Description:FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See section 10, Rule 13]

A SPRAY DRYING PROCESS FOR PREPARATION OF PRECIPITATED SUPERFINE HYDRATE

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 present invention relates to a spray drying process for preparation of precipitated superfine hydrate.

BACKGROUND OF THE INVENTION
Aluminium trihydrate is usually separated from Bauxite ore by Bayer process. In the Bayer process the ore is digested in presence of caustic soda to form a saturated solution of sodium aluminate and insoluble impurities namely red mud. Subsequently, the saturated solution is clarified to remove the insoluble impurities.

Following digestion, aluminium trihydrate is precipitated from the saturated solution by addition of aluminium trihydrate crystals as seed.

Conventional process for preparation of superfine hydrate requires powders consisting only of gibbsite phase as seed in monocrystalline form for precipitating aluminium hydrate from Bayer liquor.

US 6887454 discloses a process for preparation of aluminium hydroxide comprising the steps of seeding a liquid obtained from Bayer process with Bayerite crystals as seed having grain diameter (d50) in a range of 1.2-2.2 µm. The process described in this document involves precipitation of aluminium hydroxide at a temperature in a range of 50°C-70°C and the grain diameter (d50) of the precipitated aluminium hydroxide was in a range of 1.5- 4.5 µm. However, this process uses a higher amount of alkali and does not involve spray drying.

SUMMARY OF THE INVENTION
In an aspect, the invention relates to a spray drying process for preparation of precipitated superfine hydrate. The process comprises preparing a pregnant synthetic liquor comprising alumina and caustic in a weight ratio in a range of 0.50 to 0.65, where the amount of caustic is expressed in terms of sodium carbonate. Then, adding an unstable aluminium hydroxide seed having particle size (d50) of less than or equal to 1.0 µm to the pregnant synthetic liquor to precipitate aluminium hydrate which is then filtered. An aqueous slurry of aluminium hydrate is formed by adding water and an additive, the slurry is subjected to drying in a spray dryer and the spray dried superfine hydrate is collected from the bottom of the spray dryer.

In another aspect, the invention relates to a spray drying process for preparation of precipitated superfine hydrate. The process comprises preparing an unstable aluminium hydroxide seed by forming a pregnant synthetic liquor comprising alumina to caustic in a weight ratio in a range of 0.50 to 0.65, where the amount of caustic is expressed in terms of sodium carbonate. The pregnant synthetic liquor is neutralized with carbon dioxide at a temperature of not more than 20°C to precipitate the unstable aluminium hydroxide seed having particle size (d50) of less than or equal to 1.0 µm. Then adding the unstable aluminium hydroxide seed in a range of 1g/L to 3 g/L to the pregnant synthetic liquor at a temperature in range of 45-65°C to precipitate aluminium hydrate. The precipitated aluminium hydrate is filtered and washed to a pH in a range of 7.5-9.5 and conductivity of less than 60 µS/cm. An aqueous slurry having 50% concentration of aluminium hydrate is formed by adding water. 1.5-3% wt. of a non-ionic polymeric additive is added to the aqueous slurry. The aqueous slurry is subjected to drying in a spray dryer and the spray dried superfine hydrate having particle size (d50) in a range of 1.0 µm to 2.2 µm is collected from the bottom of the spray dryer.

In another aspect, the invention relates to a process for spray drying comprising spray drying an aqueous slurry of precipitated aluminium hydrate in a spray dryer at an inlet temperature in a range of 230-240°C, outlet temperature in a range of 110-115°C and atomizer speed of 24, 000 RPM.

DETAILED DESCRIPTION OF THE INVENTION
The term “superfine hydrate” refers to superfine aluminium hydrate having a particle size (d50) in a range of 1.0-2.2 µm. Aluminium hydrate is also known as aluminium trihydrate or alumina hydroxide. The term “caustic” generally refers to sodium hydroxide. A pregnant synthetic liquor refers to a supersaturated solution of sodium aluminate. The pregnant synthetic liquor comprises aluminium hydrate obtained from Bayer’s process. The terms “unstable aluminium hydroxide seed” and “seed” are used interchangeably throughout the specification and are intended to have the same meaning.

In an aspect, the invention relates to a spray drying process for preparation of precipitated superfine hydrate. The process comprises preparing a pregnant synthetic liquor comprising alumina and caustic in a weight ratio in a range of 0.50 to 0.65, where the amount of caustic is expressed in terms of sodium carbonate. Then, an unstable aluminium hydroxide seed having particle size (d50) of less than or equal to 1.0 µm, preferably, having a particle size (d50) in a range of 0.5 µm to 1.0 µm is added to the pregnant synthetic liquor, to precipitate aluminium hydrate which is then filtered. An aqueous slurry of aluminium hydrate is formed by adding water and an additive, the slurry is subjected to drying in a spray dryer and the spray dried superfine hydrate is collected from the bottom of the spray dryer.

The ratio of alumina to caustic (expressed in terms of sodium carbonate) in the pregnant synthetic liquor in the range of 0.5 to 0.65 provides high productivity of at least 100g/L superfine hydrate. When the ratio is lower than 0.5, productivity is decreased. A ratio higher than 0.65 results in auto precipitation.

The pregnant synthetic liquor has caustic in a concentration in a range of 225 g/L to 235 g/L, where the amount of caustic is expressed in terms of sodium carbonate.

Below the concentration of 225g/L, aluminium trihydrate get dissolved very slowly and if dissolved, the solution is quite unstable resulting in automatic and rapid uncontrolled precipitation. Above the concentration of 235 g/L, the product output is less and the resulting product has high alkali (soda) content.

The unstable aluminium hydroxide seed is added in an amount in a range of 1 g/L to 3 g/L to the pregnant synthetic liquor for precipitating aluminium hydrate. This range of the seed provides superfine hydrate with desired properties such as average particle size, high bulk density (packed and loose), specific surface area (SSA) and low oil absorption.

The precipitation of aluminium hydrate is carried out at a temperature in a range of 45°C-65°C under constant stirring at 40-60 RPM for 48 to 72 hours. The temperature range of precipitation provides superfine hydrate having particle size suitable for packing of the particles when used for applications such as but not limited to fire retardant polymeric cable, silicone insulators, or fire retardant composite.

At temperature below 45°C, chance of fine generation is high, whereby the amount product to collected in the cyclone separator will be large which is not desired.

At temperature of above 65°C, the process results in aluminium hydrate having a low soda content and a higher average particle size (d50). This will result in additional steps of milling to decrease the particle size of the aluminium hydrate to be in a desired average particle size range of 1.0-2.2 µm and it will not remain a precipitated product which cannot be used in desired applications such as but not limited to fire retardant polymeric cable, silicone insulators, or fire retardant composite.

After precipitation, the process further comprises filtering and washing the precipitated aluminium hydrate to a pH in a range of 7.5-9.5 and conductivity of less than 60 µS/cm.

An aqueous slurry of aluminium hydrate is formed with water. The slurry has a concentration of at least 50% aluminium hydrate and a non-ionic polymeric additive is added in a range of 1.5-3% wt. The additive has a viscosity at 25°C in a range of 100-500 mPas. The additive reduces the viscosity of the aqueous slurry to be suitable for pumping during spray drying.

The aqueous slurry has a viscosity in a range of 6-15 seconds at 25°C determined by Ford cup viscosity, preferably, in a range of 6-10 seconds, before spray drying.

Before spray drying, the precipitated aluminium hydrate has loose bulk density of less than or equal to 0.35 g/cc, packed bulk density of less than or equal to 0.50 g/cc.

The aqueous slurry of aluminium hydrate is dried in a fabricated spray dryer. The spray drying parameters include inlet temperature in a range of 230-240°C, outlet temperature in a range of 110-115°C and atomizer speed of 24,000 RPM. The product is collected from the bottom of the spray dryer and has minimum variation of particle size and higher bulk density (packed and loose). The product obtained after spray drying has gibbsite phase.

Spray drying the aqueous slurry provides aluminium hydrate having a reduced particle size i.e., superfine aluminium hydroxide. The use of unstable aluminium hydroxide seed provides aluminium hydroxide having a higher bulk density and lower oil absorption than that obtained from a traditional process using gibbsite seed. Also, the process provides superfine hydrate using less amount of alkali than the known process thereby providing higher efficiency during washing the product such as reducing the amount of water used for washing the product to remove the excess of the alkali.

The spray dried precipitated superfine hydrate has particle size (d50) in range of 1.0 µm to 2.2 µm, specific surface area in a range of 2 m2/g to 4 m2/g, loose bulk density of less than or equal to 0.50 g/cc, packed bulk density of less than or equal to 0.75 g/cc, and oil absorption of less than or equal to 34 g/100 g. The superfine hydrate has a desired brightness of at least 95% and a whiteness of at least 92%.

The spray dried precipitated superfine hydrate obtained by the process is useful for the preparation of preferably but not limited to fire retardant polymeric cable, silicone insulator and fire retardant composites.

In an embodiment, the spray dryer includes at least one cyclone separator, preferably two cyclone separators. In a preferred embodiment, the product obtained from the cyclone separator of the spray dryer is recycled back to the spray dryer. The recycling of the product obtained from the cyclone separator back to the spray dried chamber results in an efficient process that has reduced wastage and provides optimum particle size distribution, desired product density and oil absorption.

The unstable aluminium hydroxide seed for preparation of spray dried precipitated superfine hydrate is prepared separately from a pregnant synthetic liquor having a composition as described above.

The process for preparation of unstable aluminium hydroxide seed comprises neutralizing the above described pregnant synthetic liquor with carbon dioxide (CO2) to precipitate the seed having particle size (d50) less than or equal to 1.0 µm, preferably in a range of 0.5 µm to 1.0 µm.

Neutralization of pregnant synthetic liquor with carbon dioxide is carried out at a temperature of not more than 20°C, preferably, the temperature is in a range of 15°C-20°C.

In an embodiment, the pregnant synthetic liquor having alumina to caustic in a weight ratio in a range of 0.50 to 0.65 as described above is diluted with water in a ratio in range of 80:20 to 70:30 (pregnant synthetic liquor: water).

In an embodiment the step of neutralization of the pregnant synthetic liquor with carbon dioxide carried out till precipitation of the seed from the pregnant synthetic liquor is complete.

The seed has specific surface area of greater than or equal to 20 m2/g.

In an embodiment, the process optionally comprises reducing the particle size of the seed by milling to a particle size (d50) of less than or equal to 1.0 µm. The milling is preferably done in a bead mill.

The unstable aluminium hydroxide seed contains either unstable phase of aluminium hydroxide (bayerite) or a mixture of unstable phase of aluminium hydroxide and gibbsite as stable phase, wherein the content of unstable aluminium hydroxide phase in the mixture is more than 60%.

In an embodiment, for the process further comprises adding 2-3 g/L of the seed described above into another batch of pregnant synthetic liquor at a temperature below 20°C and stirring for 16-20 hours, preferably 18-19 hours to precipitate an unstable aluminium hydroxide seed having a particle size (d50) of less than or equal to 10 µm and specific surface area greater than or equal to 3 m2/g. This unstable aluminium hydroxide seed is subjected to bead milling to reduce the particle size to be less than or equal to 1.0 µm, preferably to reduce the particle size (d50) in a range of 0.5 µm to 1.0 µm and increase the specific surface area to be greater than or equal to 20 m2/g. This process allows mass production of unstable aluminium hydroxide seed which is then used for preparation of precipitated superfine hydrate followed by spray drying as described in preceding paragraphs.

The pregnant synthetic liquor for precipitation of unstable aluminium hydroxide seed having a particle size (d50) of less than or equal to 10 µm and specific surface area greater than or equal to 3 m2/g is undiluted and has alumina to caustic in a weight ratio in a range of 0.50 to 0.65, where the amount of caustic is expressed in terms of sodium carbonate.

In another aspect, the invention relates to a process for spray drying comprising spray drying an aqueous slurry of precipitated aluminium hydrate in a spray dryer at an inlet temperature in a range of 230-240°C, outlet temperature in a range of 110-115°C and atomizer speed of 24, 000 RPM.

EXAMPLES
Example 1: Process for mass production of unstable aluminium hydroxide seed
A pregnant synthetic liquor was produced by dissolving of aluminium trihydrate (Al2O3, 3H2O) in concentrated caustic solution where alumina/caustic ratio was 0.60-0.65. The pregnant synthetic liquor was diluted with water in a ratio of 80:20. The unstable aluminium hydroxide seed was produced from the pregnant synthetic liquor by neutralizing the pregnant synthetic liquor with carbon dioxide (CO2) at a temperature in range of 15-20°C. The neutralization step was continued till precipitation of the seed was complete. The physical properties of the unstable aluminium hydroxide seed are described in Table 1 below:

Table 1:
Properties Range
Particle size distribution measured by Sedigraph.
average required particle size (d50), (µm) =1.0
Specific surface area, SSA (m2/g) >20

Mass production of unstable aluminium hydroxide seed: 2-3g/L of the unstable aluminium hydroxide seed described above was added to another batch of undiluted pregnant synthetic liquor (alumina/caustic ratio of 0.60-0.65) at a temperature of 15-19°C. The mixture was stirred for 18-19 hours to precipitate of an unstable aluminium hydroxide seed having a particle size (d50) of less than or equal to 10 µm and specific surface area greater than or equal to 3 m2/g. This seed was milled in a bead mill to reduce the average particle size (d50) to be in a range of 0.5-1.0 µm and to increase the specific surface area to be greater than or equal to 20 m2/g. The seed contained a mixture of bayerite phase (more than 60%) and gibbsite phase.

Example 2
Preparation of superfine hydrate
A pregnant synthetic liquor was prepared by dissolving aluminium trihydrate in caustic solution having a caustic concentration in a range of 225 g/L-235 g/L (expressed in terms of sodium carbonate [Na2CO3]) and the weight ratio of aluminium trihydrate to caustic was 0.55-0.65. Then, 1 g/L of the unstable aluminium hydroxide seed prepared as described in Example 1 was added to the pregnant synthetic liquor. The resulting solution was maintained at a temperature of 45-65°C and constantly stirred at 40-60 RPM for 48 to 72 hours. Aluminium hydrate precipitated out of the solution was filtered and washed with hot water (80-90°C) to reduce the alkali content in the final product. After vigorous washing of the product in the filter press, an aqueous slurry of the filter cake (product) was formed by adding water and 1.5-3 % wt. of a non-ionic polymeric additive having viscosity in a range of 100-500mPas. The slurry had a concentration of 50% of aluminium hydrate. The viscosity of the slurry at 25°C determined by Ford cup viscosity was 6-10 seconds. The aqueous slurry was spray dried in a fabricated spray dryer at an inlet temperature in a range of 230-240°C, outlet temperature in a range of 110-115°C and atomizer speed of 24,000 RPM. Final product was collected from the bottom of the spray dryer, while cyclone product was recycled back to spray dryer.

Example 3
Aluminium hydroxide was precipitated from the pregnant synthetic liquor by adding 3 g/L of the seed described in Example 1. The process was carried out as described in Example 2.

EVALUATION
Aluminium hydroxide prepared in Example 2 (product 1) and Example 3 (product 2), was evaluated before and after spray drying for its particle size distribution, bulk density (packed and loose), oil absorption, conductivity, pH, and % brightness (reflectance at 457 nm). The %LOI (loss of ignition) of Products 1 and 2 before and after spray drying was in the range of 34.5-34.7 and % moisture was <0.25%. The elemental composition of the product was also determined by XRF (X-ray fluorescence) analysis.

The above parameters were determined by using known procedures. The result of the evaluation is described in Table 2 below:

Table 2
Properties Product 1: Before spray dying Product 2: Before spray dying Product 1: After spray dying Product 2: After spray dying
1g/l unstable aluminium hydroxide seed 3g/l unstable aluminium hydroxide seed 1 g/l unstable aluminium hydroxide seed 3 g/l unstable aluminium hydroxide seed
Particle size distribution (µm)
d90 3 1.8 3 1.6
d50 2.1 1.2 1.9 1.0
d10 1.5 0.9 0.9 0.7
SSA (m2/g) 2.26 3.08 2.19 3.75
Oil Absorption (g/100g) 33.4 32.8 33.6 33.2
Bulk Density (g/cc)
Loose (L) 0.33 0.30 0.45 0.39
Packed (P) 0.48 0.46 0.72 0.56
XRF Analysis (%)
Na2O 0.11 0.12 0.13 0.16
Fe2O3 0.020 0.020 0.027 0.019
SiO2 0.014 <0.001 0.026 0.019
TiO2 0.003 0.002 0.006 0.004
PH 8.6 7.8 8.4 9.2
Conductivity (µS/cm) 50 46 23 46
% Brightness (Reflectance @ 457 nm) 96.3 97.9 95.8 97
%Whiteness 94.7 96.9 93.2 94.8
SSA- Specific Surface Area

The spray drying process provided aluminium hydrate having a particle size (d50) in a range of 1.0 µm -2.2 µm, having oil absorption of less than 34g/100g, alkali (total soda) content of less than 0.20%, loose bulk density of less than or equal to 0.50 g/cc and packed bulk density of less than or equal to 0.72 g/cc.

Further, the results show that increasing the amount of unstable aluminium hydroxide seed to 3 g/L reduced the particle size of the spray dried precipitated aluminium hydroxide such that 50% of the particles had a particle size of 1.0 µm. Thus, the process provided superfine hydrate.

The low oil absorption and low conductivity of the Product 1 and 2 obtained after spray drying makes it suitable for including in fire retardant composites.

The higher bulk density makes the superfine hydrate suitable for application as a cable compound as it will result in increase in the density of the cable compound. Also, the higher brightness of the superfine hydrate increases the shine/colour of the finished cable compound/cable.

The foregoing description of the disclosure 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 spray drying process for preparation of precipitated superfine hydrate, the process comprising:
preparing a pregnant synthetic liquor comprising alumina and caustic in a weight ratio in a range of 0.50 to 0.65, where the amount of caustic is expressed in terms of sodium carbonate;
adding a unstable aluminium hydroxide seed having particle size (d50) of less than or equal to 1.0 µm to the pregnant synthetic liquor to precipitate aluminium hydrate;
filtering the precipitated aluminium hydrate;
adding water and an additive to aluminium hydrate to form an aqueous slurry; drying the aqueous slurry in a spray dryer; and collecting the spray dried superfine hydrate from the bottom of the spray dryer.

2. The process as claimed in claim 1, wherein the pregnant synthetic liquor has caustic in a concentration range of 225 g/L to 235 g/L, where the amount of caustic is expressed in terms of sodium carbonate.

3. The process as claimed in claim 1, wherein unstable aluminium hydroxide seed is added in an amount in a range of 1 g/L to 3 g/L.

4. The process as claimed in claim 1, wherein the unstable aluminium hydroxide seed has a particle size (d50) in a range of 0.5 µm -1.0 µm.

5. The process as claimed in claim 1, wherein the precipitation is carried out at a temperature in a range of 45°C-65°C for 48 to 72 hours.

6. The process as claimed in claim 1, comprising filtering and washing the precipitated aluminium hydrate to a pH in a range of 7.5-9.5 and conductivity of less than 60 µS/cm.

7. The process as claimed in claim 1, wherein the aqueous slurry comprises at least 50% aluminum hydrate and 1.5-3% wt. of a non-ionic polymeric additive.

8. The process as claimed in claim 7, wherein the aqueous slurry of aluminium hydrate has a viscosity in a range of 6-15 seconds at 25°C determined by Ford cup viscosity, before spray drying.

9. 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 230-240°C, outlet temperature in a range of 110-115°C and atomizer speed of 24,000 RPM.

10. The process as claimed in claim 1, wherein the spray dried precipitated superfine hydrate has particle size (d50) in range of 1.0 µm to 2.2 µm, specific surface area in a range of 2 m2/g to 4 m2/g, loose bulk density of less than or equal to 0.50 g/cc, packed bulk density of less than or equal to 0.75 g/cc, and oil absorption of less than or equal to 34 g/100 g.

11. A process for preparation of unstable aluminium hydroxide seed, the process comprising:
preparing a pregnant synthetic liquor as claimed in 1; and
neutralizing the pregnant synthetic liquor with carbon dioxide to precipitate the unstable aluminium hydroxide seed having particle size (d50) of less than or equal to 1.0 µm.

12. The process as claimed in claim 11, wherein the pregnant synthetic liquor is diluted with water in a ratio in range of 80:20 to 70:30 (pregnant synthetic liquor: water).

13. The process as claimed in claim 11, wherein the pregnant synthetic liquor is neutralized with carbon dioxide at a temperature of not more than 20°C.

14. The process as claimed in claim 11, wherein the unstable aluminium hydroxide seed has a specific surface area greater than or equal to 20 m2/g.

15. The process as claimed in claim 11, comprising subjecting the unstable aluminium hydroxide seed to milling to reduce the particle size (d50) to be less than or equal to 1.0 µm.

16. The process as claimed in claim 11, comprising:
adding 2-3 g/L of the seed to a pregnant synthetic liquor at a temperature below 20°C;
stirring for 16-20 hours to precipitate an unstable aluminium hydroxide seed having particle size (d50) of less than or equal to 10 µm and specific surface area greater than or equal to 3 m2/g; and
milling the resulting unstable aluminium hydroxide seed to a particle size (d50) of less than or equal to 1.0 µm and specific surface area greater than or equal to 20 m2/g.

17. The process as claimed in claim 1, comprising adding the unstable aluminium hydroxide seed of claim 16 to a pregnant synthetic liquor comprising alumina to caustic in a weight ratio in a range of 0.50 to 0.65, where the amount of caustic is expressed in terms of sodium carbonate.

18. A spray drying process for preparation of precipitated superfine hydrate, the process comprising:
preparing a unstable aluminium hydroxide seed by forming a pregnant synthetic liquor comprising alumina to caustic in a weight ratio in a range of 0.50 to 0.65, where the amount of caustic is expressed in terms of sodium carbonate;
neutralizing the pregnant synthetic liquor with carbon dioxide at a temperature of not more than 20°C to precipitate the unstable aluminium hydroxide seed having particle size (d50) of less than or equal to 1.0 µm;
adding the unstable aluminium hydroxide seed in a range of 1g/L to 3 g/L to the pregnant synthetic liquor at a temperature in range of 45-65°C to precipitate aluminium hydrate;
filtering and washing the precipitated aluminium hydrate to a pH in a range of 7.5-9.5 and conductivity of less than 60 µS/cm;
adding water to aluminium hydrate to form an aqueous slurry having a concentration of 50% aluminium hydrate;
adding 1.5-3% wt. of a non-ionic polymeric additive to the aqueous slurry;
drying the slurry in a spray dryer; and
collecting the spray dried superfine hydrate having particle size (d50) in range of 1.0 µm to 2.2 µm from the bottom of the spray dryer.

19. The process as claimed in claim 18, comprising:
adding 2-3 g/L of the seed to a pregnant synthetic liquor at a temperature below 20°C;
stirring for 16-20 hours to precipitate a unstable aluminium hydroxide seed having particle size (d50) of less than or equal to 10 µm and specific surface area greater than or equal to 3 m2/g;
milling the resulting unstable aluminium hydroxide seed to a particle size (d50) of less than or equal to 1.0 µm and specific surface area greater than or equal to 20 m2/g; and
adding the milled unstable aluminium hydroxide seed to the pregnant synthetic liquor as claimed in claim 18 to precipitate aluminium hydrate.

20. A process for spray dying, the process comprising:
spray drying an aqueous slurry of precipitated aluminium hydrate in a spray drier at an inlet temperature in a range of 230-240°C, outlet temperature in a range of 110-115°C and atomizer speed of 24,000 RPM.