Title of Invention

IMPROVED PROCESS FOR THE PREPARATION OF MAGNESIUM ALUMINATE SPINEL GRAINS

Abstract AN IMPROVED PROCESS FOR THE PREPARATION OF MAGNESIUM ALUMINATE SPINEL GRAINS Improved process for the preparation of dense magnesium aluminate spinel grains of both low purity (i.e. common spinels) and high purity with a bulk density of >3.35 g/cc from cheap and readily available Indian raw materials such as alumina sources including α-AI2O3, bauxite, aluminum hydroxide, diaspore, bayerite, and gibbsite, and magnesia sources including MgO, brucite, caustic MgO, and magnesite is disclosed in this process. In this process, calcination temperature has been optimized in such a way that powders obtained from a mixture of aluminum and magnesium raw materials are highly reactive, porous and friable, which facilitate the ease of grinding operation as well as avoids the powders to be ground into sub-micron size. Calcination as well as sintering temperatures used in this process to facilitate spinel phase formation and to obtain desired sintered properties are at least lower by 100 K as compared to existed commercial processes.
Full Text

This invention relates to an improved process for the preparation of magnesium aluminate spinel. The invention particularly relates to an improved process for the preparation of sintered grains of magnesium aluminate in spinel structure, for use, for example in the manufacture of refractory bricks, castables, monoliths, etc.
Magnesium aluminate in the spinel structure offers excellent spalling resistance, corrosion resistance and slag penetration resistance, which makes it one of the most favourable new generation synthetic refractory raw materials.
BACKGROUND OF THE INVENTION
Although magnesium aluminate spinel is not a naturally occurring mineral as is the chromium spinel, in the past there has been an increasing interest to apply the magnesium aluminate spinel in refractory applications. The major motivation to replace magnesite-chrome refractory by MgAl204 system stems from the fact that the environment hazard posed by the conversion of insoluble trivalent (+3) chromium to the soluble hexavalent (+6) state in the former.
Magnesium aluminate spinel find applications as precast monolithic shapes for steel plant ladles, electric furnaces, caster tundishes, degasser snorkels and lances, stoichiometric spinel addition in the range of 20-30 wt.% significantly increases the slag penetration resistance of alumina castables. Magnesia rich sintered spinel has been used for the development of magnesia spinel brick for cement rotary kilns.
Generally, it is known to mix magnesia (MgO) and alumina (AI2O3) in the stoichiometric portions required to form magnesium aluminate spinel (MgAl204), forming this admixture into shapes and then firing the shapes. In this approach, the chemical reaction between the magnesium aluminate spinel takes place during the firing of the refractory shape.
However, it has been found that the volume change-taking place concurrently with the reaction of forming spinel is so large that it tends to disrupt the refractory shape.

Thus, it is very difficult, by this method, to make a magnesium aluminate spinel shape.
In another normal practice, the mixture of AI2O3 and MgO is injected into an electric arc furnace at a temperature between 2773 and 3273K. the mixture reacts to form spinel and then melts leading to formation of dense refractory grog.
However, this process is very expensive due to high capital and power cost. Further, this process does not yield high purity spinel as lots of fusion additives are added during electrofusion.
Alternatively, it has been known to form magnesium aluminate spinel grain by admixing and firing magnesia and alumina in stoichiometric proportions. This grain is then sized, formed into shapes, and fired a second time, however, there are only a few of existing commercial processes for production of magnesium aluminate spinel using this technique.
The US patent publication No. 3, 950,504 assigned to North American Refractories Co., U.S.A., discloses a method, in which a high purity magnesium aluminate spinel is prepared by blending alumina 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 ration of about 0.4 to 0.8 and heating said blend to a temperature of about 1873 K to 2373K 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. However, this process produces a low-density product at substantially higher sintering temperatures.
Recently, Serry and co-workers (British Ceramic Transactions, 1998, Vol.97, No.6, pages 275-282, and references therein) reported the preparation of magnesium aluminate spinel, with 3.35g/cc bulk density after sintering at 1973 K, from precipitated brucite [Mg(0H)2] and gibbsite [AI(OH) 3] using double stage firing process.

Yet another process (American Ceramic Society Bulletin, 1993, Vol.72, No.4, pages 59-63, and references therein) involves wet co-grinding of alumina and magnesia, which are formed separately by calcination of its precursor like aluminum hydroxide or magnesium hydroxide or magnesite etc. The ground slurry is then fired at around 2273 K to form dense spinel. However, these processes also requfred higher sintering temperatures and high cost raw materials.
A low-grade magnesium aluminate spinel is produced in China (Xinxiang Refractory Plant, 45 Renmin Road, Xinxiang, Henan, P.R. China, 453000) by using high quality of bauxite and soft calcined magnesia. The raw mix co-ground followed by briquetting to form pellets of optimum green density, which is then electro-fused or fired around 2273 K in a rotary kiln. However, this process is limited to production of MgO rich magnesium aluminate spinel. The above process needs multiple stages of calcination or high quality of sophisticated equipment or high quantity of power and fuel or high grade raw material etc.
Considering the disadvantages of the existing processes explained above the present invention proposes an improved process overcoming the drawbacks associated with the production of a magnesium aluminate spinel refractory material in the existing known processes. This invention also describes the process for the preparation of high-density magnesium aluminate spinel at relatively low sintering temperatures from in-expensive and readily available Indian raw materials.
The present process for producing magnesium aluminate sintered spinel grains involves utilization of cheap and readily available Indian raw materials. The simultaneous calcination step, which is adopted in the present process, not only economizes but also enhances the efficiency of the process. Further, the raw materials have been selected in such a way so that the calcination temperature for required spinelization to over come volume expansion barrier during sintering is reduced by more than 100 K and also yields high porous and friable powders during simultaneous calcination process, which ultimately facilitates the ease of grinding operation. The reactivity of the calcined powder is maintained in such a way so that extensive grinding for bringing down the average particle size in sub-micron range is avoided. One of the most important features of this invention is that it is suitable for

producing both low and high purity MgAl204 spinels starting from alumina, bauxite, aluminum, hydroxide, diapsore, bayerite, gibbsite as alumina source and MgO, brucite, caustic MgO, magnesite as magnesia source. Finally, magnesium aluminate spinel grains with density >3.35 g/cc is obtained at relatively low temperatures such as 1873-1923 K according to this process.
The main objective of this present invention, accordingly, is to provide an improved process for the preparation of low purity (common spinel) and high purity magnesium aluminate spinel grains useful for refractory lining of steel making ladles and cement rotary kilns.
Another objective of the present invention is to produce the above mentioned spinel grains with purity not less than 98% and densities not less than 3.25 g/cc, apparent porosity not more than 2% and water absorption not more than 1.5% within substantially low sintering temperature range of 1923 to 1973 K.
Still another objective of the present invention is to provide an improved process for the preparation of high purity magnesium aluminate spinel refractory material or bricks with improved thermal spalling resistance, superior slag erosion and penetration resistance.
The invention has been developed based on our findings that separate calcination of the alumina as well as magnesia raw materials such as aluminum hydroxide and caustic magnesia or magnesite, respectively, will not give a reactive powder. But their simultaneous calcination results in an enhanced reactivity towards spinelization.
Accordingly the present invention provides an improved process for the preparation of low purity and high purity magnesium aluminate spinel grains which comprises :
i. Mixing or co-grinding of MgO and MgAl204 yielding materials for a period in
the range of 4 to 6 hours ii. Nodulizing or extruding the resultant raw mixture by known methods

iii. Calcining the nodules or extrudes in conventional furnace or kiln within the
temperature in the range of 1473 to 1673 K iv. Grinding the calcined materials to less than 10 micron (Dioo) V. Granulating the ground calcined powder within the range of 0.15 mm to 1.5
mm vi. Pressing or briquetting granules under the pressure in the range of 1800 to
2000 kg/cm2 vii. Sintering the pressed pellets or briquettes at temperatures in the range of
1873 K to 1973 K for 1 to 2 hours.
For high purity spinal, the magnesia yielding material may be any such material, but preferably should yield at least 97%, and most preferably 98.5%, MgO upon firing. For example, it can be a naturally occurring material such as a magnesium carbonate or it can be a synthetically produced material such as a magnesium hydroxide synthesized by reacting a magnesium salt solution such as seawater or naturally occurring brines or bitterns with lime, hydrated lime, or caustic soda. It can also be magnesia itself. A particularly suitable for m of magnesia is lightly calcined magnesia, i.e., one which has been formed by calcining magnesium hydroxide, caustic MgO, magnesium carbonate, or the like, at such a temperature, for example from 973 to 1373 K, that magnesia in a highly reactive state is formed.
For high purity spinel, the alumina yielding material may also be any such material, but again preferably will be one which yields at least 98%, and most preferably 99.5% alumina upon firing. It can be an aluminum compound such as aluminum hydroxide, for example the material produced by the Bayer process for producing alumina, or it can be alumina itself, either calcined at a relatively high temperature to form the material known as tubular alumina or calcined at a lower temperature to form a more reactive alumina.
For low purity spinel the purity of raw materials are generally eithin90 to 95% on oxide basis.
The magnesia yielding and alumina yielding materials are admixed in such a proportion so that there is slight excess of magnesia, on the fired basis, over that

required to form stoichiometric magnesium aluminate spinel. The amount of excess magnesia should be limited so that the magnesia in excess of that required to form a stoichiometric spinel with the alumina present is less, on the weight basis, than the amount of stoichiometric spinel, which could be formed in the resulting grain. Preferably the amount of magnesia will be limited so that there are not more than three moles of magnesia for each mole of alumina in the admixture.
This admixture is milled in any conventional mixer like, planetary mixer, ball mill, rod mill, Eirich mixer, etc., for a period of 4 to 6 hours. The mixture is then nodulised in the sizes of around 8-15 mm of diameter nodules or extruded into rod shape of dimension of 25 mm diameter x 150-200 mm lengths with polymeric binder like dextrin solution, polyvinyl alcohol solution, wax emulsion, methylcellulose, etc., maintaining the binder percentage between 1& 5%. The nodules / extrudes are then dried in oven within 398-623 K so that moisture content is less than 0.5%. The dried nodjule is calcined within a temperature range of 1473-1673 K to obtain 60-100% of spinel phase. The calcined material is ground in ball mill or any other conventional mill for sufficient time to reduce the particle size less than 10 micron. The ground powder is then mixed with polymeric binder like polyvinyl alcohol solution, dextrin solution, methylcellulose, wax emulsion, etc., to form dough in a conventional dough making machine prior to granulation through granulator to a definite size fraction, which lies between -8 to +325 BSS mesh. The dried granule containing less than 2% moisture is pressed in the form of pellets having volume within 15-50 cc under the pressure of 1-3 tons/cm2. The pressed pellets having green density of >2.0 g/cc are fired within 1873-2073 K and soaked for 0.5-2 hour in an electrical /gas/ oil fired kiln like rotary or tunnel or vertical shaft or down draft etc. The sintered pellets having density of >3.35 g/cc, apparent porosity The following Examples are provided merely to illustrative 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

compositions are in weight percent or weight ratios and all densities are in grams per cubic centimeter (g/cc).
EXAMPLE 1
In this experiment, aluminum trihydroxide was mixed and co-ground with caustic MgO in stoichiometric ratio in a steel ball mill using steel media for a period of around 4 hours. The chemical compositions of above alumina and magnesia raw materials are given in Table 1.

The ground mixture was then modulized into sizes of 8-10 mm of diameter nodules with dextrin solution maintaining the binder percentage in it within 3 to 5%. The nodules were then dried in oven at around 423 K to bring down the moisture content at less than 0.5%. The dried nodules were calcined in an electrically operated furnace at 1573 K to obtain not less than 70% of spinel phase. The calcined material was then ground in a ball mill to reduce the particle size (Dioo) to less than 10 microns. The ground powder was then mixed with polymeric binder like polyvinyl alcohol solution to form dough in 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 granule containing less than 2% moisture was pressed in the form of pellets having diameter 20 mm x 10 mm height under the pressure of 2000 kg /cm2. The pressed pellets having green density of 2.2 g/cc were fired at 1923 K for a soaking period of 1 hr in an electrically operated furnace.


EXAMPLE 2
In this example, Chinese Bauxite was used as an alumina raw material and Chinese magnesite was used as a magnesia source. This experiment was done to check the suitability of the present process for manufacture of low-grade spinel (known as common spinel). Both alumina and magnesia raw materials were mixed in 1:1 molar ratio with respect to alumina and magnesia in a conventional mixer for a period of 4 hours. The chemical compositions of Chinese Bauxite and Chinese magnesite are given in Table 2.

The ground mixture was nodulized with dextrin solution maintaining the binder percentage in it within 3 to 5%. The nodules were then dried and calcined in an electrically operated furnace at 1573 K. The calcined material was then ground in a ball mill to reduce the particle size (Dioo) to less than 10 microns. The ground powder was then mixed with polyvinyl alcohol solution to form dough in a conventional dough-making machine prior to granulation through granulator to a

definite size fraction. The dried granule containing less than 2% moisture was pressed in the form of pellets having diameter 20 mm x 10mm height under the pressure of 2000 kg/ cm2 . The pressed pellets having green density of 2.20 g/ cc were fired at 1923 K for a soaking period of 1 hour. The sintered properties of thus obtained magnesium aluminate spinel are as follows :

In this experiment calcined alumina, instead of aluminum tri-hydroxide as in Example 1, has been mixed and co-ground with caustic MgO in stoichiometric ratio in steel ball mill using steel media for a period of around 4 hours. The specification of calcined alumina is given below.

The ground mixture was nodulized with dextrin solution maintaining the binder wpercentage in it within 3 to 5%. The nodules were then dried and oven at around 423 K and calcined in an electrically operated furnace at 1573 K. The calcined material was then ground in a ball mill to reduce the particle size (Dioo) to less than 10 microns. The ground powder was then mixed with polymeric binder solution to form dough in a conventional dough-making machine prior to granulation through granulator to a definite size fraction. The dried granule containing less than 2% moisture was pressed in the form of pellets having diameter 20 mm x 10mm height

under the pressure of 2000 kg/ cm2. The pressed pellets having green density of 2.05 g/ CO were fired at 1923 K for a soaking period of 1 hour.

EXAMPLE 4
In this experiment the mixed and co-ground mass of aluminum, tri-hydrate and caustic MgO was nodulized and dried in oven. The dried nodules were calcined in an electrically operated furnace at 1373 K instead of 1573 K as in example 1. The extent of spinelization was found to be around 50%. The calcined material was ground in a ball mill to reduce the particle size (Dioo) to less than 10 microns followed by mixing with polymeric binder prior to granulation through granulator to a definite size fraction, which lies between 0.15 mm to 1.5 mm. The dried granule containing less than 2% moisture was pressed in the form of pellets having diameter 20 mm x 10 mm height under the pressure of 2000 kg/ cm2. The pressed pellets having green density of 1.95-g/cc were fired at 1923 K for a soaking period of 1 hour in an electrically operated furnace. The sintered properties of thus obtained magnesium aluminate spinel are as follows:


EXAMPLE 5
In this experiment the mixed and co-ground mass of aluminum, tri-hydrate and caustic MgO was nodullzed and dried in oven. The dried nodules were calcined in an electrically operated furnace at 1573 K to obtain not less than 70% of spinel phase. The calcined material was then ground in a ball mill to reduce the particle size (D100) to less than 15 microns instead of 10 microns as in earlier examples. The ground powder was then granulated and pressed in the form of pellets having diameter 20 mm x 10 mm height under the pressure of 2000 kg/ cm2. The pressed pellets having green density of 2.0-g/ cc were fired at 1923 K for a soaking period of 1 hour in an electrically operated furnace. The sintered properties of thus obtained magnesium aluminate spinel are as follows:


Advantages of the Invention
1. Utilizes cheap and readily available Indian raw materials
2. The simultaneous calcination step not only economizes but also enhances the efficiency of the process.

3. Raw materials have been selected in such a way so that the calcination
temperature for required spinelization to over come volume expansion barrier during sintering is reduced by more than 100 K.
4. The formation of highly porous and friable powders during simultaneous calcination process ultimately facilitates the ease of grinding operation.
5. The reactivity of the calcined powder is maintained in such a way so that extensive grinding for bringing down the average particle size in sub-micron range is avoided.
6. The process is suitable for producing both low and high purity MgAl204 spinels starting from alumina, bauxite, aluminum hydroxide, diaspore, bayerite, gibbsite as alumina source and MgO, brucite, caustic MgO, magnesite as magnesia source.
7. Magnesium aluminate spinel grains with density >3.35 g/cc is obtained at relatively low temperatures such as 1873-1923 K according to this process.



We Claim :
1. An Improved Process for the Preparation of high purity and low purity magnesium aluminate spinel grains which comprises:
i. Mixing or co-grinding of MgO and AI2O3 yielding materials for a period in the
range of 4 to 6 hours.
ii. Nodulizing or extruding the resultant raw mixture by known methods
iii. Calcining the nodules or extrudes in conventional furnace or kiln within the temperature In the range of 1473 to 1673 K
iv. Grinding the calcined materials to less than 10 micron (D100)
V. Granulating the ground calcined powder within the range of 0.15 mm to 1.5 mm
vi. Pressing or briquetting granules under the pressure in the range of 1800 to 2000 kg / cm2
vii. Sintering the pressed pellets or briquettes at temperatures in the range of 1873 K to 2073 K.
2. An improved process as claimed in claim 1 wherein the alumina source includes aluminum tri-hydroxide, bauxite, diaspore, gibbsite and alumina.
3. An improved process as claimed in claim si & 2 wherein the magnesia source includes MgO, caustic MgO and Magnesite.
4 An improved process as claimed in claims 1 to 3 wherein the purity of raw materials on loss free basis should not be less than 98%.

3. An improved process as claimed in claims 1 to 4 wherein the calcination temperature falls in the range of 1373 K to 1673 K preferably 1523 K to 1623 K.
6. An improved process as claimed in claims 1 to 5 wherein the extent of spinelizaiton on calcination is not less than 60% preferably more than 70%.
7. An improved process as claimed in claims 1 to 6 wherein particle size (D100) of calcined and ground mass is not be more than 10 micron preferably not more than 8 micron and average particle size (D50) is less than 2 micron, preferably less than 1.5 micron.
6. An improved process as claimed in claims 1 to 7 wherein the size fraction of granules of calcined and ground mass lies between 0.15 mm to 1.5 mm
9. An improved process as claimed in claims 1 to 8 wherein the green density of the pressed briquette is greater than 2.0 gm / cc
10. An improved process as claimed in claims 1 to 9 wherein the sintering temperature is between 1873 K and 2073 K
11. An improved process as claimed in claims 1 to 10 which is suitable for producing both low purity and high purity MgAl204 spinel.
12. An improved process for the preparation of high purity magnesium aluminate spinel grains substantially as herein described with reference to the


Documents:

29-mas-1999 abstract duplicate.pdf

29-mas-1999 abstract.pdf

29-mas-1999 claims duplicate.pdf

29-mas-1999 claims.pdf

29-mas-1999 correspondence others.pdf

29-mas-1999 correspondence po.pdf

29-mas-1999 description (complete) duplicate.pdf

29-mas-1999 description (complete).pdf

29-mas-1999 form-1.pdf

29-mas-1999 form-19.pdf

29-mas-1999 form-4.pdf

29-mas-1999 form-5.pdf


Patent Number 200272
Indian Patent Application Number 29/MAS/1999
PG Journal Number 27/2006
Publication Date 07-Jul-2006
Grant Date 02-May-2006
Date of Filing 07-Jan-1999
Name of Patentee INTERNATIONAL ADVANCED RESEARCH CENTRE
Applicant Address POWDER METALLURGY AND NEW MATERIALS OPP. BALAPUR VILLAGE, RCI ROAD, R.R. DISTRICT, HYDERABAD 500 005
Inventors:
# Inventor's Name Inventor's Address
1 MANTRAVADI CHANDRA SEKHAR RAO MPRREFRACTORIES LTD HAVING ITS REGISTERED OFFICE AT , EVEREST, ADITYA ENCLAVE, AMEERPET 500038
2 YASHWANT RAMCHANDRA MAHAJAN INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW MATERIALS, HAVING ITS REGISTERED OFFICE OPP. BALAPUR VILLAGE, RCI ROAD, RR DISTRICT, HYDERABAD 500 005
3 IBRAM GANESH INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW MATERIALS, HAVING ITS REGISTERED OFFICE OPP. BALAPUR VILLAGE, RCI ROAD, RR DISTRICT, HYDERABAD 500 005
4 SUBIR BHATTACHARJEE INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW MATERIALS, HAVING ITS REGISTERED OFFICE OPP. BALAPUR VILLAGE, RCI ROAD, RR DISTRICT, HYDERABAD 500 005
5 RAY JOHNSON INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW MATERIALS, HAVING ITS REGISTERED OFFICE OPP. BALAPUR VILLAGE, RCI ROAD, RR DISTRICT, HYDERABAD 500 005
6 BHASKER PRASAD SAHA INTERNATIONAL ADVANCED RESEARCH CENTRE FOR POWDER METALLURGY AND NEW MATERIALS, HAVING ITS REGISTERED OFFICE OPP. BALAPUR VILLAGE, RCI ROAD, RR DISTRICT, HYDERABAD 500 005
PCT International Classification Number C01F7/16
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA