FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
PROVISIONAL SPECIFICATION
(See section 10, rule 13)
L Title of the invention
A METHOD OF REMOVING FLUORIDE IMPURITIES FROM TRONA
2. Applicant(s)
Name Nationality Address
TATA CHEMICALS LTD rNDIA BOMBAY HOUSE, 24 HOM1 MODI STREET,
MUMBAI-400001

3. Preamble to the description
PROVISIONAL SPECIFICATION
The following specification particularly describes the invention.

The invention relates to the removal or reduction of fluoride impurities from trona.
DESCRIPTION OF RELATED ART
Trona is a mineral comprising sodium sesquicarbonate (Na2C03.NaHC03. 2H2O) and is found in a number of deposits around the world, e.g. in Wyoming (U.S.A.), Magadi (Kenya), Tanzania (Lake Natron), Turkey, Venezuela and Egypt.
Trona is generally used for the manufacture of soda ash by well established techniques which generally comprise milling the trona, which has been mined or dredged depending on the location of the trona deposit, and calcining the crushed material to produce soda ash. The soda ash thus produced generally incorporates a few impurities that were present in the original trona deposit and is therefore less pure than the product of the synthetic ammonia soda process for producing sodium carbonate. Various purification processes have been proposed for producing a purer form of trona-derived soda ash. However, as impurities do not adversely influence some uses of soda ash, the trona-derived soda ash can generally be used without further purification for some common uses.
It is a characteristic of the trona deposits found in Africa, and particularly with the deposit at Magadi (Kenya), that they incorporate sodium fluoride as barnacle-like encrustations on the trona crystals. The encrustations comprise large numbers of small individual crystals of sodium fluoride and the encrustations degrade into their constituent crystals during the initial milling of the trona. The sodium fluoride present in the soda ash produced from the trona is typically at a level of 0.5% to 3%, and more typically 1 % to 1.5 % by weight.
As some uses of the soda ash involve high temperatures, such as steel and glass making, the sodium fluoride gives rise to fluoride containing emissions that are undesirable from an

environmental viewpoint and run the risk of violating environment legislations or norms. On account of the high fluoride content, use of sodium carbonate or sodium bicarbonate from trona has been restricted for some high value applications. Present international standards require soda ash containing less than 0.033% fluoride for certain applications.
Sodium bicarbonate also finds wide use in industry and is often a commercially more viable product than soda ash. The production of sodium bicarbonate from trona also suffers from the limitations of fluoride content, particularly in the case of trona obtained from Lake Magadi in Africa.
Trona or crushed refined soda can be obtained naturally from Lake Magadi and is a relatively cheap material. But the presence of sodium fluoride beyond a certain threshold restricts the use of trona in detergent application and of the end products obtained from trona in high end application such as food and pharmaceuticals. There is felt a need for a process for reducing or removing fluoride in trona such that the sodium bicarbonate, or sodium bicarbonate and soda ash recovered from crude trona has reduced or insignificant levels of fluoride and capable of using as higher value products. It would be desirable to produce sodium bicarbonate with reduced amount of fluoride impurities, preferably < 0.1% by weight so that it can be used in food and pharmaceutical grade products.
DETAILED DESCRIPTION
The invention relates to a method of reducing fluoride content in crude trona. More particularly, the method relates to removal of fluoride from trona using Group II alkaline earth metal salts.

The invention also relates to production of sodium bicarbonate in pure crystalline form from crude trona containing fluoride as impurities.
In accordance with an aspect, an aqueous solution of trona containing fluoride as impurities is prepared. To the solution of trona, calcium or magnesium salt is added. The fluoride ions (F)
OA- 1-1-
present in trona solution react with the, Ca or Mg to form fluoride of calcium or magnesium. The calcium or magnesium fluoride precipitates out as they are insoluble in water. The precipitated calcium or magnesium fluoride is removed from the solution to obtain trona soda with reduced fluoride content.
In accordance with an embodiment, chlorides of calcium and magnesium are used for their easy availability and higher solubility in water compared to other alkaline earth metals.
Calcium chloride (CaCfe) is an ionic compound of calcium and chlorine. It is highly soluble in water and acts a source of calcium ions in solution. In accordance with an embodiment, calcium chloride may be produced directly from limestone (CaCOs). Alternatively calcium chloride is also produced in large amounts as a by-product of the Solvay process.
Magnesium chlorite is a group of the chemical compounds ^«h the formulas MgCl2 and i\s various hydrates MgClz(H20)x. These salts are typical ionic halides, and highly soluble in water. In accordance with an aspect the hydrated magnesium chloride may be extracted from brine or sea water.
When chloride salts of calcium and magnesium are added to the aqueous solution of trona, Ca2+ and Mg2+ ions are produced, by the following reaction:
CaCI2 = Ca2+ + 2Cl"

MgCl2=Mg2+ + 2Cr
The Ca and Mg react with 2F' ions that are present in the crude trona soda by the following reaction:
Ca2+ + 2F = CaF2 (solid)
Mg2+-f-2F = MgF2 (solid)
In accordance with an aspect, the filtrate is dried to recover sodium bicarbonate in purified form such that it contains less fluoride impurities.
The amount of sodium fluoride present can substantially be reduced and pure bicarbonate crystals can be obtained from trona. By way of a specific example, in a sample of trona soda addition of CaCl2 or MgCl2 leads to precipitation of fluoride as CaF2 or MgF2 respectively 1.2 wt%to0.35to0.5wt%.
The teachings of the document allow the use of cheap raw materials such as calcium chloride and magnesium chloride for the removal of fluoride from trona.
The following example is provided to explain and illustrate certain preferred embodiments of the process of the invention.
Examples 1
About 5 g of trona was dissolved in 23 mL water and the solution was filtered. To the solution of trona about 100 mg of CaCl2 was added. A white precipitate appeared. It was filtered and the solution was dried in a desiccator containing P2O5. The white precipitate was dried and analysed for fluoride. The amount of fluoride in the precipitate was 0.45 wt%.

Examples 2
About 5 g of trona was dissolved in 23 mL water and the solution was filtered. To the solution of trona about 180 mg of CaCk was added. A white precipitate appeared. It was filtered and the solution was dried in a desiccator containing P2O5. The white precipitate was dried and analysed for fluoride. The amount of fluoride in the precipitate was 0.35 wt%.
Dated this 6th day of October 2008