which lies between 0.15 mm to 1.5 mm. The dried granules containing less than 2% moisture were pressed in the form of pellets having diameter 20 mm and 10 mm height under the pressure of 2000 kg/cm^. The pressed pellets having green density of 2.1 g/cm^ were fired at around 1823 K for a soaking period of 1 hr in an electrically operated furnace. This result clearly shows that MAS made by the process following example 1 cannot be sintered to required dense product, normally required for refractory application, within 1823K. EXAMPLE 2 In this example 4.66 kg of aluminum tri-hydroxide and 1.33 kg of caustic MgO were mixed and co-ground in a ball mill for about 6 hours. The ground mixture was then formed into dough with 4% dextrin solution without AICI3. The dough was then subjected to extrusion in the form of rodlets prior to oven drying at around 423 K. The dried extrudate(s) were calcined at 1573 K and the spinelization amount was found to be about 70% as indicated earlier. The calcined material was ground in a bail mill to reduce the particle size (Duo) to less than 10 microns. The ground powder was then divided equally into three parts and were formed into dough separately with 300 ml of polyvinyl alcohol solution (binder concentration 6%) containing 0.01, 0.02, 0.03 mole % of AICI3 (Assay 99.0%. Laboratory Grade Reagent) with respect to final MAS. All different portions of dough were granulated to a definite size fraction, which lies between 0.15 mm to 1.5 mm. The dried granules were dry pressed in the form of pellets having diameter 20 mm and 10 mm height under the pressure of 2000 kg/cm^. The pressed pellets having green density of 2.15 g/cm were fired at around 1823 K for a soaking period of 1 hr in an electrically operated furnace. The sintered properties are given in Table II. Table II. Density, Apparent Porosity and Water Absorption of MAS sintered at 1823 K with different amount of AICI3. Table II clearly indicates that AICI3 is very effective as a sintering aid for MAS which can be sintered to a bulk density of 3.38 g/cm^, apparent porosity of 0.19% and water absorption of 0.05% by using only 0.03 mole % of AICI3 by sintering it at 1823 K. EXAMPLE 3 In this Example 4.66 kg of aluminum tri-hydroxide and 1.33 kg of caustic MgO were mixed and co-ground in a ball mill for about 6 hours. The ground mixture was then formed into dough with 4% dextrin solution without any depart. The dough was then subjected to extrusion in the form of rodlets prior to oven drying at around 423 K. The dried extrudate(s) were calcined at 1573 K and the spinelization amount was found to be about 70% as indicated earlier. The calcined material was ground in a ball mill to reduce the particle size (D100) to less than 10 microns. The ground powder was then divided equally into three parts and were formed into dough separately with 300 ml of polyvinyl alcohol solution (binder concentration 5%) containing 0.01, 0.02, 0.03 mole % of AIF3 (Assay --99.0%, Laboratory Grade Reagent) with respect to final MAS. All different portions of dough were granulated to a definite size fraction, which lies between 0.15 mm to 1.5 mm. The dried granules were dry pressed in the form of pellets having diameter 20 mm and 10 mm height under the pressure of 2000 kg/cm2. The pressed pellets having green density of 2.13 g/cm2 were fired at around 1823 K for a soaking period of 1 hr in an electrically operated furnace. The sintered properties are given in Table III. According to Table III it is understood that AIF3 is not effective at all as a sintering aid to improve sintered properties. Advantages of the Invention 1. The modified process can produce highly dense magnesium aluminate spinel grains from raw materials at relatively low sintering temperature of 1823 K by using AICI3 as a sintering additive. 2. This process can produce dense high purity MgAl204 spinel grains for refractory applications at temperatures at least 100 K less that the generally used temperature 3. The use of AICI3 as a sintering additive does not lead to any contamination of the final product. 4. The modified process does not call for any extensive grinding of the calcined powder. 5. The process is economical as the sintering temperature is very much reduced and it does not require grinding WE CLAIM: 1. A process for the production of magnesium aluminate spinel (MAS) grains which comprises: i. Mixing and co-grinding of a magnesia yielding source and an alumina yielding source for a period of less than 6 hours. ii. Making of dough of above said admixture employing a binder solution iii. Extruding the resultant dough by the process of extrusion and drying the extrudate(s) in an oil-fired or electing furnace at around 423 K, iv. Acclaiming the resultant extradites(s) in conventional furnace or kiln at around 1573 K V. Grinding the calcined extrudate(s) to less than 10-micron (Door) in a ball mill. vi. Intimately mixing of product obtained in step (V) with an aqueous solution of AICI3. vii. Granulating the ground calcined powder within the range of 0.15 mm to 1.5 mm viii Pressing or briquette granules under the pressure in the range of 1800 to 2000 kg/cm^ ix. Singeing the resulting pressed pellets or briquettes at a temperature in the range of 1823-1873 K. 2. The process as claimed in claim wherein the alumina-yielding source includes aluminum tri-hydroxide, gibbsite and alumina. 3. The process as claimed in claims 1 & 2 wherein the magnesia source includes Mow, Caustic MgO and Magnesium hydroxide. 4. The process as claimed in claims 1 to 3 wherein the purity of aluminum and magnesium yielding sources employed is greater than 98% on loss free basis The process as claimed in claims 1 to 4 wherein the magnesia yielding source and the alumina yielding source are admixed in such a proportion such that the weight ratio of MgO and AI2O3 in the fired body is 27.0 - 30.0 :: 73.0 -70.0. The process as claimed in claims 1 to 5 wherein the admixture is milled in a conventional mill like ball mill, rod mill and verb mill. The process as claimed in claims 1 to 6 wherein the admixture is converted into dough employing an aqueous solution of polymeric binder. The process as claimed in claim 7 wherein the polymeric binder employed for making the dough is selected from dextrin, polyvinyl alcohol and wax emulsion The process as claimed in claims 7& 8 wherein the polymeric binder percentage employed for making the dough is within 5%. The process as claimed in claims 1 to 9 wherein the extrudate(s) are dried in oil-fired or electrical furnace to reduce the moisture content less than 0.5% at around 423 K. The process as claimed in claims 1 to 10 wherein the dried extrudate(s) are calcined at a temperature in the range of 1373 - 1673 K to obtain 60 - 80% spinel phase. The process as claimed in claims 1 to 11 wherein the particle size (D100) of calcined and ground mass is not be more than 10 micron. The process as claimed in claims 1 to 12 wherein the calcined material is ground in ball mill or any other conventional mill for a period in the range of 4 to 16 hrs so as to reduce the particle size less than 10 micron. The process as claimed in claims 1 to 13 wherein the ground material is mixed with an aqueous solution Alban polymeric binder. The process as claimed in claims 1 to 14 wherein the solution containing AICI3 (0.01 - 0.03 mole % of final MAS) is used The process as claimed in claims 14 & 15 wherein the polymeric binder used is selected from polyvinyl alcohol and dextrin The process as claimed in claims 14 & 16 wherein the polymeric binder percentage employed for making the dough is within 6%. The process as claimed in claims 1 to 17 wherein the granules obtained in step (VII) containing less than 2% moisture are pressed in the form of pellets having volume within 15 to 50 cm^ under the pressure of 1 - 3 tons/cm^. The process as claimed in claims 1 to 18 wherein the pressed pellets are fired in a conventional electrical or oil or gas fired kiln such as muffle, tunnel or down draft at a temperature in the range of 1823 K to 1873 K. The process as claimed in claims 1 to 19 wherein the soaking time of the pressed pellets at the sintering temperature falls in the range of 0.5 to 1.5 hr. The process as claimed in claims 1 to 20 wherein the sintered pellets obtained in step (IX) have a density > 3.30g/cc, apparent porosity A process for the preparation of high purity magnesium aluminate spinel grains substantially as herein described with reference to the Example 2

Documents:

520-mas-2000-abstract.pdf

520-mas-2000-claims filed.pdf

520-mas-2000-claims granted.pdf

520-mas-2000-correspondnece-others.pdf

520-mas-2000-correspondnece-po.pdf

520-mas-2000-description(complete) filed.pdf

520-mas-2000-description(complete) granted.pdf

520-mas-2000-form 1.pdf

520-mas-2000-other documents.pdf