DESC:FIELD
The present disclosure relates to a process for preparing layered spheroidal alumina.
DEFINITIONS
As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicate otherwise.
The term “rho alumina” as used herein refers to activated alumina powder, which is an alumina transition phase. Rho alumina powder is available commercially as hydratable alumina and is used as a precursor or binder in catalysts and refractory manufacturing processes. Hydration of alumina is associated with crystallization and the bonding of particles.
The term “Rho alumina spheroidal form” as used herein refers to Rho alumina having spheroidal/ball structure.
The term “alpha alumina” as used herein refers to alumina which forms hexagonal close-packed structures in which aluminum ions fill two-thirds of the octahedral sites due to their small ionic radii (roughly 0.5 Å).
The term “alpha alumina core” as used herein refers to the porous core of spherical shaped nodule (alpha aluminium oxide nanopowder) constituting inert alpha alumina phase, and is used for proper bonding with rho alumina layers. Further, alpha alumina core has unique heat transfer properties, versatile structure, and general sturdiness make it highly valuable.
The term “alumina spheres” as used herein refers to alumina having spheroidal/ball structure.
The term “layered spheroidal alumina” as used herein refers to alumina having spheroidal shape, which is activated alumina phase coated on core composed of inert alpha alumina phase.
BACKGROUND
The background information herein below relates to the present disclosure but is not necessarily prior art.
Catalysts are used in different processes such as hydrogenation, dehydrogenation, aromatization, oxidation, and the like. The cost of catalysts used in the above mentioned processes is high and therefore, the cost of the entire process or processes increases. Typically, a catalyst comprises a support material (for example- spheroidal alumina) and active metals along with promoters and modifiers impregnated or deposited on the support material.
Conventionally, most of the hydrocarbon conversion processes use catalysts containing noble metals like platinum as the main active metal along with promoters and modifiers deposited on an active spherical alumina support. All components are uniformly dispersed throughout the alumina support. However, it is also known that in these processes selectivity towards desirable products is inhibited by larger residence time of the feed or/and the products at the active matrix of the catalyst.
Layered catalyst supports are very useful in diffusion controlled reactions where the core of the particle is inert and the active components are distributed on the outer shell to keep shorter residence time and easy access of reactant molecules resulting in high selectivity and also reduce coke formation. Such layered catalysts are mainly used in dehydrogenation of hydrocarbons, especially higher alkanes such as normal decane, which are selectively converted to monolefins namely decene.
Core-shell type catalyst support mostly constitutes a gamma alumina layer and core of inert ceramic material. Different types of raw material sources are used for preparing inert core and layer during preparation. Conventionally, the cost of core shell type catalyst is comparatively high. The cost of a particular catalyst can be reduced by reducing the cost for preparing the support material used in a particular catalyst.
There is, therefore, felt a need for an alternate process to prepare a support material which in turn reduces the cost for preparing a catalyst.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows.
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Another object of the present disclosure is to provide a process for preparing layered spheroidal alumina.
Still another object of the present disclosure is to provide a process for preparing a support material in the form of layered spheroidal alumina that is used in a catalyst system.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
The present disclosure provides a process for preparing layered spheroidal alumina. The process comprises converting rho alumina into Rho alumina spheroidal form. The rho alumina spheroidal form is converted to an alpha alumina core at a temperature in the range of 1200 oC to 1350 oC. The alpha alumina core is coated with rho alumina powder to obtain a coated alumina. The coated alumina is hydrated by steaming to obtain alumina spheres. The alumina spheres are dried at a temperature in the range of 100 °C to 120 °C to obtain dried alumina spheres. The dried alumina spheres are autoclaved to obtain an autoclaved alumina spheres. The autoclaved alumina spheres are calcined at a temperature in the range of 950 °C to 1050 °C to obtain a layered spheroidal alumina. The average diameter of the layered spheroidal alumina can be in the range of 1.4 mm to 2.4 mm.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The present disclosure will now be described with the help of the accompanying drawing, in which:
Figure 1 illustrates a graphical representation of the effect of temperature on the crushing strength of the layered spheroidal alumina in accordance with an embodiment of the present disclosure;
Figure 2 illustrates a graphical representation of the effect of calcination temperature on bulk density of the layered spheroidal alumina in accordance with an embodiment of the present disclosure;
Figure 3 illustrates a graphical representation of the effect of calcination temperature on attrition loss of the layered spheroidal alumina in accordance with an embodiment of the present disclosure;
Figure 4 illustrates a graphical representation of the effect of calcination temperature on the surface area of the layered spheroidal alumina in accordance with an embodiment of the present disclosure;
Figure 5 illustrates a graphical representation of the effect of calcination temperature on pore volume of the layered spheroidal alumina in accordance with an embodiment of the present disclosure;
Figure 6 illustrates a graphical representation of the effect of calcination temperature on average pore diameter of the layered spheroidal alumina in accordance with an embodiment of the present disclosure;
Figure 7 illustrates a graphical representation of effect of calcination temperature on meso pore size distribution of the layered spheroidal alumina in accordance with an embodiment of the present disclosure;
Figure 8 illustrates a graphical representation of the effect of calcination temperature on meso and micro pore size distribution of the layered spheroidal alumina in accordance with an embodiment of the present disclosure; and
Figure 9 illustrates a graphical representation of the effect of calcination temperature on surface acidity of the layered spheroidal alumina in accordance with an embodiment of the present disclosure.
DETAILED DESCRIPTION
Embodiments, of the present disclosure, will now be described with reference to the accompanying drawing.
Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.
The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having,” are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.
As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed elements.
The terms first, second, third, etc., should not be construed to limit the scope of the present disclosure as the aforementioned terms may be only used to distinguish one element, component, region, layer or section from another component, region, layer or section. Terms such as first, second, third etc., when used herein do not imply a specific sequence or order unless clearly suggested by the present disclosure.
As described herein above, a catalyst comprises a support material, particularly spheroidal alumina, and active metals along with promoters, and modifiers impregnated, or deposited on the support material. Moreover, the cost of any catalyst used for the processes such as oxidation, hydrogenation, dehydrogenation, and the like is high, which increases the cost of the entire process. One way to reduce the process cost is reduce the cost of the catalyst. The cost of the catalyst can be reduced by reducing the cost for preparing a support material used in the catalyst.
The present disclosure, therefore, provides a process for preparing a support material, particularly layered spheroidal alumina at reduced cost.
The layered spheroidal alumina prepared in accordance with the process of the present disclosure comprises an alpha alumina core and a coating composition for the alpha alumina core. The layered spheroidal alumina obtained by the process of the present disclosure is suitable for various catalytic applications especially long chain paraffin dehydrogenation catalyst.
The process of the present disclosure is carried out in the steps described herein below.
In the first step, rho alumina powder is converted to Rho alumina spheroidal form. Typically, the rho alumina powder is mixed with water to obtain the rho alumina in spheroidal form. The process step of mixing the powdered rho alumina can be carried out in the presence of at least one emulsifying agent selected from the group consisting of polyvinyl alcohol (PVA), polyethylene glycol (PEG), and starch. The process step of preparing the rho alumina in spheroidal form can be carried out in rotating pan mixers, i.e., the powdered rho alumina is mixed with water and/or the emulsifying agent in rotating pan mixers.
In the second step, the rho alumina spheroidal form is converted to an alpha alumina core. In accordance with the present disclosure, the spheroidal form of the rho alumina is hydrated in the presence of steam for a time period in the range of 16 hours to 24 hours to obtain a first hydrated rho alumina. The first hydrated rho alumina is hydrated in an autoclave for a time period in the range of 16 hours to 24 hours to obtain a second hydrated rho alumina. The first hydration process step of hydration can be carried out in the presence of steam at a temperature in the range of 90 °C to 110 °C. The second hydration process step can be carried out at a temperature in the range of 100 °C to 120 °C in autoclave under autogeneous pressure. The so obtained spherical beads after the second hydration process are removed from the autoclave and dried at 120 oC in the presence of air. The dried spherical beads are calcined at a temperature in the range of 1300 °C to 1400 °C to obtain the alpha alumina core. The average diameter of the alpha alumina core can be in the range of 1.4 mm to 1.6 mm.
In the third step, the alpha alumina core is coated with rho alumina powder to obtain a coated alumina. In accordance with the present disclosure, the alpha alumina core, powdered rho alumina, and water (hydration) are mixed to obtain a coated alumina. The present step of hydration is a crucial step, because hydration facilitates bonding of the alumina powder with the alpha alumina core. At least one emulsifying agent selected from the group consisting of polyvinyl alcohol (PVA), polyethylene glycol (PEG), and starch can be mixed with the alpha alumina core, the powdered rho alumina, and water to obtain a coated alumina.
In the fourth step, the coated alumina is hydrated by steaming to obtain alumina spheres. The so obtained alumina spheres are dried in the presence of air at a temperature in the range of 100°C to 120°C to obtain dried alumina spheres.
In the fifth step, the dried alumina spheres are autoclaved in the presence of water for a time period in the range of 16 hours to 24 hours to obtain autoclaved alumina spheres.
In the sixth step, the autoclaved alumina spheres are calcined at a temperature in the range of 950°C to 1050°C to obtain a layered spheroidal alumina.
The average diameter of the layered spheroidal alumina can be in the range of 1.4 mm to 2.4 mm.
The thickness of the coating of the layered spheroidal alumina can be in the range of 200 µm to 500 µm.
The coating of the layered spheroidal alumina further comprises a mixture of kappa alumina, delta alumina, theta alumina, and alpha alumina.
The kappa alumina can be present in an amount ranging from 5 wt% to 20 wt% of the total mixture. The delta alumina can be present in an amount ranging from 5 wt% to 20 wt% of the total mixture. The theta alumina can be present in an amount ranging from 20 wt% to 60 wt% of the total mixture. The alpha alumina can be present in an amount ranging from 5 wt% to 20 wt% of the total mixture.
The layered spheroidal alumina prepared in accordance with the process of the present disclosure has a crushing strength in the range of 1 kg to 3 kg, a pore volume in the range of 0.40 cc/gm to 0.55 cc/gm, a surface area in the range of 20 m2/gm to 60 m2/gm, and a bulk density in the range of 0.75 kg/lit to 0.85 kg/lit.
It is observed that the layered spheroidal alumina prepared in accordance with the process of the present disclosure has attrition loss up to 1%.
The layered spheroidal alumina prepared in accordance with the process of the present disclosure can be used as a support material in a catalyst composition, wherein the support material can be impregnated with desired active metals and promoters for carrying out different processes such as hydrogenation, dehydrogenation, oxidation, and the like.
In accordance with the present disclosure, the amorphous rho alumina phase has capacity to rehydrate back to aluminium tri hydroxides and psudoboehmite phase. This process is associated with crystallization and thus imparts the strength to particles in wet stage. These particles are strengthened further by hydrothermal treatment and ultimately converted to non-active alpha alumina. The non-reactive alpha alumina is used as core for subsequent layering by rho alumina. At this stage again, the alumina layer is hardened by hydrothermal technique but the calcination is limited to 1050 oC temperature to obtain mixture of kappa, delta, and theta type transition alumina. During catalyst preparation, the active transition alumina layer provides the surface for deposition of active metals.
In present disclosure, the core as well as shell of the spherical particle was prepared using cheaper and single source of hydratable rho alumina. The outer shell is prepared by using self-binding, hydratable rho alumina, and an organic emulsifying agent such as polyvinyl alcohol. The recrystallization process during hydration ensures the bonding between the layer and the inner core. The desired physico-mechanical properties and alumina phase composition is obtained by simple hydrothermal and thermal treatments during the formulation stage. Such treatments offers the flexibility to tune the properties like porosity, pore size distribution, surface area, bulk density, and surface acidity, which play critical role for determining the final performance of a catalyst.
The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.
EXPERIMENTAL DETAILS
Experiment 1:
440 gm of Rho alumina powder was loaded into an open pan. The pan was rotated at 25 rpm followed by spraying of 220 gm water at the rate of 6 ml/min from a peristaltic pump at an air pressure of 2-3 PSI to obtain hydrated nodules, which was dried at 110 oC to obtain 564 gm of hydrated dried nodules.
564 gm of hydrated dried nodules were calcined at 1350 oC to obtain 408 gm of calcined nodules. The so obtained calcined nodules were sieved through a sieve and a sieved fraction of 1.4 mm to 2 mm diameter (322 gm) was obtained.
100 gm of so obtained core was taken further for layering. 35 gm of water was sprayed at the rate of 6 ml/min from the peristaltic pump at an air pressure of 2-3 PSI to obtain water filled core followed by addition of 70 gm of Rho alumina powder and 22 gm of water containing 0.5% polyvinyl alcohol to obtain layered alumina spheres. The layered alumina spheres were dried at 110 oC to obtain dried layered alumina spheres (182 gm). The dried alumina spheres were calcined in ceramic tray furnace at 1050 oC to obtain calcined layered alumina spheres. The ambient hot air was recirculated in the furnace. The calcined layered alumina spheres were cooled to 25 oC before discharge and sieved through sieve to obtain layered spheroidal alumina having diameter in the range of 1.4 mm to 2.3 mm.
Table 1 – Ingredients used in the process
Stage Material Unit Qty Stage
Preparation of core Rho Alumina gm 440
Water gm 220
Hydrated dried nodules gm 564 After drying at 110 C
Calcined nodules gm 408 After calcining at 1350 C
Core, (sieved fraction 1.4 mm to 2.0 mm dia. size) gm 322

Layering on core Core gm 100
Water for pore filling of core gm 35
Rho alumina powder for layering gm 70
Water (containing 0.5% Poly Vinyl Alcohol) for layering gm 22
Dryed layered material gm 182 After drying at 110 C
Calcined layerd material gm 146 After calcining at 1050 C
Sieved material ( 1.4 to 2.3 mm dia. Size) gm 95

Experiment 2:
Experiment 1 was repeated except that the autoclaved material was calcined at 1100 °C to obtain a layered spheroidal alumina.
Experiment 3:
Experiment 1 was repeated except that the autoclaved material was calcined at 1150 °C to obtain a layered spheroidal alumina.
Experiment 4:
Experiment 1 was repeated except that the autoclaved material was calcined at 1350 °C in to obtain a layered spheroidal alumina.
The effect of temperature on different physical and mechanical properties on the layered spheroidal alumina was studied as mentioned below.
Effect of temperature on support properties:
Effect on Crushing strength
Individual particle crushing strength was tested (average of 10 particles) on 2 mm diameter layered spheroidal alumina spheres obtained at different calcination temperature and the result obtained is illustrated in Figure 1. From Figure 1 it is observed that particle crushing strength of layered spheroidal alumina involves breaking of layer as well as core. While shattering, peeling off was not observed. It was complete crushing of particle. Practically the crushing strength is remained stable.
Effect on Bulk Density
Layered spheroidal alumina was settled to 100 ml bed in measuring cylinder and weight of material was corrected for moisture correction as illustrated in Figure 2. From Figure 2, it is observed that the Bulk density of layered spheroidal alumina particles increases with calcination temperature. The rise is very sharp after 1050 oC.
Effect on Attrition Loss
Dust generation (- 850 micron) in the process of layered spheroidal alumina on account of tumbling and recorded in terms of percentage, as illustrated in Figure 3. From Figure 3 it is observed that the Attrition loss is nearly stable till 1050 oC calcination temperature.
Effect on surface area
Layered spheroidal alumina calcined at different temperatures was evaluated for BET surface area measurement by using N2 gas at Liquid nitrogen temperature, and the result obtained is depicted in Figure 4. From Figure 4, it is seen that nearly a 20% drop in surface area is observed at 1000 oC whereas an 80% drop is observed by 1150 oC.
Effect on pore volume
Mesopore volume of layered spheroidal alumina was measured by N2 adsorption technique using BJH model and total pore volume was measured by water absorption method, and the result obtained is depicted in Figure 5. From Figure.-5 it is observed that Total pore volume (water PV) goes down gradually with temperature. Only 10% drop is noticed till 1150 oC temperature. This indicates the transformation of Mesopore to Macropore with increase in temperature. In case of Mesopore volume (by BJH) sharp decline is observed after 1000 oC temperature.
Effect on average pore diameter
Variation in average pore diameter (d50) of layered spheroidal alumina by applying BJH is shown in Figure 6. From Figure 6 it is observed that average Mesopore size gradually grows from 240 Å to 450 Å with temperature in 700 oC to 1150 oC.
Effect on meso pore size distribution
Pore size distribution of layered spheroidal alumina was measured by applying BJH model to Nitrogen desorption isotherm. The model is applicable in the range of 17 A to 3000 A pore radius. The results are displayed in Figure 7. The adsorption/desorption profiles of reference (RTGM-587), ALS-10 support (RTGM-615) and support calcined at 1050 C (RTGM-609) are shown in Figure 7 for comparison. From Figure 7, it is observed that with rise in temperature, the profile clearly shows the shifting of pore size distribution to larger size. Figure 8 displays the hysteresis pattern of isotherms of Calcined ALS-10 sample (RTGM-609) shows similar pore texture like ALS-10 (RTGM 615) but with lower mesoporosity and pore widening. The pore texture in reference sample was different.
Effect on meso & Macropore size distribution
Measurement of macro pore distribution can be made reliably by mercury intrusion porosimeter. The distribution was measured for ALS-10 (RTGM 577) as such and after calcination at 1050 oC (RTGM 536) temperature as mentioned in Figure 9. From Figure 9, it is observed that Pore widening as well as Development of Macroporosity (beyond 3000 A of BJH measuring range) is clearly observed with 1050 oC sample (RTGM-577).
Effect on surface acidity
Total surface acidity (Lewis & Brönsted) was measured by temperature program desorption of adsorbed ammonia. As such ALS-10 and after calcination at 1050 oC was measured and reported in Table 1.
Table 1: Effect of calcination temperature on Surface Acidity
Sample Total acidity
RTGM-570, 700 C support 0.445
RTGM-577, 1050 C support 0.354
Reference 0.473

From Table 1, it is observed that catalyst with 1050oC support shows comparatively lower total surface acidity (by 23%).
Effect on Morphology of alumina
As mentioned above, developed supports constitutes core of alpha alumina phase coated with a mixture of transition alumina (activated alumina). The coating portion was calcined separately at 700 oC and 1050 oC and evaluated for phase composition. Powder XRD technique was used, where the diffractograms were processed by using Rietveld method to quantify the crystalline phase composition as mentioned in Table - 2.
Table 2: Effect of calcination temperature on Alumina phase composition of coating
700 C 1050C 1350 C
Gamma 46.8 0.0 0.0
Kappa 0 9.0 0.0
Delta 0 12.3 0.0
Theta 0 39.4 0.0
Alfa 0 7.3 92.6
Amorphous 53.2 31.4 7.4

From Table 2, it is observed that Conventional gamma alumina supports has phase composition similar to 700 °C calcined support. At 1050 °C temperature mixture of Kappa, delta, and theta alumina is obtained. Further, heating to 1350 °C results in conversion of the transition alumina to alpha alumina phase.
Conclusion
From the aforestated experiments, it is concluded that:
• Mechanical integrity remains reasonably intact till 1050 oC temperature (Crushing strength as well as Attrition loss). Bulk density is nearly stable till 1050 oC.
• By 1050 oC temperatures, nearly 50% drop was observed in surface area and 30% in meso pore volume and 10% in total (water) pore volume.
• With temperature, clear shift is observed from meso to macro pores. Average pore diameter increases from 232 Å to 328 Å from 700 oC to 1050 oC.
• Total surface acidity drops down by 23 % at 1050 oC temperatures.
• Gamma / delta alumina phase is reasonably stable till 900 oC. Further elevation in temperature results in transition to Theta and Alpha alumina phases.

• The alumina meets the basic requirements of mechanical integrity, pore texture, pore size, surface acidity, and morphology till calcination temperature of 1050 °C and can be used to develop catalyst to optimize the performance.
TECHNICAL ADVANCES AND ECONOMICAL SIGNIFICANCE
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of a process for preparing layered spheroidal alumina that is economical.
The disclosure has been described with reference to the accompanying embodiments which do not limit the scope and ambit of the disclosure. The description provided is purely by way of example and illustration.
The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein.
The foregoing description of the specific embodiments so fully revealed the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
,CLAIMS:WE CLAIM:
1. A process for preparing layered spheroidal alumina, said process comprising the following steps:
a. converting Rho alumina powder into Rho alumina spheroidal form;
b. converting the Rho alumina spheroidal form to an alpha alumina core;
c. coating the alpha alumina core with Rho alumina powder to obtain a coated alumina;
d. hydrating the coated alumina by steaming to obtain alumina spheres;
e. drying the alumina spheres at a temperature in the range of 100 °C to 120 °C to obtain dried alumina spheres;
f. autoclaving the dried alumina spheres to obtain autoclaved alumina spheres; and
g. calcining the autoclaved alumina spheres at a temperature in the range of 950 °C to 1050 °C to obtain a layered spheroidal alumina.
2. The process as claimed in claim 1, wherein the average diameter of said alpha alumina core is in the range of 1.4 mm to 1.6 mm.
3. The process as claimed in claim 1, wherein the coating of said layered spheroidal alumina further comprises a mixture of kappa alumina, delta alumina, theta alumina, and alpha alumina.
4. The process as claimed in claim 1, wherein the thickness of the coating of said layered spheroidal alumina is in the range of 200 µm to 500 µm.
5. The process as claimed in claim 1, wherein the average diameter of said layered spheroidal alumina is in the range of 1.4 mm to 2.4 mm.
6. The process as claimed in claim 1, wherein the process steps a) and c) are carried out in the presence of water and at least one emulsifying agent selected from the group consisting of polyvinyl alcohol (PVA), polyethylene glycol (PEG), and starch.
7. The process as claimed in claim 1, wherein the process step a) is carried out in rotating pan mixers.
8. The process as claimed in claim 1, wherein the process step d) of steaming is carried out for a time period in the range of 16 hours to 24 hours.
9. The process as claimed in claim 1, wherein the process step d) of autoclaving is carried out in the presence of water and for a time period in the range of 16 hours to 24 hours.
10. The process as claimed in claim 1, wherein the process step b) of converting the rho alumina in spheroidal form to the alpha alumina core is carried out at a temperature in the range of 1200 °C to 1350 °C.