FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
PROVISIONAL
(See section 10 and rule 13)
METHOD OF MANUFACTURING ALUMINIUM FLUORIDE
ADITYA BIRLA SCIENCE & TECHNOLOGY LIMITED
an Indian Company
of Aditya Birla Center, S. K. Ahire Marg, Worli, Mumbai 400 025,
Maharashtra, India
THE FOLLOWING SPEC IFICATION DESCRIBES THE INVENTION.

Field of invention
This invention relates to method of manufacturing aluminium fluoride.
Aluminium fluoride (AIF3) is used as a fluxing agent for the electrolysis of aluminium. The anhydrous aluminium fluoride improve the production yield of aluminum smelters, and lower its electrolytic cost. Aluminium fluoride is also used as a raw material for ceramics such as an optical glass. It was found that such fibers retain much more strength when aged in these aqueous environments than fibers of the more common zirconium fluoride-based composition. In static fatigue, the time to failure of the aluminum fluoride-based fibers is twenty times greater than that of the zirconium fluoride-based fibers.
Aluminium fluoride involves the reaction of aluminium salts with fluorine compound. There are two known methods for manufacturing aluminium fluoride, one is dry method and another is wet method. In dry method, the dry hydrofluoric acid is used as source of fluorine and in wet method where fluorosilicic acid solution or hydrofluoric acid solution is used as a source of fluorine.
A typical process for producing aluminium fluoride from fluorosilicic acid involves weighing hydrated alumina from a feed bin into batch reactor tanks, to which fluorosilicic acid solution is added by pumping from storage tanks through a heat exchanger to the batch reaction tanks. The fluorosilicic acid reacts with hydrated alumina to form a metastable, soluble form of aluminum fluoride trihydrate and silica.


US Patent: 6080210 shows a process for producing aluminum fluoride particles having a smooth surface, comprising the steps of adding aluminum fluoride seed crystals to a super saturated solution of aluminum fluoride; allowing aluminum fluoride particles to be precipitated under heating and agitation; collecting the aluminum fluoride particles by filtration; and drying them, wherein: a super-saturated solution of aluminum fluoride containing 0.6 to 1.3% of fluosilicic acid which is preferably obtained by reacting aluminum hydroxide with fluosilicic acid in an (Al(OH)3)/H2SiF6) molar ratio of 1.8 to 1.9 is used as crystallization of mother liquor.
Objects
One of the objects of the present invention is to provide a process for the preparation of aluminum fluoride anhydride having low silica content.
Another object of the present invention is to provide a process for the preparation of aluminum fluoride anhydride having large particle size.
Yet another object of the present invention is to provide a process for the preparation of aluminum fluoride anhydride, which increases the product yield.
Yet another object of the present invention is to provide a process for the preparation of aluminum fluoride anhydride, which is economical.
Further, another object of the present invention is to provide a process for the preparation of aluminum fluoride anhydride, is fast in reaction.


Further, another object of the present invention is to provide a process for the preparation of aluminum fluoride anhydride, with increased productivity.
In accordance with this invention the aluminium fluoride is produced by the following reaction
H2SiF6 + 2A1 (OH) 3 = 2 A1F3 + SiO2+4H2O
The pre heated temperature of fluorosilicic acid solution and/or its concentration increases, the reaction becomes faster. The heated/un heated aluminium hydroxide or aluminium oxide powder having low moisture content is preferably less than 1%.
In accordance with this invention there is provided a wet method for effectively manufacturing aluminium fluoride anhydride which the comprises following steps-
1) Addition of aluminium hydroxide or aluminium oxide having low
moisture content to a preheated fluorosilicic acid solution in a reactor.
2) The slurry of reaction mass from the reactor is transferred immediately in a continuous solid liquid separator to separate silica cake from aluminium fluoride solution.
3) The filtrate (aluminium fluoride solution) is transferred to crystallization vessel where seed is added after first batch transferred to the vessel by maintaining the temperature.
4) Higher particle size of aluminium fluoride crystals are prepared by adding several batches to crystallization vessel in regular interval. The stirring speed


should be such that it will just sufficient to keep all the aluminium particles in motion.
5) Then aluminium fluoride cake is filtered and subjected to flash heating to remove moisture, then calcined to get anhydrous aluminium fluoride powder having moisture content less than 1%.
In accordance with this invention aluminium hydroxide or aluminium oxide with 98.1% purity is heated to 110°C so that is to reduce moisture content to less than 0.2 %. Addition of aluminium hydroxide or aluminium oxide having low moisture content to preheated fluorosilicic acid in a reactor. The reaction is carried out the temperature between 70 to 100°C. The mole ratio of aluminium hydroxide or aluminium oxide to fluorosilicic acid is 1.90 to 1.99 in the reaction. The process is continued until fluorosilicic acid left in the reactor is less than 0.5% by weight.
The resultant aluminum fluoride solution is metastable, and the trihydrate begins to crystallize out quickly at temperature about 90°C. At the same time precipitated solid silica must removed by transferring slurry of the reaction mass from reactor to solid liquid separator to separate out silica cake from aluminium fluoride solution. The silica cake having moisture content 30% by weight and aluminum fluoride content 10 % by weight. To avoid the aluminium fluoride being contaminated with silica carried out the reaction leads to the following conditions-
a) The silica precipitates out in different forms depending on pH of the reaction mass. At low pH less than 3 the readily filtered forms of silica


predominates and at high pH the amount of fine and colloidal silica increases which makes filtration difficult.
b) The reaction time plus filtering time may not be extended to the point that aluminum fluoride crystallizes out.
c) To obtain a good yield and silica free aluminum fluoride solution, it is important that the fluosilicic acid solution is converted as quickly as possible.
The filtrate is transferred to a crystallization vessel, where aluminium fluoride anhydride crystallizes out at 80 to 95°C. A seed about 5% by weight of total aluminium fluoride is added in filtrate. Several batches of filtrate are added to crystallization vessel in a regular interval to get larger particle size aluminium fluoride crystal. The concentration of aluminium fluoride will be about more than 10% till the last reaction batch transferred to the crystallization vessel.
Aluminium fluoride particles having average diameter more than 90 micrometer can be prepared. The crystallization process is continued till the soluble aluminium fluoride content falls below about 3% by weight. The stirring speed should be such that it will be just sufficient to keep all the A1F3 particles in motion. High stirring speed may break the A1F3 crystals.
The aluminium fluoride cake is separate out by continuous filtration centrifuge is to get cake having moisture content less than 12% by weight. Wet cake of aluminium fluoride is subjected to flash heating at 160°C to remove most of the moisture, then it is calcined at a temperature range 500 to 600°C to get anhydrous aluminium fluoride powder having moisture


content less than 1%. The silica content of aluminum fluoride anhydride obtained is about 0.002% by weight. The moisture content is less than 0.4% by wt and purity is 97% by wt. The isolated product yield is about 93.5%.
In accordance with this invention for the preparation of aluminum fluoride anhydride by wet method gives cheaper process adding small amount of seed to get larger aluminum fluoride anhydride particles. Faster reaction by maintaining temperature during addition of aluminum oxide/hydroxide to hot fluorosilicic acid, and use of continuous filtration lowers the filtration time and moisture content. This will increase both productivity and product yield.
While considerable emphasis has been placed herein on the specific steps of the preferred process, it will be appreciated that many steps can be made and that many changes can be made in the preferred steps without departing from the principles of the invention. These and other changes in the preferred steps of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.