Description:

FORM 2

THE PATENTS ACT, 1970
(39 of 1970)

&

THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See section 10, Rule 13]

A PROCESS FOR PREPARATION OF ULTRAFINE ALUMINIUM HYDROXIDE

HINDALCO INDUSTRIES LIMITED, A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, WHOSE ADDRESS IS AHURA CENTRE, 1ST FLOOR, B-WING, MAHAKALI CAVES ROAD, ANDHERI (EAST), MUMBAI-400 093, MAHARASHTRA, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD OF THE INVENTION
The invention relates to a process for preparation of ultrafine aluminum hydroxide having particle size (d50) of less than or equal to 500 nm to 600 nm. More particularly, the invention relates to an ultrafine aluminum hydroxide of particle size less than or equal to 500 nm.

BACKGROUND OF THE INVENTION
Aluminum hydroxide is used as an inorganic combustion inhibitor of maximum combustion consumption in the world, and it is widely used in many industries such as plastics, building materials, macromolecular material, electronics with the fire-retardant characteristics of nontoxic, halogen and electronic cigarettes. The nano level active aluminum hydroxide specific surface area is bigger, surface energy is higher, flame-retardant effect is better, does not much influence the mechanical property and the processing characteristics of material as additive, and has compensation performance. The super-refinement of aluminum hydroxide, developing high performance synergistic agent, develop surface treatment agent efficiently, is the developing direction of aluminum hydroxide fire retardant.

The aluminum trihydroxide produced by the precipitation of a hot supersaturated solution of sodium aluminate in the presence of an auxiliary seed which has itself been obtained by the precipitation of a supersaturated solution of sodium aluminate in the presence, of ground aluminum trihydroxide having a specific surface area, measured according to the B.E.T. method, of at least one square meter per gram. It is well established in the art to carry out the precipitation of aluminum trihydroxide from a supersaturated solution of sodium aluminate by the addition of a primer consisting of previously crystallized aluminum trihydroxide. The spontaneous generation of seed crystals in a solution of this type has been found to be extremely slow and difficult to produce and even non-existent, depending on the conditions of temperature and concentration of the treated medium.

The Bayer process, which is widely described in specialist literature, is the essential process for the production of aluminum hydroxide which is intended to be converted into aluminum by igneous electrolysis. In the said process, the bauxite is treated in a hot condition by means of an aqueous solution of sodium hydroxide, thus causing solubilization of the aluminum hydroxide and the production of a supersaturated solution of sodium aluminate. After separation of the solid phase constituting the residue of the ore which has not been attacked (red mud), the super saturated solution of sodium aluminate is generally seeded with aluminum hydroxide, which is referred to hereinafter seed, in order to cause precipitation of an aluminum trihydroxide Al (OH)3. For this reason, it is common practice in the Bayer process to favor precipitation of the aluminum trihydroxide from supersaturated sodium aluminate solutions resulting from the alkaline attack of aluminous ores due to the recycling of a considerable fraction of the aluminum trihydroxide which is obtained in a previous cycle.

German Patent No.3,338,186 describes a process for the production of aluminum hydroxide with an average grain diameter under 4 microns and a specific surface between 10 and 25 m2/g. In this process, the actual crystallization is performed at a constant temperature over the entire absorptive precipitation time and crystals are obtained, which exhibit a pronounced platelet shape. This platelet structure is disadvantageous in different uses and also in production. Thus, for example, by the greatly platelet-shaped character, the filtration and washing of the absorptivity precipitated aluminum hydroxide suspension is made more difficult. Suspensions, dispersions, or plastic mixtures, which contain greatly platelet-shaped aluminum hydroxide as fillers, exhibit very high viscosities and cannot be easily filtered. Other drawbacks are the poor hiding power and the poor coating volume in the brushing paint as well as less gloss.

US 4,582,697 describes process for the production of aluminum trihydroxide having controlled median diameter of 2 to 100 microns, with a unimodal distribution and minimum deviation, by precipitation a hot supersaturated sodium aluminate solution in the presence of seed aluminum trihydroxide, separating the resultant solid and liquid phases and recovering the solid phase constituted by precipitated aluminum trihydroxide. The drawback of the said process is that the process provides unsatisfactory solutions as they lead to the production of aluminum trihydroxide whose size is inadequately controlled. Another major drawback of the said process is that the said process is energy extensive, very expensive and requires more labour while producing the aluminium hydroxide.

US 5,127,529 describes a process for the production of a short-prismatic aluminum hydroxide as well as the use of the aluminum hydroxide as spread coating pigment in paper and cardboard coating, as flame-retardant filler in plastics and as white pigment in paints and enamels. The said process also had a lot of drawbacks such as it was a very time-consuming process and it was performed at very high temperatures maintained for long duration, making it cost intensive.

However, from above it can be observed that the process for production of aluminum hydroxide particle having bigger particle had shortcomings. The application or use of final product having bigger particle size, had lower surface area and increased gap separation or voids. Therefore, the particle size of aluminum hydroxide usually used in the day-to-day activities, ranges either in between 100-200 nm or more than 1000 nm.

By the number of publications issued in this field, the specialist literature reveals the importance and the complexity of the research carried out by those skilled in the art to try to provide industrially viable solutions to the above-mentioned problems and to control the size of the aluminum trihydroxide particles. It can also be clearly said that the conventional processes used for the production of aluminum hydroxide having particle size of 100-200 nm cannot be used for mass production. Further, to add if the conventional processes used for the production of aluminum hydroxide having particle size of 100-200 nm is performed for mass production, it shall have the mentioned drawbacks:
• the process usually provides an unsatisfactory solution as they lead to the production of aluminum trihydroxide whose size is inadequately controlled; and
• extremely very cost extensive and energy intensive process.

Since already known in the related prior art documents it can be clearly understood that the particle size is inversely proportional to the surface area. The bigger the particle size the lower will be the surface area and vice versa. The bigger the particle size the more void is seen in been the two particles while placing them together, however, in a finetuned ultrafine ball milled particle, having low surface area, the void size reduces, and the final product appears more intact without voids while placed together. The well-known conventional method for producing aluminium hydroxide having low particle size had drawbacks, such as:
It is difficult to produce and maintain consistency while producing aluminium hydroxide of particle size 100-200 nm. It is also very expensive, requires more labour while producing the low particle sized aluminium hydroxide and while utilising it for the final product and is a very energy intensive process.

The inventors of the present invention have tried overcoming the aforesaid drawbacks that existed in the state of the art.

SUMMARY OF INVENTION:
In one embodiment, the present invention relates to a process for preparation of ultrafine aluminium hydroxide of particle size (d50) of less than or equal to 500 nm to 600 nm. The process for preparation comprises of:
mixing an aluminium hydroxide seed having a particle size (d50) of less than or equal to 500 nm and a surfactant with a pregnant synthetic liquor at a temperature in a range of 55-60°C to precipitate aluminium hydroxide;
adding water to the precipitated aluminum hydroxide to form an aqueous slurry;
drying the aqueous slurry in a spray dryer; and
collecting the spray dried ultrafine aluminum hydroxide from the spray dryer.

Another embodiment of the present invention describes a process for preparation of ultrafine aluminum hydroxide having particle size (d50) of less than or equal to 500 nm to 600 nm, the process comprising:
preparing an aqueous solution of aluminum hydroxide seed comprising 10% by weight of aluminum hydroxide having a particle size (d50) of less than or equal to 500 nm and PEG 400;
preparing a pregnant synthetic liquor comprising alumina and caustic in a weight ratio in a range of 0.60 to 0.70 and PEG 400;
adding the aqueous solution of aluminum hydroxide seed to the pregnant synthetic liquor at a temperature in a range of 55-60°C to precipitate aluminum hydroxide;
filtering and washing the precipitated aluminum hydroxide to a pH range of 7-9.5 and conductivity of less than 100 µS/cm;
adding water to the precipitated aluminium hydroxide to form an aqueous slurry having precipitated aluminium hydroxide in an amount of 20-30% by weight and 2%-5) % by weight of a polymeric additive;
drying the aqueous slurry in a spray dryer at an inlet temperature in a range of 230 °C-250 °C, outlet temperature is in a range of 110 °C-120 °C; and
collecting the spray dried ultrafine aluminium hydroxide from the of the spray dryer.
Another embodiment of the present invention is the product ultrafine aluminum hydroxide particle. The ultrafine particle has specific surface area in the range of 7 m2/g – 15 m2/g; bulk density in the range of 0.20-0.35 g/cc, preferably 0.30 g/cc; packed bulk density in the range of 0.35-0.50 g/cc, preferably 0.42 g/cc and oil absorption is less than or equal to 35-60 g/100 g.

DETAILED DESCRIPTION OF THE INVENTION:
The term “ultrafine aluminum hydroxide/ ultrafine hydrate” refers to superfine aluminum hydroxide having a particle size (d50) less than or equal to 500 nm to 600 nm.

“Pregnant Synthetic Liquor” refers to the solution of alumina and caustic in a weight ratio range of 0.40 to 0.80. In the present invention pregnant synthetic liquor has weight ratio of alumina to caustic in the range of 0.60-0.80 and the amount of caustic used is expressed in terms of sodium carbonate.

“Ball milling” is a grinding method that grinds nanotubes into extremely fine powders. During the ball milling process, the collision between the tiny rigid balls in a concealed container will generate localized high pressure. Usually, ceramic, flint pebbles and stainless steel are used. The ball milling process used for grinding aluminium hydroxide particles are attrition milling or bead milling process.

“Aluminium hydroxide”, “Aluminium trihydrate”, “Al2(OH)3” “Hydrate” are all the synonyms of aluminium hydroxide.

“SDS” stands for sodium dodecyl sulfate.
“CTAB” stands for cetyl trimethyl ammonium bromide.

One embodiment of the present invention describes a process for preparation of ultrafine aluminum hydroxide having particle size (d50) of less than or equal to 500 nm to 600 nm, the process comprising:
mixing an aluminum hydroxide seed having a particle size (d50) of less than or equal to 500 nm and a surfactant with a pregnant synthetic liquor at a temperature in a range of 55-60°C to precipitate aluminum hydroxide;
adding water to the precipitated aluminum hydroxide to form an aqueous slurry;
drying the aqueous slurry in a spray dryer; and
collecting the spray dried ultrafine aluminum hydroxide from the spray dryer.

The aluminum hydroxide seed is obtained from bayerite or gibbsite and is an aqueous dispersion, comprising 10% w/v of aluminum hydroxide.

In one aspect of the present invention mixing of the aforementioned solutions in a manner that half of the total amount of surfactant is added to the aqueous dispersion of aluminum hydroxide seed and the other half of the total amount of surfactant is added in pregnant synthetic liquor before mixing the aluminum hydroxide seed and pregnant synthetic liquor.

The surfactant is selected from a group comprising of polyethylene glycol (PEG 400), cationic cetyl trimethyl ammonium bromide (CTAB), Sodium dodecyl Sulfate (SDS) or a combination thereof, preferably PEG 400.

Another embodiment of the invention describes aluminum hydroxide seed added in an amount of 3g/L-5g/L of the pregnant synthetic liquor; and the surfactant is added in an amount of 3g/L-5 g/L of the pregnant synthetic liquor. The pregnant synthetic liquor used comprises of alumina and caustic in a weight ratio range of 0.60 to 0.70, where the amount of caustic is expressed in terms of sodium carbonate, and the amount of caustic is in a range of 230 – 250 g/L. The precipitation of aluminum hydroxide from the pregnant synthetic liquor is carried out at a temperature in a range of 55-60°C for a period of 12 to 16 hours followed by filtering and washing at a pH range of 7-9.5 and conductivity of less than 100 µS/cm.

The aqueous slurry described in the above embodiment comprises 20% to 30% by weight of the aluminum hydroxide and 2% - 5% by weight of a polymeric additive. The spray drying is carried in a spray dryer at an inlet temperature in a range of 230 °C-250 °C, outlet temperature is in a range of 110 °C-120 °C.

Another embodiment describes a process for preparation of ultrafine aluminum hydroxide having particle size (d50) of less than or equal to 500 nm to 600 nm, the process comprising:
preparing an aqueous solution of aluminum hydroxide seed comprising 10% by weight of aluminum hydroxide having a particle size (d50) of less than or equal to 500 nm and PEG 400;
preparing a pregnant synthetic liquor comprising alumina and caustic in a weight ratio in a range of 0.60 to 0.70 and PEG 400;
adding the aqueous solution of aluminum hydroxide seed to the pregnant synthetic liquor at a temperature in a range of 55-60°C to precipitate aluminum hydroxide;
filtering and washing the precipitated aluminum hydroxide to a pH range of 7-9.5 and conductivity of less than 100 µS/cm;
adding water to the precipitated aluminium hydroxide to form an aqueous slurry having precipitated aluminium hydroxide in an amount of 20-30% by weight and 2%-5% by weight of a polymeric additive;
drying the aqueous slurry in a spray dryer at an inlet temperature in a range of 230 °C-250 °C, outlet temperature is in a range of 110 °C-120 °C; and
collecting the spray dried ultrafine aluminium hydroxide from the of the spray dryer.
Another embodiment of the invention describes that while mixing the aqueous aluminium hydroxide seeds in surfactant, the mixture was constantly stirred. Rotational frequency or rotational speed for the aforesaid mixture was kept constant throughout the precipitation i.e., 20-50 RPM and precipitation time considered was in the range of 12-16 hrs. The precipitation was carried out at 60°C. After the precipitation, the product was filtered and washed with hot water 80-90°C to reduce the soda content in the final product. After vigorous washing of the product in the filter press, the cake was re-slurry in the water. In normal traditional process, 500 nm particles could not be produced.
Another embodiment of the present invention is the ultrafine aluminium hydroxide having particle size (d50) less than or equal to 500 nm to 600 nm, preferably 500 nm. The said particle has specific surface area in the range of 7 m2/g – 15 m2/g. The properties of the said ultrafine aluminium hydroxide particle are - loose bulk density equal to 0.20-0.35 g/cc, packed bulk density is 0.35-0.50 g/cc and oil absorption is less than or equal to 35-60 g/100 g.
Example 1: Preparation of seed particles
The aluminium hydroxide particles were milled to a particle size of d50= <500 nm. The milling process was carried out through attrition milling or bead milling process. Effect of ball milling (BM) on an aluminium powder through a reaction with hot water was investigated. BM increased surface area of the aluminium particles increased crystalline imperfections in the aluminium lattice and removed a native oxide film on surface of the particles. The increase in surface area of the particles was studied by measurement of particle size, SSA (Specific surface area by BET).

The seed obtained was further used for the final precipitation. The specification of milled seed is given bellow in table 1.

Properties Range
Particle size distribution measured by Segigraph
d50, (nm) =500 nm
Specific surface area, SSA (m2/g) >15

Example 2: Process for preparation of ultrafine aluminium hydroxide as per the present invention
Initially, a pregnant liquor was prepared at an alumina to caustic weight ratio of 0.65 and caustic concentration of 240 g/L. An aqueous dispersion of 10% wt of aluminium hydroxide seeds as prepared in Example 1 in water was prepared. 3 g/L of the aqueous aluminium hydroxide seed dispersion was taken, and 1.5 g/L surfactant (PEG 400) was added in the aqueous dispersion under constant stirring condition for 30 minutes. Parallelly, 1.5 g/L surfactant was added in the pregnant synthetic liquor under stirring conditions with a constant stirring for 30 minutes. Both the mixings are carefully perused parallelly in different containers at a temperature of 55-60 °C. Then the aqueous dispersion containing seed was added to the pregnant synthetic liquor for precipitation of aluminium hydroxide.
After performing the mixing of seed mix to the pregnant synthetic liquor the medium was stirred for a period of 15 minutes 400-700 RPM and shall slowly start mixing 20-50 RPM. The said mixture is left for a period of 12-15 hours for precipitation process.
The precipitated batch is stopped and filtered for fine tuning further. To the precipitated batch water is added and an additive to form a re-slurry. The said re-slurry is dried in a spray dryer at an inlet temperature in a range of 230 °C-250 °C, outlet temperature is in a range of 110 °C-120 °C; and collecting the spray dried superfine hydrate from the spray dryer.

Table 2 shows the physical and chemical properties of ultrafine aluminium hydroxide particles obtained by the said process.

Analysis Results of Ultrafine ATH

Elemental analysis by XRF, %
Fe2O3 0.022
Na2O 0.28
SiO2 0.020
TiO2 0.001
PSD (nm) by Sedigraph
d90 900
d50 500
d10 < 500
SSA, m2/g 8.1
XRD Gibbsite
B.D. (Loose), g/cc 0.30
B.D. (Packed), g/cc 0.42
pH 7.5
Oil absorption (linseed oil), g/100g 39
Conductivity, µS/cm 85
Whiteness, % 98
Brightness, % 98.4
Moisture, % 0.24
LOM, % 35.9

Example 3: Process for preparation of ultrafine aluminium hydroxide particle using CTAB as a surfactant
A similar experiment as example 2 was conducted with CTAB (a cationic surfactant) being used a surfactant. However, using CTAB had its disadvantages such as CTAB has a very peculiar smell and is volatile in nature, which causes irritation and sneezing, therefore, the inventors preferred using PEG 400 over CTAB.
Example 4: Process for preparation of ultrafine aluminium hydroxide particle using Sodium dodecyl sulfate (SDS) as a surfactant
A similar experiment as example 2 was conducted with SDS (an anionic surfactant) being used a surfactant. However, using SDS had its disadvantages such as it consumed more water and time for removal of foam during washing. The amount of water and time SDS consumed added additional cost to this process, therefore, the inventor’s preferred using PEG 400.
Results and Discussions:
The obtained ultrafine aluminium hydroxide particle has a particle size (d50) is less than equal to 500 nm to 600 nm, preferably 500 nm; having specific surface area in the range of 7 m2/g – 15 m2/g. The physical properties of the said particles were - loose bulk density equal to 0.20-0.35 g/cc, packed bulk density is 0.35-0.50 g/cc and oil absorption is less than or equal to 35-60 g/100 g.
CTAB when used as a surfactant has a very peculiar smell, which is irritating in nature and causes sneezing, Sodium dodecyl sulfate (SDS) when used for obtaining an ultrafine aluminium hydroxide particle consumed higher amount of water and time, to remove the foam during washing. PEG 400 was preferred as it was safe, water soluble non-ionic and washing of precipitated & finished product – consumed less water as it did not form foam during washing.
The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to a person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.
, Claims:
1. A process for preparation of ultrafine aluminum hydroxide having particle size (d50) of less than or equal to 500 nm to 600 nm, the process comprising:
mixing an aluminum hydroxide seed having a particle size (d50) of less than or equal to 500 nm and a surfactant with a pregnant synthetic liquor at a temperature in a range of 55-60°C to precipitate aluminum hydroxide;
adding water to the precipitated aluminum hydroxide to form an aqueous slurry;
drying the aqueous slurry in a spray dryer; and
collecting the spray dried ultrafine aluminum hydroxide from the spray dryer.
2. The process, as claimed in claim 1, wherein the aluminum hydroxide seed is an aqueous dispersion, comprising 10% w/v of aluminum hydroxide.
3. ``The process, as claimed in claim 1 or 2, wherein half of the total amount of surfactant is added to the aqueous dispersion of aluminum hydroxide seed and the other half of the total amount of surfactant is added in pregnant synthetic liquor before mixing the aluminum hydroxide seed and pregnant synthetic liquor.
4. The process, as claimed in any one of the claims 1-4, wherein the surfactant is selected from a group comprising of polyethylene glycol (PEG 400), cationic cetyl trimethyl ammonium bromide (CTAB), Sodium dodecyl Sulfate (SDS) or a combination thereof.
5. The process, as claimed in claim 4, wherein the surfactant is PEG 400.
6. The process, as claimed in claim 1, wherein aluminum hydroxide seed is added in an amount of (3-5) grams per liters of the pregnant synthetic liquor; and the surfactant is added in an amount of 3-5 grams per liters of the pregnant synthetic liquor.
7. The process, as claimed in claim 1, wherein the pregnant synthetic liquor comprises of alumina and caustic in a weight ratio range of 0.60 to 0.70, where the amount of caustic is expressed in terms of sodium carbonate, and the amount of caustic is in a range of 230 – 250 grams per liter.
8. The process, as claimed in claim 1, wherein the precipitation is carried out for a period of 12 to 16 hours.
9. The process, as claimed in claim 1, comprising filtering and washing aluminum hydroxide to a pH in a range from 7-9.5 and conductivity of less than 100 µS/cm.
10. The process as claimed in claim 1, wherein the aqueous slurry comprises 20% to 30% by weight of the aluminum hydroxide and 2% - 5% by weight of a polymeric additive.
11. The process, as claimed in claim 1, wherein the spray drying is carried in a spray dryer at an inlet temperature in a range of 230 °C-250 °C, outlet temperature is in a range of 110 °C-120 °C.
12. The process, as claimed in claim 1, wherein the yield of the aluminum hydroxide of the process is 140 g/L or more.
13. A process for preparation of ultrafine aluminum hydroxide having particle size (d50) of less than or equal to 500 nm to 600 nm, the process comprising:
preparing an aqueous solution of aluminum hydroxide seed comprising 10% by weight of aluminum hydroxide having a particle size (d50) of less than or equal to 500 nm/0.5 µm and PEG 400;
preparing a pregnant synthetic liquor comprising alumina and caustic in a weight ratio in a range of 0.60 to 0.70 and PEG 400;
adding the aqueous solution of aluminum hydroxide seed to the pregnant synthetic liquor at a temperature in a range of 55-60°C to precipitate aluminum hydroxide;
filtering and washing the precipitated aluminum hydroxide to a pH range of 7-9.5 and conductivity of less than 100 µS/cm;
adding water to the precipitated aluminium hydroxide to form an aqueous slurry having precipitated aluminium hydroxide in an amount of 20-30% by weight and (2-5)% by weight of a polymeric additive;
drying the aqueous slurry in a spray dryer at an inlet temperature in a range of 230 °C-250 °C, outlet temperature is in a range of 110 °C-120 °C; and
collecting the spray dried ultrafine aluminium hydroxide from the of the spray dryer.
14. An ultrafine aluminum hydroxide having particle size (d50) is less than equal to 500 nm to 600 nm, preferably 500 nm.
15. The ultrafine aluminum hydroxide particle, as claimed in claim 14, having specific surface area in the range of 7 m2/g – 15 m2/g.

16. The ultrafine aluminum hydroxide particle, as claimed in claim 14 or 15, having loose bulk density equal to 0.20-0.35 g/cc, packed bulk density is 0.35-0.50 g/cc and oil absorption is less than or equal to 35-60 g/100 g.