FORM 2THE PATENTS ACT, 1970 (39 of 1970) &THE PATENTS RULES, 2003PROVISIONAL SPECIFICATION (See section 10, rule 13)
1. Title of the inventionA METHOD OF REMOVING FLUORIDE IMPURITIES FROM TRONA
2. Applicants)
Name Nationality Address
TATA CHEMICALS LTD INDIA BOMBAY HOUSE, 24 HOMI MODI STREET, MUMBAI-400001


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

A METHOD OF REMOVING FLUORIDE IMPURITIES FROM TRONA
The invention relates to the preparation of sodium bicarbonate and soda ash from trona with a reduction of fluoride. In particular, the invention relates to the removal or reduction of fluoride in trona and the preparation of sodium bicarbonate from trona.
DESCRIPTION OF RELATED ART
Trona is a mineral comprising sodium sesquicarbonate (Na2CO3.NaHCO3. 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 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.
Some techniques for the removal of impurities from trona are known, including for example US 2887360, US 3498744, and more specifically methods of removing fluoride from soda ash have been discussed in GB 2337515, GB 1502850 and GB 1495530. However, these documents do not disclose removal of soda ash to levels below the current accepted international standards. Moreover, the processes described are complex requiring heating and cooling and are for the removal of fluoride from the soda ash obtained after the treatment of trona.
Trona or crushed refined soda can be obtained naturally form Lake Magadi and is a relatively cheap material. It would be desirable to convert the trona or crushed refined soda to a higher value product. There is felt a need for a process for obtaining sodium bicarbonate, or sodium bicarbonate and soda ash, from trona such that the sodium

bicarbonate obtained has reduced or insignificant levels of fluoride. As the presence of sodium fluoride beyond a certain threshold restricts the use of the end products in high end application such as food and pharmaceuticals, it is necessary to remove sodium fluoride or reduce it to acceptable levels.
DESCRIPTION OF PREFERRED EMBODIMENTS
The invention relates to a method of producing sodium bicarbonate from trona. The invention relates to a method of producing soda ash from trona. The invention relates to a method of removing or reducing fluoride in trona in the manufacture of sodium bicarbonate or sodium carbonate.
In accordance with an aspect, a solution of trona in water is prepared. Through the solution CO2 is passed such that sodium carbonate in the trona is converted to sodium bicarbonate. Sodium bicarbonate precipitates as it is less soluble as compared to sodium carbonate. The sodium fluoride present in the trona is dissolved in the solution along with the sodium carbonate.
Sodium carbonate can be recovered from the supernatant liquid by the addition of any non-solvent including an organic solvent which will cause precipitation of the sodium carbonate. Alternatively, the supernatant liquid may be dried in vacuum to obtain solid materials.
The solubility of sodium fluoride in water is 4.2 g/100 mL at 18 deg C, with a higher solubility at room temperature. As trona on an average contains about 1 wt % of sodium fluoride, the level of sodium fluoride in the trona solution formed is below the maximum solubility. By way of specific example, a solution of trona obtained by dissolving 5 gm of trona in 23 ml of water corresponds to approximately 50 mg of sodium fluoride which is

soluble in 23 mL of water. In accordance with solubility data, the maximum amount of sodium fluoride that can be dissolved in 25 mL of water is ~lg. As 50 mg is significantly lower than the maximum limit, the level of sodium fluoride in the trona is easily soluble in the solution, particularly so at room temperature or higher temperatures.
Sodium carbonate has a solubility of 30-33 g /100mL of water at 25° C whereas sodium bicarbonate has solubility around 7.2 g/100 mL of water at 25°C. Thus the sodium bicarbonate tends to precipitate out as compared to sodium carbonate.
It is preferred that a saturated solution of trona is prepared before carbon dioxide is bubbled through it.
As trona is alkaline, a saturated solution of trona typically has a pH in the range of 10 to 11. When carbon dioxide is bubbled through the solution, the carbon dioxide reacts with water to form carbonic acid, which reacts with sodium carbonate to form sodium bicarbonate that precipitates out. There exists a limit to the amount of carbon dioxide that can dissolve in the solution at room temperature and pressure. As the reaction proceeds it brings down the pH from approximately 11 to 8. After this, further passing of carbon dioxide will not provide any significant changes.
The sodium bicarbonate obtained is crystalline bicarbonate. This can be achieved at atmospheric pressure as well as under pressure. The bicarbonate thus produced can be utilized in food and pharmaceuticals applications. This is a very cheap way of removing fluoride ion by exploiting the solubility of carbonate and bicarbonate.
The bicarbonate is crystalline with a particle size ranging from 10 to 70 nm and free flowing

The carbon dioxide may optionally be bubbled through the trona solution at a pressure varying between 1 to 10 bar. The carbon dioxide is bubbled through the trona solution at room temperature and optionally the temperature may be in the range of room temperature to 80°C. The sodium bicarbonate as produced according to the method has a purity level from 90 to 98 %. The carbon dioxide may optionally be bubbled through the trona solution for a period from 15 min to 2 hours, and preferably for a time period ranging from 20 minutes to 1hour. The pH of the trona solution may be brought down from 12 to 7, and typically is brought down from 11 to 8.
The teachings of the document allow the use of a cheap raw material such as carbon dioxide to precipitate a pure form of sodium bicarbonate from a saturated solution of trona. The amount of sodium fluoride present can substantially be reduced and pure bicarbonate crystal can be obtained from trona or crushed refined soda. The sodium bicarbonate produced may find various applications including in flue gas treatment and specialty chemical applications such as food grade and pharmaceutical grade.
The following examples are 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. Through the solution CO2 was bubbled till the pH comes down from 9.8 to 8.3. A whitish precipitate is obtained which is filtered. Residue was dried and kept for further characterization for fluoride analysis and also SEM analysis.
The bicarbonate thus produced has fluoride content 0.011% and yield was 55%. The bicarbonate thus produced has a particle size ranging from 30 to 60 nm.

Example 2
About 4.95 g of trona was dissolved in 23 mL water and the solution was filtered to remove the insoluble fraction. Through the solution carbon dioxide was bubbled till the pH comes down from 10.2 to 8.3. A white precipitate was obtained which was filtered and dried and kept for further characterization.
Amount of sodium fluoride in the sodium bicarbonate was 0.025 % with a yield around 40%. Amount of fluoride in the supernatant liquid was 1.3 %
Example 3
About 5 g of trona was dissolved in 23 mL water and the solution was filtered to remove the insoluble fraction. To the saturated solution of trona gaseous CO2 was bubbled to bring down the pH from 10.48 to 8.4. A white precipitate was obtained which was filtered and dried and kept for further characterization.
The supernatant liquid was dried and the solid obtained was kept for further characterization. Amount of sodium fluoride in the bicarbonate was 0.02 % with a yield around 40%. Amount of fluoride in the supernatant liquid was 1.3 %.
Table 1: Fluoride analysis in different trona samples by Ion chromatogram.

S. No Weight of trona (g) Actual Fluoride (%) Fluoride in precipitate bicarbonate (%) Fluoride in supernatant liquid (%)
1 5 0.07 0.01 0.066
2 4.95 1 0.025 1.3
3 5 1.1 0.02 1.3