FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICA TION
(See section 10, rule 13)
1. Title of the invention
A METHOD OF PREPARING SODIUM BICARBONATE FROM TRONA
2. Applicant(s)
Name Nationality Address
TATA CHEMICALS LTD. INDIA BOMBAY HOUSE, 24 HOMI MODI STREET, MUMBAI-400001


3. Preamble to the description
COMPLETE SPECIFICA TION
The following specification particularly describes the invention and the manner in which it is
to be performed.

The invention relates to the preparation of sodium bicarbonate from trona. In particular, the invention relates to the preparation of fine particle size sodium bicarbonate with reduced amount of impurities such as fluoride.
BACKGROUND
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 or sodium bicarbonate by well established techniques. The sodium bicarbonate 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 bicarbonate.
Sodium bicarbonate finds wide use in industry and the production of sodium bicarbonate from trona suffers from the limitations of fluoride content, particularly in the case of trona obtained from Lake Magadi in Africa. 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.

Sodium bicarbonate with fine particle size is required for some applications such as health related, personal care, household, and specialty type products. Sodium bicarbonate with very fine particle size is useful in flue gas treatment, deodorizer preparation, and polyurethane or PVC foam generation. Bicarbonate with higher surface area and lower particle size is effective for flue gas treatment. Conventionally, fine sodium bicarbonate is prepared by milling of pure bicarbonate. However, this process has limitation as it is an energy intensive process.
Trona or crushed refined soda can be obtained naturally form Lake Magadi and is a relatively cheap material. But the presence of sodium fluoride beyond a certain threshold restricts the use of the end products derived from the trona or crushed refined soda. Moreover, obtaining high quality sodium bicarbonate having a fine particle size from trona is energy intensive and expensive.
SUMMARY
The invention relates to a method of preparing sodium bicarbonate from trona comprising preparing a saturated solution of trona and filtering the trona solution to remove insoluble impurities; adding a surfactant to the trona solution; introducing carbon dioxide to the trona solution till the pH of the solution reaches the range of 7.5 to 8.75 and at least some sodium carbonate in the trona solution is converted to sodium bicarbonate that precipitates; and recovering the precipitated sodium bicarbonate from the trona solution.

The invention also relates to sodium bicarbonate with reduced fluoride impurities produced by preparing a saturated solution of trona and filtering the trona solution to remove insoluble impurities; adding a surfactant to the trona solution; introducing carbon dioxide to the trona solution till the pH of the solution reaches the range of 7.5 to 8.75 and sodium carbonate in the trona solution is converted to sodium bicarbonate that precipitates; and recovering the precipitated sodium bicarbonate from the trona solution.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The accompanying drawings illustrate the preferred embodiments of the invention and together with the following detailed description serve to explain the principles of the invention.
FIG. 1 is a schematic illustration of the method of manufacturing sodium bi carbonate with fine particle size and reduced fluoride content and production of sodium carbonate in accordance with an embodiment of the invention.
FIG. 2 is a comparative chart illustrating the particle sizes of the sodium bicarbonate obtained with and without the use of a surfactant as provided by the method. DETAILED DESCRIPTION
This invention as described herein is an improvement over the applicant's earlier filed patent application number 535/MUM/2008 dated March 18, 2008.

To promote an understanding of the principles of the invention, reference will be made to the embodiment illustrated in the drawing and specific language will be used to describe the same. It will nevertheless be understood that no limitation of scope of the invention is thereby intended, such alterations and further modifications in the illustrated method and such further applications of the principles of the inventions as illustrated therein being contemplated as would normally occur to one skilled in art to which the invention relates.
A process of preparing sodium bicarbonate having a fine particle size and substantially reduced fluoride impurities is described.
The process involves the removal of fluoride from trona by first forming a solution of trona in water. To this trona solution a surfactant is added. Carbon dioxide is passed through the trona solution such that at least some 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. Precipitated sodium bicarbonate is separated from the supernatant to obtain sodium bicarbonate with reduced amount of fluoride content. The size of the sodium bicarbonate particles formed is found to include substantial portions having particle size lower than 100 micron and particularly lower than 75 micron.
The solubility of sodium fluoride in water is 4.2 g/100 mL at 18°C, with a higher solubility at room temperature. As trona on an average contains about 1 to 3 weight percent 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/100 mL 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.
In accordance with an aspect a saturated solution of trona including a surfactant is prepared before carbon dioxide is bubbled through it.
With reference to figure 1, the process of manufacturing sodium bicarbonate having fine particle size from trona containing fluoride impurities is illustrated. At step 101 the crude trona is milled to obtain a generally uniform powder and this powdered trona is used to prepare a saturated solution of trona at step 102. Insoluble impurities are removed from the trona solution before further processing. To the trona solution a surfactant is added as indicated by step 103. Carbon dioxide is introduced into the trona solution containing surfactant as indicated at step 104 till the pH of the trona solution is brought down in the range of 7.5 to 8.75 and sodium bicarbonate is precipitated, as indicated at step 105, and

a supernatant including the dissolved fluoride impurities is obtained as indicated at step 106. The supernatant is decanted and contains some amount of sodium carbonate which may be recovered from it either by the addition of a non solvent or by vacuum drying as indicated at step 107.
In accordance with an aspect the crude trona is milled to obtain a fine power. The milled trona is then dissolved in water to obtain a saturated solution of trona. The saturated solution of trona is filtered to remove insoluble impurities.
The amount of surfactant added to the trona solution varies from 0.1 to 2.0 % by weight with respect to trona and particularly 0.1 to 1 % by weight with respect to trona.
The surfactant used may be cationic or anionic. Anionic surfactant may be any anionic surfactant with sulphate group including but not limited to sodium Iauryl sulphate (SLS) or sodium dodecyl sulphate (SDS) (C12H25S04Na), sodium undecyl triethoxy sulphate, sodium polyoxy ethylene alkyl ether sulphate, didodecyl dimethyl ammonium sulphate, sodium n-dodecyl sulphate, sodium Iauryl ether sulphate and sodium tetradecyl sulphate. Equally, any anionic surfactant with sulphonate or sulphosuccinate group may be added to the trona solution. By way of specific example, a linear alkyl sulphonate may be added to the trona solution. Alternatively, sodium dodecyl triethoxy sulphonate may be used.
In accordance with an aspect, a cationic surfactant may be added to the trona solution. By way of specific example, the cationic surfactant may be cetyl

trimethyl ammonium bromide or hexadecyl trimethyl ammonium bromide (CTAB). Alternatively, hexadecyl amine may be added as a surfactant to the trona solution.
The sodium bicarbonate as produced according to the method has a purity level from 92 to 98% and it is crystalline with a substantial portion having particle size lower than 100 microns and particularly having particle size ranging from 30 to 75 micron. With reference to figure 2, a comparative chart illustrating the particle sizes of the sodium bicarbonate obtained with and without the use of a surfactant as provided by the method is provided. The amount of product obtained is 2.52 gm. Analysis of the sodium bicarbonate obtained without using a surfactant in the manner as provided by the inventive method shows more than forty percent of the sodium bicarbonate particle having particle size greater than 500 micron.
As trona is alkaline, a saturated solution of trona typically has a pH in the range of 10 to 11 and sometimes higher. 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 from 11 to 8 or 7.5.
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 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 lhour. The pH of the trona solution may be brought down from 12 to 7.5, and typically is brought down from 11 to 8.
In accordance with an aspect the precipitated sodium bicarbonate may be separated from the supernatant by any method including but not limited to decanting or filtration. In accordance with a preferred embodiment the precipitated sodium bicarbonate is separated from the supernatant by decanting.
In accordance with an aspect, the sodium carbonate present in the supernatant may be recovered. In accordance with an embodiment the sodium carbonate may be recovered from the supernatant by the addition of any non-solvent. The non-solvent may include an organic solvent which will cause precipitation of the sodium carbonate.
In accordance with an embodiment, the supernatant liquid may be dried to obtain solid sodium carbonate. The drying of the supernatant may be carried out by any means including but not limited to vacuum drying.
The following examples are provided to explain and illustrate certain preferred embodiments of the process of the invention. Example 1
About 5 gm of crushed refined trona was taken in a beaker, to that about 23 ml of distilled water was added, stirred for 10 minutes, followed by filtration to remove the insoluble impurities. To the filtered trona solution 100.5 mg cetyl

trimethyl ammonium bromide is added and stirred. Carbon dioxide (C02) gas is bubbled for 45 minutes (approximate). Formation of white precipitate of sodium bicarbonate is observed. The solution is filtered to obtain sodium bicarbonate particles that are dried and weighed. Weight of sodium bicarbonate is 2.21 grn.
Example 2
About 5 gm of crushed refined trona was taken in a beaker, to that about 23 ml of distilled water was added, stirred for 10 minute, followed by filtration to remove the insoluble impurities. In filtered trona solution 50.8 mgofcetyl trimethyl ammonium bromide (CTAB) is added and stirred. Carbon dioxide (C02) gas is bubbled for 45 minute (approximate). Formation of white precipitate of sodium bicarbonate is observed. The solution is filtered to obtain sodium bicarbonate particles that are dried and weighed. Weight of sodium bicarbonate is 2.28 gm.
Particle size analysis of the sodium bicarbonate obtained by using cetyl trimethyl ammonium bromide is tabulated in the table 1 below. It is observed that over fifty percent of the sodium bicarbonate formed has particle size lower than 75 micron.


Table 1
Example 3
About 5 gm of crushed refined trona was taken in a beaker, to that about 23 ml of distilled water was added, stirred for 10 minute, followed by filtration to remove the insoluble impurities. In filtered trona solution 100.2 mg of sodium dodecyl sulphate (SDS) was added and stirred. Carbon dioxide (C02) gas is bubbled for 45 minute (approximate). Formation of white precipitate of sodium bicarbonate is observed. The solution is filtered to obtain sodium bicarbonate particles that are dried and weighed. Weight of sodium bicarbonate is 2.09 gm.
Example 4
About 5 gm of crushed refined trona was taken in a beaker, to that about 23 ml of distilled water was added, stirred for 10 minute, followed by filtration to remove the insoluble impurities. In filtered trona solution add 50.6 mg of sodium

dodecyl sulphate was added and stirred. Carbon dioxide (C02) gas is bubbled for 45 minute (approximate). Formation of white precipitate of sodium bicarbonate is observed. The solution is filtered to obtain sodium bicarbonate particles that are dried and weighed. Weight of sodium bicarbonate is 2.23 gm.
Example 5
About 5 gm of crushed refined trona was taken in a beaker, to that about 23 ml of distilled water was added, stirred for 10 minute, followed by filtration to remove the insoluble impurities. In filtered trona solution 25.4 mg of sodium dodecyl sulphate was added and stirred. Carbon dioxide (C02) gas is bubbled for 45 minute (approximate). Formation of white precipitate of sodium bicarbonate is observed. The solution is filtered to obtain sodium bicarbonate particles that are dried and weighed. Weight of sodium bicarbonate is 2.40 gm.
Example 6
About 5 gm of crushed refined trona was taken in a beaker, to that about 23 ml of distilled water was added, stirred for 10 minute, followed by filtration to remove the insoluble impurities. In filtered trona solution 10.6 mg of sodium dodecyl sulphate was added and stirred. Carbon dioxide (CO2) gas is bubbled for 45 minute (approximate). Formation of white precipitate of sodium bicarbonate is observed. The solution is filtered to obtain sodium bicarbonate particles that are dried and weighed. Weight of sodium bicarbonate is 2.16 gm.

Particle size analysis of the sodium bicarbonate obtained by using sodium dodecyl sulphate is tabulated in the table 2 below. It is observed that over fifty percent of the sodium bicarbonate formed has particle size lower than 75 micron.

Example 7
About 5 gm of crushed refined trona was taken in a beaker, to that about 23 ml of distilled water was added, stirred for 10 minute, followed by filtration to remove the insoluble impurities. In filtered trona solution 50.3 mg of hexadecyl amine was added and stirred. Carbon dioxide (C02) gas is bubbled for 45 minute (approximate). Formation of white precipitate of sodium bicarbonate is observed.

The solution is filtered to obtain sodium bicarbonate particles that are dried and weighed. Weight of sodium bicarbonate is 2.16 gm.
Example 8
About 5 gm of crushed refined trona was taken in a beaker, to that about 23 ml of distilled water was added, stirred for 10 minute, followed by filtration to remove the insoluble impurities. In filtered trona solution 25.1 mg of hexadecyl amine was added and stirred. Carbon dioxide (C02) gas is bubbled for 45 minute (approximate). Formation of white precipitate of sodium bicarbonate is observed. The solution is filtered to obtain sodium bicarbonate particles that are dried and weighed. Weight of sodium bicarbonate is 2.21 gm.
Particle size analysis of the sodium bicarbonate obtained by using hexadecyl amine is tabulated in the table 3 below. It is observed that over fifty percent of the sodium bicarbonate formed has particle size lower than 75 micron.

I

Particle Size Distribution
1 10
Particle Size (ym)

1000 3000

Size (pm) Vo«Owr% Size (pm] VoIOwf% Size (pm) VoiOwr% Size (tim) Vol Over % Stzeftjm) VclOwr%
2.000 100.00
37.000 57.54
125.000 11.81
250.000 5.37
500.000 0.53
5.000 98.62 53.000 38.40 149.000 10.10 297.000 3.59 595.000 0.00
10.000 94.17 74.000 23.70 177.000 9.08 354.000 2.53 707.000 0.00
20.000 82.50 105.000 14.79 210.000 7.57 420.000 1.61 841.000 0.00
Table 3 INDUSTRIAL APPLICAPABILITY
The method describes allows the use of a cheap raw material such as carbon dioxide to precipitate a pure form of sodium bicarbonate having a fine particle size 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 sodium bicarbonate obtained may also be used a deodorizer and for generating CO2 to be used in foam generation and fungicide application.

The use of a surfactant in the manner as described by the inventive method gives more than forty percent of the sodium bicarbonate having particle size distribution below 40 to 60 micron. Pure sodium bicarbonate with less fluoride impurities and having particle size below 37 micron can be easily obtained by the inventive method.
Sodium bicarbonate with very fine particle size can be useful in flue gas treatment, deodorizer preparation, and polyurethane/PVC foam generation. Sodium bicarbonate with higher surface area and lower particle size are effective for flue gas treatment.
The particle size of sodium bicarbonate also affects the bubble size of carbon dioxide obtained by the decomposition of sodium bicarbonate. Carbon dioxide with controlled bubble size is useful for synthesizing polyurethane/PVC foam. Sodium bicarbonate with fine particle size may work as a substitute for azodicarbonamide, an expensive organic compound as compared to sodium bicarbonate.

We claim:
1. A method of preparing sodium bicarbonate from trona comprising:
preparing a saturated solution of trona and filtering the trona solution to remove insoluble impurities;
adding a surfactant to the trona solution;
introducing carbon dioxide to the trona solution till the pH of the solution reaches the range of 7.5 to 8.75 and at least some sodium carbonate in the trona solution is converted to sodium bicarbonate that precipitates;
recovering the precipitated sodium bicarbonate from the trona solution.
2. A method as claimed in claim 1 wherein between 0.1 to 2 % by weight of surfactant with respect to the trona is added to the trona solution.
3. A method as claimed in claim 1 wherein the surfactant is any one of sodium dodecyl sulphate, sodium undecyl triethoxy sulphate, sodium polyoxy ethylene alkyl ether sulphate, didodecyl dimethyl ammonium sulphate, sodium n-dodecyl sulphate, sodium lauryl ether sulphate, sodium tetradecyl sulphate, linear alkyl sulphaonate, sodium dodecyl triethoxy sulphonate, sulphosuccinate, cetyl trimethyl ammonium bromide and hexadecyl amine.

4. A method as claimed in claim 1 wherein the saturated trona solution is prepared from milled trona powder.
5. A method as claimed in claim 1 wherein carbon dioxide is introduced in to the trona solution at a pressure in the range of 1 bar to 10 bar.
6. A method as claimed in claim 1 wherein the trona solution is heated to a temperature up to 75 degrees before the introduction of carbon dioxide.
7. A method as claimed in claim 1 wherein carbon dioxide is introduced into the trona solution for a period of 15 min to 2 hours, and preferably for a period of 20 minutes to 1 hour.
8. A method as claimed in claim 1 further comprising recovering sodium carbonate from the trona solution.
9. A method as claimed in claim 1 wherein the sodium bicarbonate prepared is used as a deodorizer; in a fungicidal application, in flue gas treatment; in specialty chemical applications including food grade and pharmaceutical grade applications or for foam generation.

10. Sodium bicarbonate with reduced fluoride impurities produced by preparing a
saturated solution of trona and filtering the trona solution to remove
insoluble impurities;
adding a surfactant to the trona solution;
introducing carbon dioxide to the trona solution till the pH of the solution reaches the range of 7.5 to 8.75 and sodium carbonate in the trona solution is converted to sodium bicarbonate that precipitates;
recovering the precipitated sodium bicarbonate from the trona solution.
11. Sodium bicarbonate with reduced fluoride impurities as claimed in claim 10 having particle size less than 75 micron.
12. A method substantially as herein described with reference to and as illustrated by the accompanying drawing.
13. Sodium bicarbonate substantially as herein described with reference to and as illustrated by the accompanying drawing.

Dated this 3rd day of December, 2008