DESC:FORM 2

THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See section 10, Rule 13]

PROCESS FOR DEVELOPMENT OF PURE (3N) CALCINED ALUMINA

HINDALCO INDUSTRIES LIMITED, an Indian Company, having its address at 21st Floor, One International Center, Tower 4, Prabhadevi, Near Prabhadevi Railway Station, Senapati Bapat Marg, Mumbai-400013, Maharashtra, India.

Preamble to the description:

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
[001] The present invention relates to a process for preparing high purity alumina, more particularly to a process for preparing at least 99.90% pure calcined alumina for high-end electronic ceramic applications.

BACKGROUND OF THE INVENTION
[002] The requirement for high purity alumina (HPA) has increased because of the increase in market demand for high end ceramic products. The properties of alumina play a vital role, especially particle size, surface area and purity, which are the key factors for the qualification of material. The purity of alumina determines its superior mechanical and electrical properties. Advanced ceramic products, like ceramic electronic products, are the major application where a minimum of 99.90 % pure calcined alumina is required as it will help to have desired electronic or electrical properties of the product made from 3N alumina.
[003] Conventionally known alumina are all costly and require old technology to produce 3N alumina. For instance, US6203773B1 discloses a process for mineralization of alumina. Herein, conversion of alumina to alpha alumina by heating at elevated temperatures in the presence of a mineralizer is disclosed. However, the major drawback is that it discloses the use of fluoride solutions such as potassium fluoride, ammonium fluoride, sodium fluoride, etc. which are harmful.
[004] WO2005123590A1 discloses a process for producing a low-soda alumina comprising calcining aluminum hydroxide in a calciner in the presence of a soda-removal agent, wherein the alumina dust produced in the calciner is sorted by particle size and collected in a dust collector and at least a portion of the collected alumina dust is subjected to a soda-removal process and is then returned to the calciner. The soda-removal agent includes chlorine-based compounds. The main drawback of the disclosure is that after calcination, washing is carried through sulfuric acid which is not cost effective.
[005] In view thereof, there is a need to provide a cost-effective and robust process i.e., to minimize the impact of variation of alumina by producing a minimum of 99.90% pure calcined alumina for high-end electronic ceramic applications. Furthermore, there is a requirement for a process which is easy to scale, and utilizes less harmful or harmless materials, without compromising the properties and impurity level of the resulting 3N alumina.
[006] Thus, there is a need in the art for a process for preparing high purity or 3N alumina which addresses at least the aforementioned problems.

SUMMARY OF THE INVENTION
[007] In one aspect, the present invention is directed to a process for preparing a high purity alumina. The process comprises the steps of a) pre-calcining a hydrate of alumina to obtain a pre-calcined hydrate (PCH); b) leaching the pre-calcined hydrate (PCH) in presence of an acidic solution; and calcining a leached pre-calcined hydrate of step b) in presence of a mineralizer and/or an additive to obtain the high purity alumina.
[008] In an embodiment, the hydrate of alumina is selected from Al(OH)3, AlO(OH), and AlO(OH).
[009] In another embodiment, the pre-calcined hydrate (PCH) has impurities deposited on a surface thereof, the impurities include sodium oxide and other metal oxides such as iron oxide.
[010] In still another embodiment, the pre-calcination in step a) is carried out at a temperature ranging between 400oC to 450oC for a duration ranging between 2 h to 3 h.
[011] In yet another embodiment, the pre-calcined hydrate (PCH) has a BET surface area of more than 150 m2/g determined according to ISO 9277.
[012] In still further embodiment, the acidic solution is an aqueous acidic solution comprising acetic acid.
[013] In a further embodiment, the molar ratio between the acidic solution and the pre-calcined hydrate (PCH) is in the range of 1:1 to 22:1.
[014] In another embodiment, the calcination in step (c) is carried out at a temperature ranging between 1400 °C to 1550 °C for a duration ranging between 1 h to 3 h.
[015] In yet another embodiment, the mineralizer is boric acid, and the additive is mullite.
[016] In a further embodiment, the calcination is carried out in presence of the mineralizer and the additive, the amount of mineralizer ranging between 0.25 wt.% to 0.75 wt.% whereas the amount of additive ranging between 2.5 wt.% to 5.0 wt.%, both wt.% based on the total weight of leached pre-calcined hydrate.
[017] In another embodiment, the high purity alumina has a purity of at least 99.90%. Further, the high purity alumina is used in electro-ceramic applications.
[018] In another aspect, the present invention is directed to a high purity alumina obtained by the above process. The high purity alumina has a BET surface area ranging between 1.0 to 1.5 m2/g determined according to ISO 9277, and particle size Dv50 ranging between 50 microns to 60 microns determined according to ISO 13320.

BRIEF DESCRIPTION OF DRAWINGS
[019] Figure 1 shows a process flow diagram for preparing at least 99.90% pure (3N) calcined alumina, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION
[020] Before the compositions and formulations of the present invention are described, it is to be understood that this invention is not limited to particular compositions and formulations described, since such compositions and formulations may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting since the scope of the present invention will be limited only by the appended claims.
[021] The terms “comprising”, “comprises” and “comprised of” as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms “comprising”, “comprises” and “comprised of” as used herein comprise the terms “consisting of”, “consists” and “consists of”.
[022] Furthermore, the terms “first”, “second”, “third” or “(a)”, “(b)”, “(c)”, “(d)” etc. and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. In case the terms “first”, “second”, “third” or “(A)”, “(B)” and “(C)” or “(a)”, “(b)”, “(c)”, “(d)”, “i”, “ii” etc. relate to steps of a method or use or assay there is no time or time interval coherence between the steps, that is, the steps may be carried out simultaneously or there may be time intervals of seconds, minutes, hours, days, weeks, months or even years between such steps unless otherwise indicated in the application as set forth herein above or below.
[023] In the following passages, different aspects of the present invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.
[024] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some, but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention and form different embodiments, as would be understood by those in the art. For example, in the appended claims, any of the claimed embodiments can be used in any combination.
[025] Furthermore, the ranges defined throughout the specification include the end values as well, i.e., a range of 1 to 10 implies that both 1 and 10 are included in the range. For the avoidance of doubt, the applicant(s) shall be entitled to any equivalents according to applicable law.
[026] As used herein, the term "3N alumina" refers to a type of alumina (aluminum oxide) that has a high level of purity. The "3N" designation represents the purity level, and in this context, "3N" typically stands for 99.90% purity. The "N" stands for "nine," indicating the number of nines after the decimal point. Therefore, 3N alumina means that the material is at least 99.90% pure aluminum oxide, with only trace amounts of impurities. This high level of purity makes it suitable for applications where a clean and uncontaminated material is required, such as in the production of advanced ceramics, precision components, and certain electronic applications. Further in the present context, “high purity alumina” refers to “3N alumina” and therefore, the terminologies may be used interchangeably throughout the description below.
[027] An aspect of the present invention relates to a process for preparing a high purity alumina.
[028] In an embodiment, the process comprises the steps of:
a) pre-calcining a hydrate of alumina to obtain a pre-calcined hydrate (PCH);
b) leaching the pre-calcined hydrate (PCH) in the presence of an acidic solution; and
c) calcining a leached pre-calcined hydrate of step b) in the presence of a mineralizer and/or an additive to obtain the high purity alumina.
[029] In an embodiment, the pre-calcination step is carried out at a temperature ranging between 400oC to 450oC for a duration ranging between 2 h to 3 h. During this step, the hydrate of alumina is soaked when the temperature ranging between 400oC to 450oC is reached. Soaking ensures complete decomposition of the hydrate into PCH and water vapor, promotes homogeneity by distributing heat evenly throughout the PCH, and helps to achieve the desired properties in the final product, i.e., high purity alumina. PCH obtained in this step has a BET surface area of more than 150 m2/g determined according to ISO 9277. Preferably, the BET surface area ranges between 150 m2/g to 200 m2/g.
[030] In another embodiment, the hydrate of alumina is selected from Al(OH)3, AlO(OH), and AlO(OH). Preferably, the hydrate is Al(OH)3. Further in this regard, a specific type of Al(OH)3 is preferred in the present invention - Bayer hydrate or aluminum trihydrate. Bayer hydrate is another name for aluminium hydroxide (Al(OH)3), a white, odorless, and crystalline solid. It is the main product of the Bayer process, the industrial process for extracting aluminum from bauxite ore. Typically, the Bayer process involves dissolving bauxite ore in a hot, concentrated solution of sodium hydroxide (NaOH). This dissolves the aluminum oxide in the bauxite, forming sodium aluminate (NaAlO2). The solution is then cooled, causing the aluminum hydroxide to precipitate out of solution. The precipitate is filtered and washed and then dried to produce Bayer hydrate.
[031] As the surface of the hydrate, after pre-calcination, becomes porous, the acid penetrates onto the surface easily and can remove or reduce the impurities. This is known as leaching PCH with acid. The impurities include major amounts of soda (sodium oxide, Na2O) and other metal oxides such as iron oxide in minor amounts. Thus, the impurities are leached from PCH during reaction with acid. For this, PCH is subjected to leaching in an acidic solution.
[032] In an embodiment, the acidic solution is an aqueous acidic solution comprising acetic acid. Other acids for similar purposes, such as but not limited to citric acid, oxalic acid, and tartaric acid, may also be employed. The person skilled in the art is aware of alternate possibilities in this regard.
[033] Exemplary leaching reaction may be represented as:
2CH3COOH + Na2O = 2CH3COONa + H2O

[034] In another embodiment, a molar ratio between the acidic solution and PCH is in the range of 1:1 to 22:1. Preferably, the molar ratio ranges between 1:1 to 20:1, or 1:1 to 15:1, or 1:1 to 12:1.
[035] Leached PCH, as obtained in step b) is then subjected to washing and filtration prior to calcining in step c). Suitable means for washing and filtration are known to the person skilled in the art.
[036] In another embodiment, the calcination in step c) is carried out at a temperature ranging between 1400 °C to 1550 °C for a duration ranging between 1 h to 3 h. During calcination, the mineralizer and/or additive is added.
[037] Mineralizers have an impact on the calcination temperature (saves energy and risk of thermal decomposition), increase the rate of reaction (by shortening the calcination time), improve the morphology of alumina (control on size, shape, and surface area), and further reduce the impurities (preventing formation of volatile impurities, if any). Suitable mineralizers known in the art include mostly fluoride compounds. However, owing to the harmful impact of fluorides, the use of such compounds is avoided in the present invention. Therefore, the present invention utilizes boron compounds for this purpose. Exemplary boron compounds suitable as mineralizers include borates such as sodium borate, potassium borate, lithium borate, barium borate, calcium borate, rare earth borates; and boron acid such as boric acid.
[038] In a preferred embodiment, the mineralizer is boric acid or H3BO3.
[039] Calcination, being high temperature application, often requires the presence of additives capable of withstanding the temperature. For this purpose, additives having excellent thermal and mechanical properties are preferred. Suitable additive for calcination in the present invention includes mullite.
[040] While Mullite is generally known to be a refractory ceramic material having the chemical formula Al6Si2O13, or more precisely, 3Al2O3·2SiO2, it primarily serves to reduce the impurities present after the pre-calcination and leaching step in the present invention. In particular, Mullite reduces the amount of soda in leached PCH being subjected to calcination. Mullite is often formed during the firing of certain aluminosilicate raw materials, such as kaolinite and alumina, under specific temperature conditions.
[041] Preferably, a combination of the mineralizer and additive is used during calcination. The combination of mineralizer and the additive is added in an amount ranging between 2.0 wt.% to 6.0 wt.% based on the total weight of leached PCH. Preferably, the amount of the mineralizer is in the range of 0.25 wt.% to 0.75 wt.% whereas the amount of additive is in the range of 2.5 wt.% to 5.0 wt.%.
[042] An embodiment of the present invention is shown in Figure 1, wherein PCH is obtained from hydrate, which is then formed as a slurry by adding the aqueous acidic solution during leaching. Thereafter, the leached PCH is subjected to filtering and washing to remove any impurity, prior to subjecting calcination in the presence of mineralizer and/or additive to obtain the high purity alumina or 3N alumina.
[043] Advantageously, the present invention is a robust and cost-effective process for producing 3N alumina. The present invention process is easy to scale, utilizes less harmful or harmless materials, and does not compromise the properties and impurity level of the resulting 3N alumina. The process also minimizes the impact of variation of alumina by producing at least 99.90% pure calcined alumina for high-end electronic ceramic applications.
[044] Another aspect of the present invention relates to a high purity alumina obtained by the above process. Accordingly, the embodiments pertaining to the process are applicable here as well.
[045] The high purity alumina or 3N alumina is at least 99.90% pure. Further, the alumina has a BET surface area ranging between 1.0 to 1.5 m2/g determined according to ISO 9277, and particle size Dv50 ranging between 50 microns to 60 microns determined according to ISO 13320. The high purity alumina can be used in electro-ceramic applications like ceramic electronic substrates, faucets, valves, etc.
[046] EXAMPLES
[047] The following examples are illustrative of the present invention but not limitative of the scope thereof:
[048] General synthesis of 3N alumina in accordance with the present invention:
[049] Bayer hydrate was used as feed which was pre-calcined in a calciner at 430°C for 2 h to 3 h to obtain PCH with BET surface area in the range of 150 m2/g to 160 m2/g. This was followed by leaching PCH with 17.4(N) acetic acid, in different amounts to remove impurities. As shown in Table 1 below, different amounts of acetic acid were diluted in 450 ml of water. In diluted acetic acid, 300g of PCH was added to make a slurry. After leaching for a minimum one hour, the leached PCH was filtered, and washed with 2 liters (2000ml) of hot water twice.
[050] Elemental analysis of samples was carried out using X-ray fluorescence (XRF). Table 1 below represents the process parameters and results for removal or reduction of the impurities on PCH. The process flow diagram of Figure 1 was followed to prepare at least 99.90% pure (3N) calcined alumina.
[051] Table 1: Evaluation of leached PCH post filtering and washing
Sample No. T1 T2 T3 T4 T5 T6 T7 T8 T9
Acetic acid (ml) 1.8 1.8 1.8 5.4 5.4 5.4 9.0 9.0 9.0
Molar Ratio 2:1 2:1 2:1 6:1 6:1 6:1 10:1 10:1 10:1
Elemental analysis by XRF (%)
Fe2O3 0.017 0.019 0.016 0.017 0.019 0.017 0.017 0.018 0.017
Na2O 0.066 0.065 0.06 0.061 0.072 0.066 0.068 0.064 0.061
SiO2 0.028 0.029 0036 0.031 0.027 0.031 0.013 0.014 0.015
TiO2 0.003 0.003 0.004 0.003 0.003 0.002 0.003 0.003 0.003
CaO 0.032 0.032 0.03 0.016 0.016 0.016 0.014 0.014 0.014
SO3 0.031 0.033 0.032 0.033 0.04 0.043 0.03 0.028 0.028

[052] As noted above, it can be observed that the amount of impurities such as soda (Na2O) was reduced to as low as 0.06% due to leaching by acetic acid.
[053] The leached PCH was calcined at 1450°C to 1550°C for 2 h in presence of different amounts of the mineralizer and additive, to obtain at least 99.90% pure (3N) calcined alumina. Various process parameters and results in terms of elemental analysis through X-ray fluorescence (XRF) are summarized in Table 2 below:
[054] Table 2: Development of 3N alumina
Experiments T1 T2 T3 T4
H3BO3 (%) 0.5 0.5 0.5 0.75
Mullite Ball (%) 2.5 2.5 5.0 5.0
Temperature / time 1500 °C/ 2 hours 1500 °C/ 2 hours 1550 °C/ 2 hours 1450 °C/ 2 hours
Elemental Analysis by XRF (%)
Al2O3 99.908 (3N) 99.906 (3N) 99.909 (3N) 99.902 (3N)
Fe2O3 0.018 0.016 0.018 0.017
Na2O 0.01 0.012 0.015 0.017
SiO2 0.029 0.031 0.019 0.024
TiO2 0.002 0.002 0.003 0.002
CaO 0.02 0.018 0.016 0.02
SO3 0.013 0.015 0.02 0.015

[055] From the above Table, it can be concluded that all the impurities have been reduced to an amount of 0.01% and a minimum of 99.90% pure (3N) calcined alumina can be obtained.
[056] Comparative examples
[057] Comparative samples were prepared in accordance with the general synthesis procedure above. However, these samples were prepared in the absence of the additive (refer CE1) or mineralizer (CE2). Results are summarized in Table 3 below:
[058] Table 3: Comparative examples
Experiments CE1 CE2
H3BO3(%) 0.25 0
Mullite Ball (%) 0 5
Temp. 1550 °C/2 hour 1550 °C/2 hour
Elemental Analysis by XRF (%)
Al2O3 99.885 (2N) 99.797 (2N)
Fe2O3 0.017 0.03
Na2O 0.02 0.05
SiO2 0.029 0.06
TiO2 0.002 0.003
CaO 0.033 0.03
SO3 0.014 0.03
SSA 1.72 1.85
[059] As noted above, both CE1 and CE2 resulted in inferior (2N) alumina, which is not in accordance with the present invention. The 2N alumina is not useful for application in high-end electronic ceramic applications, which requires at least 3N alumina.
[060] The effect of leaching was also studied. Herein, PCH was directly subjected to calcination. The results are summarized in Table 4 below.
[061] Table 4: Effect of leaching
Experiments T10 T11
H3BO3(%) 0.5 0.75
Mullite Ball (%) 5 5
Temp. 1550°C/2 hour 1550°C/2 hour
Elemental analysis by XRF (%)
Al2O3 99.847 (2N) 99.856 (2N)
Fe2O3 0.02 0.02
Na2O 0.05 0.04
SiO2 0.03 0.03
TiO2 0.003 0.004
CaO 0.03 0.02
SO3 0.02 0.03
SSA 1.62 1.58
[062] As noted above, the absence of leaching also results in 2N alumina being formed, which is not useful for application in high-end electronic ceramic applications.
[063] Thus, the present invention is a robust and cost-effective process for producing 3N alumina, which is easy to scale, utilizes less harmful or harmless materials, and does not compromise the properties and impurity level of the resulting 3N alumina. Further, the process also minimizes the impact of variation of alumina by producing at least 99.90% pure calcined alumina for high-end electronic ceramic applications.
[064] The foregoing description of the present invention has been set merely to illustrate the present invention and is not intended to be limiting. Since the modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to the person skilled in the art, the present invention should be construed to include everything within the scope of the disclosure.
[065] Further, it will be apparent to the person skilled in the art that various changes and modifications may be made without departing from the scope of the present invention as defined in the following claims.
,CLAIMS:WE CLAIM:
1. A process for preparing a high purity alumina comprising the steps of:
a) pre-calcining a hydrate of alumina to obtain a pre-calcined hydrate (PCH);
b) leaching the pre-calcined hydrate (PCH) in presence of an acidic solution; and
c) calcining a leached pre-calcined hydrate of step (b) in presence of a mineralizer and/or an additive to obtain the high purity alumina.

2. The process as claimed in claim 1, wherein the hydrate of alumina is selected from Al(OH)3, AlO(OH), and AlO(OH).

3. The process as claimed in claim 1 or 2, wherein the pre-calcined hydrate (PCH) has impurities deposited on a surface thereof, the impurities include sodium oxide (majority) and iron oxide (minor).

4. The process as claimed in claims 1 to 3, wherein the pre-calcination in step a) is carried out at a temperature ranging between 400oC to 450oC for a duration ranging between 2 h to 3 h.

5. The process as claimed in claims 1 to 4, wherein the pre-calcined hydrate (PCH) has a BET surface area of more than 150 m2/g determined according to ISO 9277.

6. The process as claimed in claims 1 to 5, wherein the acidic solution is an aqueous acidic solution comprising acetic acid.

7. The process as claimed in claims 1 to 6, wherein a molar ratio between the acidic solution and the pre-calcined hydrate (PCH) is in the range of 1:1 to 22:1.

8. The process as claimed in claims 1 to 7, wherein the calcination in step (c) is carried out at a temperature ranging between 1400 °C to 1550 °C for a duration ranging between 1 h to 3 h.

9. The process as claimed in claims 1 to 8, wherein the mineralizer is boric acid, and the additive is mullite.

10. The process as claimed in claims 1 to 9, wherein the calcination is carried out in presence of the mineralizer and the additive, the amount of mineralizer ranging between 0.25 wt.% to 0.75 wt.% whereas the amount of additive ranging between 2.5 wt.% to 5.0 wt.%, both wt.% based on the total weight of leached pre-calcined hydrate.

11. A high purity alumina obtained by the process as claimed in one or more of claims 1 to 10 having a BET surface area ranging between 1.0 to 1.5 m2/g determined according to ISO 9277, and particle size Dv50 ranging between 50 microns to 60 microns determined according to ISO 13320.

Dated this 11th day of April 2023

Hindalco Industries Limited
By their Agent & Attorney

(Nisha Austin)
of Khaitan & Co
Reg No IN/PA-1390