WO2021234497A1

WO2021234497A1 - Process for the purification of 2 mercaptobenzothiazole

Info

Publication number: WO2021234497A1
Application number: PCT/IB2021/053893
Authority: WO
Inventors: Jayabalan LAKSHMANAN; Janardanan Kapyur NAMBOODIRI; Monojit CHAUDHURI
Original assignee: Nitrex Chemicals India Limited
Priority date: 2020-05-16
Filing date: 2021-05-07
Publication date: 2021-11-25

Abstract

The present disclosure relates to a process for the purification of a crude 2-mercaptobenzothiazole. The process comprises purifying the crude 2-mercaptobenzothiazole by using a suitable fluid medium to obtain pure 2-mercaptobenzothiazole and filtrate. The filtrate containing a dissolved portion of 2-mercaptobenzothiazole is further treated with a base and neutralized by using an acid to obtain pure 2-mercaptobenzothiazole. The process of the present disclosure is simple, economical, and produces pure 2-mercaptobenzothiazole with comparatively high yields.

Description

PROCESS FOR THE PURIFICATION OF 2 MERCAPTOBENZOTHIAZOLE

FIELD

The present disclosure relates to a process for the purification of 2-mercaptobenzothiazole.

BACKGROUND

The background information herein below relates to the present disclosure but is not necessarily prior art.

2-mercaptobenzothiazole is a commercially important benzene-fused aromatic heterocyclic compound with nitrogen and sulphur heteroatoms and a thiol substituent.

2-mercaptobenzothiazole and its derivatives are important vulcanization accelerators in the rubber industry. 2-mercaptobenzothiazole is also used as an intermediate in pharmaceuticals, agrochemicals, leather processing chemicals, and water treatment chemical industries. It is also used as a corrosion inhibitor in oils, greases, cooling fluids and the like, as a stabilizer against air and ozone in polyether polymers, and as a component in skin medication for dogs.

Currently, the process for the preparation of 2-mercaptobenzothiazole involves condensing aniline, sulphur, and carbondisulphide at high temperature and pressure, which yields about 86-91% of 2-mercaptobenzothiazole along with certain byproducts, intermediates, and unreacted reagents. The main byproduct formed is hydrogen sulphide which is released and scrubbed with a dilute caustic solution to generate sodium hydrogen sulphide solution. Alternatively, the hydrogen sulphide is subjected to sulphur recovery by controlled ignition with oxygen techniques. Crude 2-mercaptobenzothiazole need to be purified to obtain the specified purity required for commercial applications or use as an intermediate for downstream thiazole and sulfenamide accelerator products.

Conventionally, crude 2-mercaptobenzothiazole is purified by converting it to its sodium salt solution by reacting crude 2-mercaptobenzothiazole with dilute sodium hydroxide solution to obtain sodium salt of 2-mercaptobenzothiazole. Subsequently, sodium salt of 2- mercaptobenzothiazole is further purified by extraction with organic water-immiscible hydrocarbons such as toluene, xylene, and the like followed by liquid-liquid countercurrent extraction with the same solvent, oxidation, and/or chemical oxidation to render certain water insoluble impurities, which can be removed by filtration.

The sodium salt of 2-mercaptobenzothiazole must be further treated with acids or acid salts like zinc chloride either to precipitate it as 2-mercaptobenzothiazole, or a salt of 2- mercaptobenzothiazole or to convert into its sulfenamide derivatives by reaction with primary or secondary amines, which inevitably generates a huge amount of chloride or sulphate salts, depending on the acids/acid salts used, after the neutralization reaction. Additionally, the required purity level of 2-mercaptobenzothiazole is only achieved at certain dilution levels of the sodium salt solution of 2-mercaptobenzothiazole and a large quantity of water must be used as a diluent. This results in reduced batch output and higher volumes of wastewater that must be treated by expensive effluent treatment processes before disposal. The presence of higher levels of salts as total dissolved solids (TDS) also renders the effluent untreatable by conventional biological activated sludge techniques.

Therefore, there is felt a need to provide a process for the purification of 2- mercaptobenzothiazole that mitigates the aforestated drawbacks.

OBJECTS

Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:

An object of the present disclosure is to ameliorate one or more problems of the prior art or to at least provide a useful alternative.

Another object of the present disclosure is to provide a simple process for the purification of 2-mercaptobenzothiazole.

Yet another object of the present disclosure is to provide a process for the purification of 2- mercaptobenzothiazole that is more cost-efficient.

Still another object of the present disclosure is to provide a process for the purification of 2- mercaptobenzothiazole with high yield.

Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.

SUMMARY

The present disclosure relates to a process for the purification of 2-mercaptobenzothiazole. The process comprises heating a first fluid medium to obtain a heated first fluid medium. Mixing a predetermined amount of a crude 2-mercaptobenzothiazole slowly to the heated first fluid medium for a first predetermined time period to obtain a mixture. The mixture is heated at a second predetermined temperature for a second predetermined time period to obtain slurry. The slurry is subjected to cooling to a third predetermined temperature for a third predetermined time period to obtain a resultant mixture. The resultant mixture is subjected to filtration to obtain a first wet cake and a first filtrate. The first wet cake is mixed with a predetermined amount of a second fluid medium followed by filtration to obtain a first wet mass and a second filtrate. The process of mixing the first wet mass with the second fluid medium is iterated for a predetermined number of times to obtain second wet mass and subsequent filtrates. The so obtained second wet mass is dried at a fourth predetermined temperature to obtain pure 2-mercaptobenzothiazole. Separately mixing the first filtrate, second filtrate, and subsequent filtrates and reacting with a base to obtain a salt of 2- mercaptobenzothiazole. The salt of 2-mercaptobenzothiazole is neutralized with acid, followed by filtration to obtain a third wet mass. The third wet mass is dried at a fifth predetermined temperature to obtain pure 2-mercaptobenzothiazole. Yield of the so obtained pure 2-mercaptobenzothiazole is comparatively higher.

DETAILED DESCRIPTION

Embodiments, of the present disclosure, will now be described in detail as follows. Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.

The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having,” are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units, and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.

As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed elements.

The terms first, second, third, etc., should not be construed to limit the scope of the present disclosure as the aforementioned terms may be only used to distinguish one element, component, region, layer, or section from another component, region, layer or section. Terms such as first, second, third, etc., when used herein do not imply a specific sequence or order unless clearly suggested by the present disclosure.

2-mercaptobenzothiazole is an organosulphur compound used in the vulcanization of rubber. It is also used in polymerization reactions as a radical polymerization inhibitor, chain transfer agent, reforming agent, and additive for photo-initiators.

Conventional processes for the purification of 2-mercaptobenzothiazole generate large amounts of salt in the downstream accelerators production that renders the effluent less viable for treatment (such as by tertiary biological oxidation treatment). Other problems associated with conventional processes are high cost, not able to remove all impurities, and the use of hazardous solvents.

The process of the present disclosure provides a simple process for the purification of 2- mercaptobenzothiazole, environment friendly, efficiently removes majority of the impurities, reduces salt load in downstream operations, improved recovery and higher purity of the final product.

In an aspect of the present disclosure, there is provided a process for the purification of 2- mercaptobenzothiazole.

The process is described in detail.

In a first step, a first fluid medium is heated at a first predetermined temperature to obtain a heated first fluid medium. A predetermined amount of a crude 2-mercaptobenzothiazole is slowly added to the heated first fluid medium for a first predetermined time period to obtain a mixture.

In accordance with an embodiment of the present disclosure, the first fluid medium is selected from toluene, o-xylene, m-xylene, p-xylene, mixed xylene, straight-chain C2-C6 alkylbenzenes, and aliphatic Ce- o hydrocarbons. In an exemplary embodiment, the first fluid medium is toluene.

In accordance with an embodiment of the present disclosure, the first predetermined temperature is in the range of 45 °C to 60 °C. In an exemplary embodiment, the first predetermined temperature is 50-55°C.

In accordance with an embodiment of the present disclosure, the first predetermined time period is in the range of 50 minutes to 70 minutes. In an exemplary embodiment, the first predetermined time period is 60 minutes.

In accordance with an embodiment of the present disclosure, the predetermined amount of the crude 2-mercaptobenzothiazole to the first fluid medium is in the range of 1:1 to 1:10 (w/v). In an exemplary embodiment, the predetermined amount of the crude 2- mercaptobenzothiazole to the first fluid medium is 1:4 (w/v).

In a second step, the mixture is heated at a second predetermined temperature for a second predetermined time period to obtain a slurry. The slurry is subjected to cooling at a third predetermined temperature for a third predetermined time period to obtain a resultant mixture.

In accordance with an embodiment of the present disclosure, the second predetermined temperature is in the range of 80 °C to 105 °C. In an exemplary embodiment, the second predetermined temperature is 92 °C.

In accordance with an embodiment of the present disclosure, the second predetermined time period is in the range of 50 minutes to 70 minutes. In an exemplary embodiment, the second predetermined time period is 60 minutes.

In accordance with an embodiment of the present disclosure, the third predetermined temperature is in the range of 35 °C to 55°C. In an exemplary embodiment, the third predetermined temperature is 45 °C.

In accordance with an embodiment of the present disclosure, the third predetermined time period is in the range of 20 minutes to 40 minutes. In an exemplary embodiment, the third predetermined time period is 30 minutes.

In a third step, the resultant mixture is subjected to filtration to obtain a first wet cake and a first filtrate. The first wet cake is mixed with a predetermined amount of a second fluid medium followed by filtration to obtain a first wet mass and a second filtrate. The process of mixing the first wet mass with the second fluid medium is iterated for a predetermined number of times to obtain a second wet mass and subsequent filtrates.

In accordance with an embodiment of the present disclosure, the second fluid medium is selected from toluene, o-xylene, m-xylene, p-xylene, mixed xylene, straight-chain C2-C6 alkylbenzenes, and aliphatic C6-C10 hydrocarbons. In an exemplary embodiment, the second fluid medium is toluene.

In accordance with the present disclosure, the first fluid medium and the second fluid medium are the same or different.

In accordance with an embodiment of the present disclosure, the ratio of the predetermined amount of the first wet cake to the second fluid medium is in the range of 1:0.3 to 1:1.10. In an exemplary embodiment, the ratio of the predetermined amount of the first wet cake to the second fluid medium is 1:0.4.

In a fourth step, the second wet mass is dried at a fourth predetermined temperature to obtain a pure 2-mercaptobenzothiazole.

In accordance with an embodiment of the present disclosure, the fourth predetermined temperature is in the range of 95 °C to 115 °C. In an exemplary embodiment, the fourth predetermined temperature is 105 °C.

In a fifth step, separately mixing the first filtrate, the second filtrate, and subsequent filtrates and reacting with a base to obtain a salt of 2-mercaptobenzothiazole.

In accordance with an embodiment of the present disclosure, the base is selected from sodium hydroxide, and potassium hydroxide. In an exemplary embodiment, the base is sodium hydroxide.

Further, the salt of 2-mercaptobenzothiazole is neutralized with an acid, followed by filtration to obtain a third wet mass. The third wet mass is dried at a fifth predetermined temperature to obtain a pure 2-mercaptobenzothiazole.

In accordance with an embodiment of the present disclosure, the acid is selected from sulphuric acid and hydrochloric acid. In an exemplary embodiment, the acid is sulphuric acid.

In accordance with an embodiment of the present disclosure, the fifth predetermined temperature is in the range of 95 °C to 115 °C. In an exemplary embodiment, the fifth predetermined temperature is 105 °C. Finally, the so obtained combined crop of the pure 2-mercaptobenzothiazole is having a yield greater than 99.5%.

The process for the purification of 2-mercaptobenzothiazole described herein has significant commercial benefits owing to the reduction in effluent volumes, reduction in salt load in the effluent due to which the effluent can be treated with cheaper conventional treatments, better recovery of product, and increase in per batch output while it is produced as such and/or is used as an intermediate in the downstream production of other rubber accelerators.

The foregoing description of the embodiments has been provided for purposes of illustration and is not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.

The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.

EXPERIMENTAL DETAILS

Experiment I: Process for the preparation of 2-mercaptobenzothiazole The crude 2-mercaptobenzothiazole was obtained by reacting aniline, carbon disulphide, and sulphur in an autoclave at a temperature up to 265°C and a pressure of 57-59bar. To maintain the pressure range, the hydrogen sulphide generated is accordingly released through a caustic alkali containing scrubbing arrangement.

The crude 2-mercaptobenzothiazole thus produced was analysed to identify its major constituents and the constituents were as given below in Table -1.

Table -1

Experiment II: Purification of 2-mercaptobenzothiazole in accordance with the process of the present disclosure:

1275 cc of toluene was charged into a reactor and heated to 50-55°C to obtain heated toluene. 318 gm of crude 2-mercaptobenzothiazole ground powder was added slowly over a time period of one hour to the heated toluene to obtain a mixture.

The temperature of the mixture was raised to 92 °C and maintained at 92 °C under stirring for a period of one hour to obtain a slurry. The slurry was cooled to 45 °C for 30 minutes to obtain a resultant mixture. The so obtained resultant mixture was filtered at a temperature of 40-50 °C to obtain a first wet cake (280.5 gm.) and a first filtrate. The wet cake was slurried by mixing with 115 ml of toluene and sucked under vacuum to obtain a first wet mass and a second filtrate. The process of re-slurrying and separation was repeated one more time with additional 115 ml of fresh toluene to obtain a second wet mass and a third filtrate. The so obtained second wet mass containing about 7 -10 % of toluene was subjected to drying in a hot air oven at 105 °C until the toluene content in 2-mercaptobenzothiazole reaches less than 0.1% w/w. The pure 2-mercaptobenzothiazole was obtained as pale- yellow crystals. The yield of 2-mercaptobenzothiazole was 89.95% (256.0 gm) and purity was 98.6%.

Further, 1440 cc of the combined toluene filtrate (the first filtrate, the second filtrate, and the third filtrate) was mixed with 45 ml of aqueous 5% w/v sodium hydroxide solution to obtain a biphasic mixture. The aqueous layer was separated to obtain a first caustic extract and a toluene extract. The process of extraction with 45 ml of dilute 5% w/v sodium hydroxide solution was repeated twice to obtain subsequent caustic extracts. To the combined caustic extracts (first caustic extract and subsequent caustic extracts), 2 ml of dilute 5% w/v sodium hydroxide was added to adjust the pH to 10.0-10.5 to obtain sodium salt of 2- mercaptobenzothiazole solution. The aqueous solution containing sodium salt of 2- mercaptobenzothiazole was further treated with a stream of air under the surface of the solution for one hour, filtered, and acidified with 32.8 ml of dilute 25 % w/v sulphuric acid to obtain a third wet mass. The so obtained third wet mass was subjected to drying in a hot air oven at 105 °C to obtain a pure 2-mercaptobenzothiazole. The yield of 2- mercaptobenzothiazole was 27.61 gm and purity was 98.2%.

The yield of the combined crop of pure 2-mercaptobenzothiazole was 99.65 %. Experiment III: Purification of 2-mercaptobenzothiazole in accordance with the process of the present disclosure:

1275 cc of toluene was charged into a reactor and heated to 50-55°C to obtain a heated toluene. 318 g of a crude 2-mercaptobenzothiazole in the molten state (melting was carried out at a temperature of 200-210 °C) was mixed with the heated toluene by using a jacket- heated funnel arrangement slowly for a time period of one hour. The temperature was raised to 65 °C at the end of addition to obtain a mixture. The temperature of the mixture was further raised to 92 °C and maintained at 92 °C under stirring for one hour to obtain a slurry. The slurry was cooled to 45 °C for 30 minutes to obtain a resultant mixture. The so obtained resultant mixture was filtered at 40-50 °C to obtain a first wet cake (281 gm) and a first filtrate. The first wet cake was slurried by mixing with 115 ml of toluene and sucked to obtain a first wet mass and a second filtrate. The process of re-slurrying and separation was repeated one more time with additional 115 ml of fresh toluene to obtain a second wet mass and a third filtrate. The so obtained second wet mass containing about 7 -10 % of toluene was subjected to drying in a hot air oven at a temperature of 105 °C until the toluene content in 2- mercaptobenzothiazole reaches less than 0.1% w/w. The pure 2-mercaptobenzothiazole was obtained as pale- yellow crystals. The yield of 2-mercaptobenzothiazole was 89.95% (256.0 gm) and purity was 98.6%.

Further, 1440 cc of the combined toluene filtrate (the first filtrate, the second filtrate, and the third filtrate) was mixed with 45 ml of aqueous 5% w/v sodium hydroxide solution to obtain a biphasic mixture. The aqueous layer was separated to obtain a first caustic extract and a toluene extract. The process of extraction with 45 ml of dilute 5% w/v sodium hydroxide solution was repeated twice to obtain subsequent caustic extracts. To the combined caustic extracts (first caustic extract and subsequent caustic extracts), 2 ml of dilute 5% w/v sodium hydroxide was added to adjust the pH to 10.0-10.5 to obtain sodium salt of 2- mercaptobenzothiazole solution. This aqueous solution containing sodium salt of 2- mercaptobenzothiazole was further treated with a stream of air under the surface of the solution for one hour, filtered, and acidified with 32.8 mL of dilute 25 % w/v sulphuric acid to obtain the third wet mass. The so obtained third wet mass was subjected to drying in a hot air oven at a temperature of 105 °C to obtain pure 2-mercaptobenzothiazole. The yield of 2- mercaptobenzothiazole was 27.61 gm and purity was 98.2%.

The yield of the combined crop of pure 2-mercaptobenzothiazole was 99.65 %.

Experiment IV : The crude 2-mercaptobenzothiazole (MBT) was purified in a similar manner as disclosed in Experiment II, varying the temperature conditions. The results are tabulated below in Table - 2.

It is evident from the above table that 2-mercaptobenzothiazole obtained in Experiment II has better yield, purity, and quality.

Experiment V :

The crude 2-mercaptobenzothiazole (MBT) was purified in a similar manner as 5 disclosed in Experiment II, with the same temperature conditions to ascertain the reproducibility. The results are tabulated below in Table -3

It is evident from the above table that the process parameters result in consistent performance producing pure 2-mercaptobenzothiazole with a yield of greater than 99.5%.

TECHNICAL ADVANCEMENTS

The present disclosure described hereinabove has several technical advantages including, but not limited to, the realization of a process for the purification of 2-mercaptobenzothiazole that:

• is cost-efficient;

• reduces the level of Total Dissolved Solids (TDS) in the downstream effluents;

• environment friendly; and

• provides a higher yield of 2-mercaptobenzothiazole in comparison to the conventional processes.

The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

The foregoing description of the specific embodiments so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary. While considerable emphasis has been placed herein on the components and component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments of the disclosure 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 disclosure and not as a limitation.

Claims

CLAIMS:

1. A process for purifying crude 2-mercaptobenzothiazole, said process comprising the following steps: a) heating a first fluid medium at a first predetermined temperature to obtain a heated first fluid medium; b) mixing said crude 2-mercaptobenzothiazole slowly for a first predetermined time period with said first heated fluid medium to obtain a mixture; c) heating the mixture at a second predetermined temperature for a second predetermined time period to obtain a slurry; d) cooling said slurry at a third predetermined temperature for a third predetermined time period to obtain a resultant mixture; e) filtering the resultant mixture to obtain a first wet cake and a first filtrate; f) mixing said first wet cake with a predetermined amount of a second fluid medium followed by filtration to obtain a first wet mass and a second filtrate; g) iterating the step f) for a predetermined number of times to obtain a second wet mass and subsequent filtrates; h) drying the second wet mass at a fourth predetermined temperature to obtain a pure 2- mercaptobenzothiazole ; i) separately mixing said first filtrate, said second filtrate, and said subsequent filtrates and reacting with a base to obtain a salt of 2-mercaptobenzothiazole; j) neutralizing said salt of 2-mercaptobenzothiazole with an acid followed by filtration to obtain a third wet mass; and k) drying said third wet mass at a fifth predetermined temperature to obtain a pure 2- mercaptobenzothiazole. wherein the total yield of the pure 2-mercaptobenzothiazole is >99.5%.

2. The process as claimed in claim 1, wherein said first predetermined temperature is in the range of 45 °C to 60 °C.

3. The process as claimed in claim 1, wherein said first predetermined time period is in the range of 50 minutes to 70 minutes.

4. The process as claimed in claim 1 , wherein said second predetermined temperature is in the range of 80 °C to 105 °C.

5. The process as claimed in claim 1, wherein said second predetermined time period is in the range of 50 minutes to 70 minutes.

6. The process as claimed in claim 1, wherein said third predetermined temperature is in the range of 35 °C to 55°C.

7. The process as claimed in claim 1, wherein said third predetermined time period is in the range of 20 minutes to 40 minutes.

8. The process as claimed in claim 1, wherein said first fluid medium and said second fluid medium is independently selected from the group consisting of toluene, o-xylene, m- xylene, p-xylene, mixed xylene, straight-chain C2-C6 alkylbenzenes, and aliphatic C6-C10 hydrocarbons.

9. The process as claimed in claim 1, wherein a ratio of said crude 2-mercaptobenzothiazole to said first fluid medium is in the range of 1:1 to 1:10 (w/v).

10. The process as claimed in claim 1, wherein said predetermined number of times is in the range of 2 times to 5 times.

11. The process as claimed in claim 1, wherein said fourth predetermined temperature is in the range of 95 °C to 115 °C.

12. The process as claimed in claim 1, wherein said fifth predetermined temperature is in the range of 95 °C to 115 °C.

13. The process as claimed in claim 1, wherein the ratio of said first wet cake to said second fluid medium is in the range of 1:0.3 to 1:1.10.

14. The process as claimed in claim 1, wherein said base is at least one selected from sodium hydroxide, and potassium hydroxide.

15. The process as claimed in claim 1, wherein said acid is at least one selected from sulphuric acid, and hydrochloric acid.