FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
THE PATENTS RULES, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION (See section 10 and rule 13)
TITLE OF THE INVENTION A PROCESS FOR ALUMINA PRECIPITATION
APPLICANTS
Aditya Birla Science and Technology Company Pvt Ltd
of address
Plot No 1 and 1-A/1, Taloja, MIDC, Taluka-Panvel, District Raigad Maharashtra – 410208, India
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes this invention and the manner in which it is
to be performed:

FIELD OF THE INVENTION
[001] The present invention relates to a process for precipitation of alumina using a multistage agitator.
DESCRIPTION OF THE BACKGROUND ART
[002] Efficiency of Bayer’s process strongly depends on the efficiency of alumina precipitation (Loh et al., 2000). Alumina precipitation circuit consist of growth, agglomeration and coarse/fine seed tanks. These tanks use an external stirring medium to suspend the solid particles and uniformly mix throughout the tank to achieve a homogenized alumina concentration which determines hydrate particle size distribution and crystal morphology.
[003] The degree of mixing obtained in given duration depends on the energy supplied to the agitator. It is well known that the geometrical parameters like impeller blade design and tank dimensions strongly influences the flow patterns, mixing behavior and shear rate inside the tank which in turn determines hydrate suspension rate, particle size distribution and alumina yield.
[004] Various types of single and multistage agitators are used in alumina precipitation process. The LIGHTNIN® draft tube impeller fitted with single stage impeller which is most widely used for alumina hydrate suspension in agglomeration and growth stages in alumina plants. Draft tube agitators generate an effective circulation loop top to bottom and can suspends solids with minimum energy requirement (Lane., 2006). However, draft tubes are not effective at higher solid percentage loadings because of single impeller driven flow.
[005] Airlift agitators are being used in a few alumina plants. These agitators do not have any moving parts in which the slurry containing alumina hydrate is lifted by the air itself hence the solid loading capacity of airlift agitators is lower than the draft tube agitators and, they are not useful for plants with high production rates. Few alumina plants adapted Swirl flow technology for precipitation tank mainly to avoid the scales formation. Swirl flow agitator also consists of a single impeller dipped inside the slurry surface and it draws the slurry from bottom to top and discharges in the tangential direction.

[006] For alumina precipitation process, multistage agitators have an upper hand compared to the conventionally used agitators. Many studies are available in literatures focusing different impeller blade designs to improve the mixing efficiency. Kresta., 2012 et al. has done a fundamental analysis on solid transport and off bottom suspension analysis in the mixing tank. Their study revealed that both turbulent eddies and mean kinetic energy are important for solid suspensions.
[007] Based on the flow pattern generated by an impeller agitator are classified either axial flow or radial flow or axial and radial flow agitators. Rushton turbine R100, R130, R200, R300, R510 are some of the most used radial flow impellers. Pitch blade turbines such as Lightnin A200, A310, A312, A620 and Chemineer HE-3 are some of the conventional axial impellers. The flow pattern developed by an axial and radial flow are entirely different hence depending the on the application one should select the agitator.
[008] In an axial downward flow impellers flow recirculates from top to bottom of the tank. When fluid leaves at the bottom, solids present at base are pushed towards walls by the radial velocities and then lifted from there by the help of baffles and container walls. Hence, it requires to have higher radial velocities at the base to avoid solid settling at bottom [ Kresta,2012]. In case of radial flow agitator solids are pushed towards center of base from periphery of tanks where they are lifted.
[009] In view of higher production rates and huge dimensions of alumina precipitation tanks axial impellers with high flow efficiency and minimal power consumption agitators are needed for alumina precipitation process.
SUMMARY OF THE INVENTION
[010] The present invention is conceived to solve the aforementioned problems.
[011] It is an object of the present invention to address the technical, process difficulties of alumina precipitation circuit by adopting to a new multistage agitator combined with broad bladed hydro foil and a pitch bladed turbine at the bottom as shown in Figure 1.

[012] Thus, in an aspect, the present invention is a hydrodynamically and energy efficient agitator is being presented for alumina precipitation circuit.
[013] In an aspect, the present invention provides a method for precipitating alumina using the multistage agitator system, that enhances the flow recirculation by generating a continuous flow circulation loop.
[014] In an aspect, the method for precipitating alumina by using a mixing system having a multi-stage agitator, said agitator consisting of a vertical drive shaft having an impeller assembly coupled to said drive shaft, said impeller assembly comprise of a plurality of impellers having at least three hydrofoil blades extending radially outwardly and oriented downwardly, wherein the outer edge of a hydrofoil blade is 1.5-2.5 times higher than the inner edge and the inner edge is 5-8% shorter than the leading inner edge.
[015] In an aspect, ratio of impeller diameter to hydrofoil blade width is 1-1.5.
[016] In an aspect, the impeller assembly comprises of a bottom impeller having a pitch blade turbine.
BRIEF DESCRIPTION OF THE DRAWINGS
[017] The foregoing summary, as well as the following detailed description of the invention will be better understood when read in conjunction with the appended drawings. For the purpose of assisting in the explanation of the invention, there are shown in the drawings embodiments which are presently preferred and considered illustrative. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown therein. In the drawings:
[018] Figure 1 is a schematic representation of mixing tank right side view, in accordance with the embodiment of present invention.
[019] Figure 2 is a schematic representation of mixing tank left side view, in accordance with the embodiment of present invention.

[020] Figure 3 shows the top plane view of three bladed hydrofoil impeller in accordance with the embodiment of present invention.
[021] Figure 4 shows the side elevational view of three bladed hydrofoil impeller in accordance with the embodiment of present invention.
[022] Figure 5 shows the side elevational view of bottom impeller in accordance with the embodiment of present invention.
[023] Figure 6 illustrates the schematic representation of an ideal flow pattern desired for an energy efficient mixing inside a vertical tank.
DESRIPTION OF THE INVENTION
[024] In describing and claiming the invention, the following terminology will be used in accordance with the definitions set forth below. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. As used herein, each of the following terms has the meaning associated with it in this section. Specific and preferred values listed below for individual process parameters, substituents, and ranges are for illustration only; they do not exclude other defined values or other values falling within the preferred defined ranges.
[025] As used herein, the singular forms "a," "an," and "the" include plural reference unless the context clearly dictates otherwise.
[026] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful, and is not intended to exclude other embodiments from the scope of the invention

[027] As used herein, the terms “comprising” “including,” “having,” “containing,” “involving,” and the like are to be understood to be open-ended, i.e. to mean including but not limited to.
[028] Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described. All publications and other references mentioned herein are incorporated by reference in their entirety. Numeric ranges are inclusive of the numbers defining the range.
[029] The embodiments of present invention are related to improve the yield, efficiency and consistency of solid liquid mixing systems with improved flow distribution and solid suspension. In another embodiment of the present invention, the solid-liquid system may be selected from any solid-liquid system including pharma, process and mineral industry slurry.
[030] In the context of present invention, solid-liquid system is alumina hydrate particles in sodium aluminate solution.
[031] Accordingly, the invention provides a method for precipitation of alumina during using a multistage agitator tank having improved flow efficiency and solid suspension.
[032] The solid-liquid system of the present invention is sodium aluminate solution containing caustic concentration of 235-250 gpl in terms of Na2CO3 with liquor density of ~1300kg/m3 and viscosity of 5-20 Cp.
[033] This solid liquid system is used for continuous operation; these agitation systems are cooled from 100OC to 25~30 OC temperature. The speed of agitator is below 50 revolutions per minute but also depends upon on the solid density and solid loading in the slurry solution.

[034] The mixing tank used in the present invention is a vertical mixing tank as shown in the Figure 1.
[035] The vertical mixing tank with the newly proposed multistage agitator is shown in Figure 1. It contains a side wall 3 and a bottom wall 2, the bottom wall shown in the Figure 1 is a circular, but it may be curved as well. In an embodiment, said mixing tanks may have a top wall 1. The dimensions of the mixing tank vary based on the scale of operation.
[036] The tank has an inlet 4 and an outlet 5 with a riser pipe 6 dipped inside the tank for preventing the short-circuiting whose length is decided based on the working fluid and feeding system. Some of the mixing tanks may not be having the riser pipes 6. The location of inlet and outlet not necessarily be fixed one it can be side entry/exit or top entry/exit.
[037] The mixing tank usually contains either three or four baffles 7, the mixing tank considered in the present invention is most widely employed in the white side precipitation circuit of Bayer’s process. It has only three baffles 7 and the riser pipe 6 will play the role of fourth baffle. The placement of baffles 7 may or may not be equally spaced.
[038] Various embodiments of present invention are shown in Figure 2. The dimensions described here are merely representative one should not treat as limited. The mixing tank comprises of a vertically mounted shaft 8 located at the centre of the tank in some application it can be placed at off-centre as well. The shaft is equipped with gearbox and motor to drive the shaft which are not shown here as they are out of scope of this invention.
[039] In an embodiment, there are 5 stages of impellers 9-13 as shown in Figure 2 which can be directly welded to the shaft 8 or mounted on the hub (not shown here). Impellers are named based on their sequence of arrangement on the shaft 8 topmost one 9 is named as Impeller-1, 10 as Impeller-2, 11 as Impeller-3, 12 as Impeller-4 and bottom most one 13 is named as Impeller-5. The first four stage of impellers 9-12 in Figure 2 are hydro foil blades, identical in the shape and size. However, their sizes are

need not to be same always, it depends on the various constraints such as power consumption, solid loading and the slurry properties.
[040] As shown in Figure 2 first four stages of impellers 9-12 consist of 3 hydrofoil blades at each stage. The shape of these blades shown in Figure 3 and Figure 4 is designed based on inventers vast experience on the mixing systems to achieve an efficient flow distribution and solid suspension of alumina hydrates.
[041] The shaft 8 and impellers 9-13 together called as an agitator, the impeller blades 9-12 are hydrofoils whereas the bottom impeller 13 is a pitch blade turbine which is an embodiment of the present invention generates a continuous flow recirculation loop as shown in Figure 6 which is most preferred for vertical mixing tank. The hydrofoil blades at the top direct the flow from top to bottom whereas bottom pitch blade turbine changes the downward flow to an upward flow by converting axial velocity to radial velocity. This unique shape of impeller blades and agitator can save the energy by almost 20-30% with improved hydrate suspension in precipitation circuit.
[042] The process of precipitation of alumina using the multistage agitator enhances the flow recirculation by generating a continuous flow circulation loop. The improved mixing system is a multi-stage agitator is shown in Figure 6.
[043] The agitator consists of a vertical shaft 8, four stages of hydrofoil impellers with three blades at each stage and a pitch blade turbine 13 at the bottom. In an embodiment of the invention the agitator is having 5 stages of impellers with first four stages 9-12 being hydrofoils and the bottom one being a pitch blade turbine 15. The agitator for an example may have combination of two stages of pitch blade turbines and three stages of hydrofoil blades based on the productivity and power consumption limits.
[044] The fluid flow in the alumina precipitator tank is from the top to towards the bottom of the tank in a continuous circulation manner thereby improving the homogeneity of the suspension.
[045] The multi-stage agitator is more efficient in terms of pumping capability and power consumption is reduced by 20-30%. Another advantage of performing the alumina

precipitation process using such an agitator provides more mean kinetic energy and less turbulence which reduces the alumina particle breakage there by narrow downs the alumina particle size distribution.
[046] The impeller according to the present invention has rounded corners and edges to prevent the particle breakage and to maintain lower shear rates at the edges. The impeller according to present invention can handle slurry with solid loadings of up to 900 GPL.
[047] The ratio of impeller diameter to blade width is 1-1.5. Outer edges of the hydrofoil blades 14 are curved as shown in Figure 4. In a preferred embodiment these blades can directly welded at 5 to the shaft 8 or attached with the hub. The inner edge 21 makes a predetermined angle with the central axis of shaft about 35-50 deg. The length of outside edge 14 is 1.5-2.5 times the leading inner edge 21. The leading inner edge 21 is having approximately 1-1.3 m and is smaller than the trailing inner edge 22 by 30-80 mm. The distance between the leading inner edge 16 to leading outer edge 17 is approximately 1.5-1.8 m. The angle between the first hydrofoil blade and second hydrofoil blade is 110-130 degree. The distance between each impeller stage also known as separation distance which is 0.8-1.5 times the impeller diameter.
[048] The bottom impeller 13 is a pitch blade turbine with four impeller blades as shown in Figure 5. It is purposefully created the bottom impeller to be a narrow bladed to minimize the power consumption. The diameter of bottom impeller is around 0.3 to 0.6 times the tank diameter. In a preferred embodiment the diameter of the bottom impeller may be bigger/smaller than the hydrofoils at the top. The tip width 26 of each blade is roughly 7-10% of individual blade length. The ratio of outer edge length 26 to inner edge 27 is 0.4- 0.6.
[049] The angle between each blade a is 75-90 degrees. The inner edge 27 spans an angle of 25-45 degree with central axis of shaft. In an embodiment the bottom impellers can directly welded to shaft or attached to the hub. The leading edge 25 is flat whereas
lower end of trailing edge 23 is curved and makes an angle * 95-120 degree with outer edge 26. The clearance between the tank bottom and central line of bottom

impeller may vary from 0.3-1 m. This bottom turbine plays a vital role in generating high radial velocities at the tank bottom which helps in lifting the bottom settled solids. This bottom turbine also helps in quickly re-suspending bottom settled solids back into the fluid after the restart of an agitator.
[050] The inventors of the present invention assume that even though alumina precipitation tank is used, any person who is skilled in this art can directly relate embodiments of the present invention to any process involving solid-liquid mixing systems of any other industry. Henceforth, the terms used such as alumina hydrate suspension, alumina slurry may be interpreted to solid-liquid systems of any industry.
IMPROVED ALUMINA SLURRY RECIRCULATION
[051] Precipitation/agglomeration efficiency of alumina hydrate strongly depends on the flow pattern inside the mixing tank. Most of the alumina hydrate precipitation tanks will be 10-20 m by height with L/D being 1.5-2. It is recommended in literature to use an axial downward pumping agitator for this kind of geometries. Axial downward pumping agitators creates a single recirculation loop covering top to bottom of precipitator shown in Figure 6. This single recirculation helps in maintaining the homogenized super saturation levels with less energy input.
[052] In order to prove the mixing efficiency pumping efficiency of this new agitator performance is calculated and compared with conventional two blade agitators of the
alumina refinery in Table 1. Pumping efficiency is calculated as Q/ND3 where Q is downward flow rate generated by an impeller, N is agitator speed and D is impeller diameter. Pumping value is directly proportional to pumping efficiency, an impeller blade with higher pumping efficiency pumps more volume of fluid at the same rpm. In case of an axial flow impellers pumping efficiency also depicts how effective an impeller blade in discharging the flow towards the next impeller. It could be seen that for the same agitator rpm the alumina slurry pumping efficiency is improved by almost 5 times with the current invention.

Pumping Efficiency (-) Conventional agitators Present invention
Impeller-1 0.21 1.08

Impeller-2 0.37 1.19
Impeller-3 0.52 1.85
Impeller-4 0.26 2.1
Table 1
UNIFORM FLOW DISTRIBUTION/ HIGHER MEAN VELOCITY
[053] The nature of axial impellers is to form a single recirculation from top to bottom as shown in Figure 6. Which is more predominant with the present invention because of higher pumping capability compared to conventional agitators in alumina refinery. The jet length of a single recirculation loop observed is 8-10% more with the present invention compared to the conventional two blade agitators. Solid settling at the base is most common issue to the alumina plants, bottom settling has adverse effect on the particle size distribution and hence on product quality. This problem would become better if higher velocities are maintained at the tank bottom. Because of higher pumping capability and strong single recirculation loop, more connected flow pattern with increased mean velocity is achieved with the present invention.
REDUCED POWER CONSUMPTION
[054] To further describe the efficiency of this new agitator agitation Index and power requirement is calculated. The interesting characteristics of this invention is it can suspend the solids uniformly inside the tank at a lower power than conventional used two blade agitator. Table 2 compares the agitation index and power requirement for the conventional two bladed agitators and the newly proposed hydrofoil impeller. With the new impeller agitation index is improved by 50% and power requirement is reduced by almost 30 %. This agitation index reveals how efficient an agitator in imparting the energy supplied to the agitator to alumina slurry. This increased energy and agitation efficiency of new agitator is because of more streamlined mixing with lower turbulence generated by the new impeller blade design. Streamlined mixing also reduces the flow divergence in the core region of mixing tank there by reduces the formation of recirculation vertices and hence power consumption. Decrease in the turbulence level is also an added advantage to the product quality. At higher turbulence level alumina particles are subjected to higher shear rates this will break the particle and creates fines issue to the plants which can be avoided with the new

agitator. The Agitation index is measured as a ratio of volume avg. velocity to impeller tip velocity multiplied by 100.

Conventional agitators Present invention
Power (Kw) 69 47
Agitation Index(-) 12 18
Table 2

We claim:
1) A method for precipitation of alumina in a mixing system comprising of a multistage
agitator,
said agitator comprising of a vertical drive shaft having an impeller assembly coupled to said drive shaft,
an impeller assembly coupled to said drive shaft to be rotated by the drive shaft, said impeller assembly comprising
a plurality of impellers having at least three hydrofoil blades extending radially outwardly and oriented downwardly;
said hydrofoil blade comprising of outer edge, leading outer edge, inner edge, leading inner edge, trailing edge,
wherein the ratio of impeller diameter to hydrofoil blade width is 1-1.5,
wherein each hydrofoil blade of the impeller curves from its inner edge to outer leading edge so that said hydrofoil blade rotates at predetermined angle respect to the central axis of drive shaft,
wherein an outer edge of the hydrofoil blade is longer than the inner edge and the inner edge is shorter than the leading inner edge.
2) The method as claimed in claim 1, wherein said feed in the process is a sodium aluminate solution
3) The method as claimed in claim 1, wherein concentration of said sodium aluminate is in the range of 235-250 gpl and density in the range of 1200-1300kg/m3.
4) The method as claimed in claim 1, wherein viscosity of said sodium aluminate is in the range of 5-20Cp.
5) The method as claimed in claim 1, wherein the process is carried out at a temperature of 100 to 30oC.
6) The method as claimed in claim 1, wherein the process is carried out at a speed of 20 -50rpm.