DESC:FIELD OF THE INVENTION
[001] The present invention relates to fine alumina trihydrate (ATH) and the process of preparing the same. More particularly, the invention relates to a process of preparing fine alumina trihydrate having a particle size less than 90 µm, preferably less than 50 µm with reduced soda content. The fine alumina trihydrate (ATH) exhibits better surface finish and higher mechanical strength.

BACKGROUND OF THE INVENTION
[002] Alumina trihydrate (ATH) is a critical material in various industries. Traditionally, alumina trihydrate has been produced through methods such as the Bayer process, where bauxite is processed using an alkaline solution to extract alumina.
[003] While effective, these conventional methods often result in coarse particle sizes and higher soda content, both of which can negatively impact the material’s performance in non-metallurgical applications. Larger particle sizes reduce the material's surface area, making it less effective for reactions in water treatment and surface coating. Additionally, excessive soda content can interfere with chemical processes and compromise the stability and reactivity of the final product. Further, many non-metallurgical applications require finer particles with higher purity ATH.
[004] Fine ATH is especially valuable for applications where small, consistent particles and low impurity levels are crucial. However, conventional methods often struggle to strike the right balance between particle size reduction and maintaining a low soda content, leading to suboptimal quality for specific industrial uses.
[005] A key factor in the precipitation process of alumina trihydrate is controlling the conditions under which it forms from an aqueous caustic solution. The temperature, supersaturation levels, seed charge, and residence time all significantly influence the particle size distribution and purity of the resulting ATH. When the process conditions are not optimized, it can lead to the formation of larger particles with higher soda content. Conversely, fine particles require precise control over nucleation and growth, which is influenced by the initial seed charge and the degree of supersaturation of the solution.
[006] Historically, there have been attempts to improve the precipitation process through various modifications, such as adjusting the temperature, supersaturation, and seeding techniques. However, these methods often result in either excessive particle growth or inefficient nucleation, leading to products with unsatisfactory properties for certain applications.
[007] Currently, the finer alumina trihydrate (ATH) particles are produced by milling the Bayer precipitate hydrate and then separating them based on their size. However, milling the coarse calcined alumina hydrate of ~90-100 µm in size to less than 50 µm size breaks the hydrate particles, resulting in reduced flowability and other application-related issues.
[008] Further, the addition of milled hydrates to resin and other polymer-based systems leads to a higher viscosity of resin-filler-mix in many non-metallurgical applications. When milled hydrates are added to resin and other polymer-based systems, they exhibit a higher viscosity of resin-filler-mix in many non-metallurgical applications.
[009] The cable industry prefers ATH products produced through the precipitation route, as precipitated hydrate offers advantages over milled hydrates, such as better surface finish and higher mechanical strength.
[010] Currently, the precipitation operation used produces alumina trihydrate particles with a median diameter between 90-120µm and a precipitation yield of about 60-90 g/L. Further, this process results in ATH having a soda content in the range of 0.20-0.40% in the final product.
[011] As industries continue to demand higher performance and more cost-effective materials, the need for a reliable method of producing fine alumina trihydrate with smaller particle sizes and low soda content has become more critical. Advances in this area would not only enhance the performance of alumina-based products in various industrial processes but also provide a more scalable and efficient approach to meeting the evolving needs of non-metallurgical applications.
[012] Therefore, there exists a need for an improved method to produce fine alumina hydrate which is industrially feasible and eliminates the drawbacks associated with the production of alumina trihydrate particles.
[013] Owing to the foregoing problems, the method to produce the fine alumina trihydrate having a particle size < 50 µm is provided which addresses the issues known in the art. Particularly, to address the issues regarding the milling of hydrate particles, reduction in soda content and to provide a better surface finish and mechanical strength.

SUMMARY OF THE INVENTION
[014] In one aspect, the present invention relates to a method for preparing fine alumina trihydrate (ATH), comprising the following steps: (a) leaching or digesting bauxite at a first predetermined temperature with an aqueous alkali solution to produce an alkali aluminate solution; (b) cooling the alkali aluminate solution to a second predetermined temperature; (c) filtering the cooled alkali aluminate solution to remove the insoluble residues; (d) cooling the resulting filtrate to a third predetermined temperature and supersaturating the resulting filtrate solution with respect to alumina trihydrate to obtain supersaturated alkali aluminate liquor; (e) seeding the supersaturated alkali aluminate liquor with an alumina trihydrate seed charge, followed by agitation of the resulting slurry in a reactor for a predetermined time and initiating precipitation from the supersaturated liquor at a fourth predetermined temperature; (f) separating the precipitated fine alumina trihydrate from the slurry followed by washing to achieve the fine alumina trihydrate (ATH).
[015] In another aspect, the invention relates to fine alumina trihydrate (ATH), comprising a median particle size (d50) less than 90 µm, preferably a median particle size (d50) less than 50 µm, and median particle size (d10) less than 30 µm and a soda content (Na2O) less than 0.150 wt%.
[016] In a further aspect, the invention is directed for the use of the fine alumina trihydrate (ATH) for non-metallurgical applications, especially alum, PAC (Poly aluminium chloride) and solid surface applications.
BRIEF DESCRIPTION OF DRAWINGS
[017] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in the context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments.
Figure 1 shows a diagrammatic representation of the method for producing fine alumina trihydrate, according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION
[018] 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.
[019] 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”.
[020] 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. 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.
[021] 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.
[022] 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.
[023] 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 shall be entitled to any equivalents according to applicable law.
[024] In an aspect, the present disclosure relates to a method for preparing fine alumina trihydrate (ATH), comprising the following steps:
(a) leaching or digesting bauxite at a first predetermined temperature with an aqueous alkali solution to produce an alkali aluminate solution;
(b) cooling the alkali aluminate solution to a second predetermined temperature;
(c) filtering the cooled alkali aluminate solution to remove the insoluble residues;
(d) cooling the resulting filtrate to a third predetermined temperature and supersaturating the resulting filtrate solution with respect to alumina trihydrate to obtain supersaturated alkali aluminate liquor;
(e) seeding the supersaturated alkali aluminate liquor with an alumina trihydrate seed charge, followed by agitation of the resulting slurry in a reactor for a predetermined time and initiating precipitation from the supersaturated liquor at a fourth predetermined temperature;
(f) separating the precipitated fine alumina trihydrate from the slurry followed by washing to achieve the fine alumina trihydrate (ATH).
[025] In a preferred embodiment of the method, the aqueous alkali solution in step a) is selected from an Na2CO3 solution. Sodium carbonate is chosen for its lower cost and its ability to produce a more easily manageable caustic solution.
[026] In a preferred embodiment of the method, the aqueous alkali solution is caustic soda (Na2CO3) having concentration in the supersaturated sodium aluminate liquor is in the range of 200 g/L -240 g/L. The concentration of caustic soda in the liquor plays a critical role in achieving the desired supersaturation levels for the precipitation process, ensuring high-quality alumina trihydrate production.
[027] In a preferred embodiment, the ratio of Al2O3 to Na2CO3 in the supersaturated sodium aluminate liquor is in the range of 0.60 to 0.70. The precise ratio ensures the liquor remains sufficiently supersaturated while minimizing the formation of unwanted by-products. This balance plays a key role in optimizing the efficiency and yield of the process, ultimately enhancing the overall quality of the final product.
[028] In a preferred embodiment of the method, the alkali aluminate solution in step a) is selected from a sodium aluminate solution. The sodium aluminate solution is a crucial intermediate in the production of alumina trihydrate, and its composition is carefully controlled to ensure efficient precipitation during the subsequent steps of the process.
[029] In an embodiment, the first predetermined temperatures during the leaching step (a) are in the range of 68°C to 74°C.
[030] In an embodiment, the cooling of the alkali aluminate solution in step (b) is done to a second predetermined temperature range of 67 to 74 oC.
[031] In a preferred embodiment, in step (b) the cooling is done by flashing preferably at a temperature of 140oC to about 100oC.
[032] In an embodiment, flash cooling or flashing helps to rapidly lower the temperature of the alkali aluminate solution, creating favourable conditions for the subsequent precipitation process. This method of cooling aids in the formation of supersaturated solutions, which is essential for the efficient precipitation of alumina trihydrate. Flashing is an energy-efficient cooling method that reduces the overall energy costs associated with the process.
[033] In a preferred embodiment, the cooled alkali aluminate solution is filtered according to step (c) to remove the insoluble residues, primarily iron oxides, titania and sodium aluminum silicate. The filtrate, free from these residues, is then ready for supersaturation and precipitation.
[034] In an embodiment, the degree of supersaturation directly influences the rate of nucleation and precipitation, with higher supersaturation levels generally leading to finer particle sizes. The process parameters are carefully controlled to maintain the desired supersaturation, maximizing the efficiency of the precipitation step.
[035] In a preferred embodiment of the method, the alumina trihydrate supersaturation is in the range of 150 to 160 g/L.
[036] In a preferred embodiment, the predetermined temperature in step (d) is in the range 60 to 75oC.
[037] In an embodiment of the method, the agitation of the resulting slurry is done for a period of about 24 hours or more, preferably 24 to 40 hours.
[038] In a preferred embodiment, the agitation of the resulting slurry is done for a period of 24 hours. The agitation step is done in a continuously stirred tank reactor (CSTR) or a batch reactor.
[039] The precipitation of fine alumina trihydrate (ATH) from the supersaturated liquors is carried out. In an embodiment of the method, the temperature at the start of precipitation is in the range of 60°C to 80°C and the seed charge in the range of 150-600 g/L.
[040] The precipitation of alumina trihydrate from an aqueous caustic solution is carried out at a lower temperature and with a higher seed charge. This higher seed charge triggers the nucleation process, which is important for the production of alumina trihydrate. To achieve this, a previously precipitated alumina trihydrate is added to the supersaturated filtrate of sodium aluminum solution containing alumina trihydrate.
[041] Further, the amount of seed used, and the specific surface area of the seed have an impact on the particle size of the resulting product. According to the method of the invention finer-sized particles are produced with higher seed quantities and higher seed-specific surface areas. Further, the degree of rate of nucleation is also associated with the formation of finer particles, while the level of supersaturation controls the rate of nucleation.
[042] In a preferred embodiment of the method, the median particle size of the seed charge is in the range of 40- 50 µm.
[043] In another aspect, the present invention relates to fine alumina trihydrate (ATH) having median particle size less than 50 µm and soda content less than 0.150 wt%.
[044] The fine alumina trihydrate produced according to the method is separated from the slurry by filtration or centrifugation post precipitation and is useful for non-metallurgical applications.
[045] In an embodiment, the final product is washed with hot water to reduce any residual caustic soda content.
[046] In an aspect, the present invention relates to fine alumina trihydrate (ATH), comprising a median particle size (d50) less than 90 µm and a soda content (Na2O) less than 0.150 wt%.
[047] In an embodiment, the fine alumina trihydrate (ATH) comprises a median particle size (d50) less than 50 µm, and median particle size (d10) less than 30 µm and a soda content (Na2O) less than 0.150 wt%.
[048] In an embodiment, the fine alumina trihydrate (ATH) comprises the soda content (Na2O) less than 0.120 wt%.
[049] In an embodiment, the fine alumina trihydrate (ATH) has a specific surface area in the range of 5 to 12 m²/g.
[050] In an embodiment, the bulk density of the fine alumina trihydrate (ATH) is less than 1.5 g/cm³, and the particle size distribution has a narrow span (d90/d10) of less than 3.
[051] In another aspect, the present invention relates to use of the fine alumina trihydrate (ATH) in non-metallurgical applications.
EXAMPLES
[052] The following experimental examples are illustrative of the invention but not limitative of the scope thereof.
[053] Process of preparing fine alumina trihydrate (ATH): The method for preparing fine alumina trihydrate comprising the following steps:
Step 1- Leaching the hydrate of alumina in the presence of aqueous alkaline solution: Alumina trihydrate contained in bauxite is extracted by leaching or digesting it in an aqueous caustic solution (Na2CO3) under pressure and at an elevated temperature above 100°C. The purpose of this step is to extract the alumina content of the ore as sodium aluminate. During the digestion process, enough alumina is dissolved in the aqueous caustic solution to produce an aluminate solution that was just below the saturation level.
Step 2 - Cooling and filtering saturated alumina extract solution: The obtained solution or slurry is cooled by flashing or pressure reduction to atmospheric pressure to about 100 °C. The cooled solution was then filtered to remove the insoluble residues, primarily iron oxides, titania, and sodium aluminum silicate. The resulting filtrate, sodium aluminum solution is further supersaturated with respect to gibbsite, alumina trihydrate. The solution was further cooled to a temperature in the range 75°C to 60°C, at which temperature the solution is supersaturated with alumina trihydrate. The Na in the super saturated liquor is in the range of 200-260g/L with the ratio of Al2O3 to Na2CO3 ranging between 0.60 to 0.70, preferably ~0.670.
Step 3 - Seeding saturated alumina solution with seed charge: A seed charge of previously precipitated alumina trihydrate is added to the supersaturated liquor. The hydrate precipitation was carried out at the starting temperature range of 80 to 60 °C (precipitation at lower temperature) and the seed having a median particle size in the range of 40 - 50 µm was charged in the range of 150-600 g/L (higher seed charge) at the start of the precipitation. The total circulation time for agitation was in the range of 24 to 40 hours and alumina supersaturation was in the range of 150 to 160 g/L to cause precipitation of alumina trihydrate on the added alumina trihydrate seed particles. The obtained product has median particle size of < 50 µm and soda content (Na2O) < 0.150 wt%. The productivity was thus, increased > 80 g/L due the lower particle size of the alumina trihydrate.
[054] The diagrammatic representation of the method for producing fine alumina trihydrate has been demonstrated in Figure 1. It can be seen that supersaturated liquor (1) is provided with a seed charge (2) and the resulting agglomeration (3) mixed and agitation under circulation for around 6 hours. The agglomeration (3) is allowed to grow (4) for 18 hours to produce the product hydrate (5) having median particle size of < 50 µm and soda content (Na2O) < 0.150 wt%.
[055] The experimental details for preparing fine alumina trihydrate according to the method of the present invention are provided as follows:

Table 1: Precipitation Conditions for Tests
Seed Size (d50), µm 40±5
Seed Charge, g/L As per the DoE
Filling Liquor Caustic, g/L 230
Filling Liquor A/C 0.670
Precipitation Start Temperature, °C As per DoE
Circulation Time, h 24

Table 2: Precipitation Test Results
Test No. Precipitation Conditions Product
d50, um Na2O,
% Productivity,
g/L
1,3 Precipitation Start Temperature, °C 68 - - -
Seed Charge, g/L 329 66 0.152 79
471 58 0.143 80
2,4 Precipitation Start Temperature, °C 73 - - -
Seed Charge, g/L 329 65 0.138 73
471 58 0.134 76
5, 6, 9,10 Precipitation Start Temperature, °C 71 - - -
Seed Charge, g/L 300 66 0.147 72
400 60 0.130 73
500 57 0.123 76
7 Precipitation Start Temperature, °C 67 - - -
Seed Charge, g/L 471 56 0.170 77

Table 3: Precipitation Test Results
Test No. Precipitation Conditions Product
D50, um Na2O,
% Productivity,
g/L

Conf-2 Precipitation Start Temperature, °C 60 - - -
Seed Charge, g/L 471 51.8 0.140 78
500 52.4 0.153 78

550
49.8 \
0.140
80

Conf-3 Precipitation Start Temperature, °C 60 - - -
Seed Charge, g/L 550 49.3 0.137 80

Precipitation Start Temperature, °C 60
Precipitation End Temperature, °C 45
filling Liquor Caustic, g/L 230
Filling Liquor Alumina to Caustic ratio, 0.670
Seed Charge, g/L 550
Seed Size (d50), µm 40±5
Circulation Time, h 24
Table 4: Precipitation Conditions for Producing the Finer Hydrate according to present invention.

[056] Advantageously, the improved method for preparing fine alumina trihydrate (ATH) offers several significant benefits over traditional processes, particularly for non-metallurgical applications. One of the primary benefits is the ability to produce ATH with a particle size distribution that meets the specific demands of industries requiring fine and consistent particles.
[057] In addition to the reduction in particle size, the method also effectively reduces the soda content in the final product. The resulting ATH has a soda content of less than 0.150 wt%, which is a notable improvement over conventional methods that often yield products with higher soda content. Lower soda content improves the chemical stability and reactivity of the alumina trihydrate, ensuring its suitability for sensitive applications where high purity is required, such as in the production of polyaluminum chloride (PAC) and solid surface materials.
[058] Furthermore, the precipitation-based approach used in this method provides a superior surface finish and higher mechanical strength compared to milled ATH products. This is especially important for the cable industry, where fine, precipitated ATH is preferred for its ability to deliver better performance in terms of both aesthetics and durability. The improved mechanical strength and surface finish result in enhanced processing characteristics and end-product quality in a range of applications.
[059] Another key advantage is the industrial feasibility of the process. The method described utilizes carefully controlled parameters such as temperature, supersaturation levels, and seed charge to achieve precipitation conditions, leading to fine ATH production without the need for extensive milling or complex post-processing steps. This makes the process more scalable and cost-effective, offering a reliable way to meet the increasing demand for fine ATH with low soda content in non-metallurgical sectors.
[060] Additionally, the final product’s narrower particle size distribution and low bulk density (less than 1.5 g/cm³) further enhance its handling and processing characteristics, particularly when incorporated into resin and polymer-based systems. The fine ATH exhibits lower viscosity when mixed with these materials, reducing the flow-related issues that are often encountered with milled products. This makes it an ideal choice for applications requiring precise formulation control and ease of processing.
[061] Overall, the method not only improves the physical properties of alumina trihydrate by achieving smaller particle sizes and lower soda content but also enhances its performance in non-metallurgical applications. The resulting fine ATH provides better reactivity, higher purity, superior surface finish, and mechanical strength, while also being more cost-effective and industrially feasible to produce. These advantages position the product as an ideal solution for the growing needs of industries such as water treatment, coatings, and specialty chemical manufacturing.
[062] The foregoing description of the invention has been set merely to illustrate the 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 invention should be construed to include everything within the scope of the disclosure.
,CLAIMS:1. A method for preparing fine alumina trihydrate (ATH), comprising the following steps:
a) leaching or digesting bauxite at a first predetermined temperature with an aqueous alkali solution to produce an alkali aluminate solution;
b) cooling the alkali aluminate solution to a second predetermined temperature;
c) filtering the cooled alkali aluminate solution to remove the insoluble residues;
d) cooling the resulting filtrate to a third predetermined temperature and supersaturating the resulting filtrate solution with respect to alumina trihydrate to obtain supersaturated alkali aluminate liquor;
e) seeding the supersaturated alkali aluminate liquor with an alumina trihydrate seed charge, followed by agitation of the resulting slurry in a reactor for a predetermined time and initiating precipitation from the supersaturated liquor at a fourth predetermined temperature;
f) separating the precipitated fine alumina trihydrate from the slurry followed by washing to achieve the fine alumina trihydrate (ATH).
2. The method as claimed in claim 1, wherein the concentration of aqueous alkali solution in alkali aluminate solution is between 200 g/L to 240 g/L.
3. The method as claimed in claim 1, wherein the aqueous alkali solution in step (a) comprises sodium carbonate (Na2CO3).
4. The method as claimed in claim 1, wherein the alkali aluminate solution obtained in step (a) is a sodium aluminate solution.
5. The method as claimed in claim 1, wherein the ratio of Al2O3 to Na2CO3 in the alkali aluminate solution is in the range of 0.60 to 0.70
6. The method as claimed in claim 1, wherein the first predetermined temperature in step (a) is in the range of 68°C to 74°C.
7. The method as claimed in claim 1, wherein step (b) the cooling of the alkali aluminate solution is at a second predetermined temperature range of 67 to 74oC.
8. The method as claimed in claim 1, wherein step (d) the amount of alumina trihydrate used for supersaturation of the filtrate solution is in the range of 150 to 160 g/L.
9. The method as claimed in claim 1, wherein step (d) the third predetermined temperature in is in the range 60 to 75oC.
10. The method as claimed in claim 1, wherein step (e) the amount alumina trihydrate seed charge is in the range of 150 to 600 g/L, preferably between 550 to 600 g/L.
11. The method as claimed in claim 1, wherein step (e) the alumina trihydrate seed charge has a median particle size in the range of 40 to 50 µm.
12. The method as claimed in claim 1, wherein step (e) the fourth predetermined temperature to initiate precipitation is in the range of 60°C to 80°C, preferably 60°C.
13. The method as claimed in claim 1, wherein step (e) the reactor is selected from a continuously stirred tank reactor (CSTR) or a batch reactor.
14. The method as claimed in claim 1, wherein step (f) the separation of the fine alumina trihydrate from the slurry is by filtration or centrifugation post precipitation.
15. A fine alumina trihydrate (ATH), comprising a median particle size (d50) less than 90 µm and a soda content (Na2O) less than 0.150 wt%.
16. The fine alumina trihydrate (ATH) as claimed in claim 15, comprising a median particle size (d50) less than 50 µm, and median particle size (d10) less than 30 µm and a soda content (Na2O) less than 0.150 wt%.
17. The fine alumina trihydrate (ATH) as claimed in claim 15 and 16, wherein the soda content (Na2O) is less than 0.120 wt%, and the fine alumina trihydrate (ATH) has a specific surface area in the range of 5 to 12 m²/g.
18. The fine alumina trihydrate (ATH) as claimed in claim 15 to 17, wherein the bulk density of the fine alumina trihydrate (ATH) is less than 1.5 g/cm³, and the particle size distribution has a narrow span (d90/d10) of less than 3.