Form 2
THE PATENTS ACT 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2002
COMPLETE SPECIFICATION
(See Section 10; Rule 13)
TITLE
A process and device for selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids
APPLICANTS
Reliance Industries Limited, Maker Chamber IV, Nariman Point, Mumbai 400021, Maharashtra, India, an Indian Company
INVENTORS Under Section 28(2)
Mathew Thomas, Fotedar Sunil, Dr Narawane Shreerang Raghunath, Chougule Subhash Anna and Kumbhar Ramdas Bhau, all of Reliance Industries Limited, B-4, MIDC Industrial Area, Patalganga 410220, Raigad, Maharashtra, India and Malshe Vinod Chitamani, University, Department of Chemical Technology, Matunga, Mumbai 400019, Maharashtra, India, all Indian Nationals

The following specification particularly describes and ascertains the nature of this invention and the manner in which it is to be performed:


FIELD OF INVENTION
This invention relates to a process and device for selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids.
PRIOR ART
Polycarboxylic acids such as terephthalic acid, isophthalic acid, trimellitic acid or pyromellitic acid find applications, among other things, in the manufacture of fibres, dye and pigment intermediates, photographic films, paints or plastic articles. Terephthalic acid, in particular, is primarily used for manufacturing polyester fiber used in making cloth. Polycarboxylic acids are usually manufactured by catalytic liquid phase oxidation of the corresponding polyalkyl aromatics such as paraxylene, m-xylene, mesitylene or durene. The liquid phase is typically provided by the solvent acetic acid and the catalysts used are groups VI, VII and VIII elements of the Periodic Table, usually cobalt and manganese. A bromide activator for the metal acetate catalysts is provided preferably as hydrobromic acid which is usually fed with stoichiometric quantity of sodium hydroxide to prevent corrosion of the reactor. Most of the
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polycarboxylic acids precipitate out from the mother liquor and are isolated by filtration.
The mother liquor comprises mainly acetic acid and organic impurities like undesirable isomers of phthalic acid or higher derivatives of polycarboxylic acid and inorganics like cobalt and manganese compounds along with corrosion products like iron, nickel, chromium, calcium or sodium. Acetic acid in the mother liquor is distilled out and recycled to the oxidation reactor. To scale down the level of impurities in the mother liquor being recycled into the oxidation reactor, a purge stream from the mother liquor is subjected to distillation and the distillate containing mainly acetic acid is recycled to the reactor. The residue from the purge stream distillation comprising valuable cobalt and manganese catalysts together with corrosion products, is incinerated to burn off the organic impurities. Cobalt and manganese catalysts in the ash are recovered by acid leaching followed by filtration and treatment of the resulting filtrate with a bicarbonate like sodium bicarbonate. This method gives poor recovery of cobalt and manganese metals and is also time consuming and expensive.
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In another method, the metal catalysts in the purge stream
distillation residue are precipitated out by treating the residue with
aqueous metal sulfide such as sodium sulfide or sodium or
potassium or ammonium hydrogen sulfide as precipitating agent at
70°C. The sulfide precipitating agents are not cobalt and
manganese selective thereby resulting in poor recovery of the
desired cobalt and manganese catalysts (Japanese Patent No
7344841).
As per US Patent No 4910175, cobalt and manganese are recovered from the purged stream by precipitation thereof using monoalkali metal oxalate or oxalic acid in combination with alkali metal hydroxides. This patent does not teach recovery of cobalt and manganese from the filtrate obtained post filtration of the precipitated metals. Thus valuable metal catalysts in the filtrate are lost. Besides this patent requires alkali metal hydroxides for precipitation of cobalt and manganese.
In yet another method, the recovery of cobalt and manganese metals have been achieved by passing the purge stream through an ion-exchange bed comprising a cationic exchanger such as sulfonic acid type resin followed by eluting the ion exchange bed
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with a mineral acid. The sulfonic acid type resins exchange almost all metal ions with the result that all the other metals along with cobalt and manganese are also eluted thus resulting in poor separation of cobalt and manganese. Due to the non-selective nature, these resins also demand lengthy resin column to attain equilibrium especially under the low pH conditions of the purge stream. Besides, the equilibrium concentration of metals in the liquid phase is fairly high under low pH conditions i.e. in acetic acid medium, when compared to the equilibrium concentration of metals in the resin phase. Therefore all the metal ions loaded are not absorbed on the resin at low pH. These resins exhibit low capacity/total capacity and are not suitable for efficient separation of cobalt and manganese.
OBJECTS OF INVENTION
An object of the invention is to provide a process for selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids, which gives improved recovery of cobalt and manganese catalysts.
Another object of the invention is to provide a process for selective recovery of cobalt and manganese catalysts from purge stream in the
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manufacture of polycarboxylic acids, which is efficient and economical.
Another object of the invention is to provide a device for selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids, which provides improved recovery of cobalt and manganese catalysts.
Another object of the invention is to provide a device for selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids, which provides efficient recovery of cobalt and manganese catalysts in an economic manner.
DESCRIPTION OF INVENTION
According to the invention there is provided a process for selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids comprising:
a) precipitating cobalt and manganese from the purge stream with a selective precipitating agent at40- 100o|C, the molar ratio of the precipitating agent to the cobalt and manganese in the
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purge stream being 0.7 -0.95 : 1, followed by filtration of the purge stream to separate the precipitate from the filtrate.
b) regenerating the cobalt and manganese from the precipitate by acidification; and
c) recovering cobalt and manganese from the filtrate by running the filtrate through an ion exchange bed comprising a chelating ion exchange resin followed by eluting the ion exchange bed with a mineral acid.
Preferably the purge stream is obtained from terephthalic acid manufacture.
The precipitating agents are cobalt and manganese selective and are for example, tartaric acid, oxalic acid or sodium sulfite.
Preferably the molar ratio of the precipitating agent to the cobalt and manganese in the purge stream is 0.9:1.
The precipitation is carried out preferably at 60 - 90°C.
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The acidification of the precipitate is carried out using hydrochloric acid, hydrobromic acid or sulfuric acid, preferably hydrobromic acid.
The chelating ion exchange resin selectively chelates with cobalt and manganese. The chelating ion exchange resin is imidodiacetate, imidophosphonate, dithiocarbamate or 2-amino pyridyl type, preferably imidophosphonate.
The eluants used to elute the metals absorbed in the ion exchange bed may be a mineral acid such as hydrochloric_jcid or hydrobromic acid, preferably hydrobromicjicid.
According to the invention there is also provided a device for carrying out the process of selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids as described above comprising a precipitation vessel (2) provided with heating means (2a) and connected to a filtration vessel (5) which in turn is connected to a dissolution vessel (6) and an ion exchange column (7) comprising an ion exchange bed of chelating ion exchange resin (8).
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The following is a detailed description of the invention with reference to the accompanying drawings in which the sole Fig 1 is a schematic diagram of the device used for carrying out the process for selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids according to an embodiment of the invention.
The device 1 of Fig 1 of the accompanying drawings comprises a precipitation vessel 2 provided with purge stream inlet 3 and precipitating agent inlet 4. The precipitation vessel is also provided with a steam jacket 2a. The precipitation vessel 2 is connected to a filtration vessel 5 which may comprise a filter bed or centrifuge. The filtration vessel 5 is connected to a dissolution vessel 6 and an ion exchange column 7 comprising an ion exchange bed of chelating ion exchange resin 8. Cobalt and manganese in the purge stream (not shown) in the precipitation vessel maintained at 65°C by steam circulated through the jacket 2a are selectively precipitated by introducing a selective precipitating agent (not shown) thereinto. The slurry obtained is filtered in the filtration vessel 5. The precipitate from the filtration vessel 5 is acidified in the dissolution vessel 6. The resulting solution comprising predominantly cobalt and manganese catalysts maybe
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directly or after processing recycled to the oxidation reactor for reuse in the manufacture of polycarboxylic acids. The filtrate from the filtration vessel 5 is passed through the ion exchange column 7. The ion exchange column is eluted with a mineral acid like hydrobromic acid to obtain an eluate comprising predominantly cobalt and manganese catalysts which may be recycled to the oxidation reactor for reuse.
According to the invention valuable cobalt and manganese catalysts in the purge stream from the manufacture of polycarboxylic acids are selectively recovered both from the precipitate and filtrate and recycled and reused in the oxidation reactor. Because cobalt and manganese are recovered from both the precipitate and filtrate, practically the entire cobalt and manganese content in the purge stream is recovered therefrom avoiding losses. Therefore, efficiency of recovery is improved to the order of 99% with resulting cost benefits. The invention eliminates use of alkali metal hydroxide during precipitation. The chelating ion exchange resins are highly selective of cobalt and manganese. Therefore, the equilibrium concentration in the resin phase is high and it is not necessary to use lengthy resin column to attain equilibrium. These resins have good capacity at low pH of the purge stream.
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The following experimental examples are illustrative of the invention but not limitative of the scope thereof.
Example 1
Composition of a typical purge stream from crude terephthalic acid manufacture by liquid phase catalytic oxidation of paraxylene was as follows:
Cobalt - 242 ppm, manganese - 356 ppm, iron -1 ppm, nickel - 2 ppm, chromium - 0.8 ppm, calcium - 0.5 ppm, sodium - 105 ppm, copper - 0.1 ppm, bromide - 550 ppm, acetic acid - 90 - 92%, water -8- 10% trimellitic acid -2000 ppm, paratoluic acid - 620 ppm, benzoic acid -11000 ppm, terephthalic acid -20,000 ppm, 4-carboxy benzaldehyde - 320 ppm, isophthalic acid - 2700 ppm, o-phthalic acid - 1100 ppm.
250 ml of the above purge stream was treated in a typical device of Fig 1. 0.36 gms of crystalline tartaric acid (tartaric acid: Cobalt / Manganese : : 0.9 : 1) was used as the precipitating agent. Precipitation was carried out at 65°C for 30 seconds under stirring.
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After filtration the tartarate precipitate was dissolved with 15% HBr (50 ml). The cobalt and manganese content in the solution was found out by Atomic Absorption Spectroscopy (AAS) was as follows:
Cobalt = 720 ppm
Manganese = 870 ppm
The filtrate was run through a 25 mm diameter, 55 mm length glass column filled with imido phosphonate resin (Indion BSR, Mfd by Ion Exchange, Mumbai, exchange capacity - 2eq/l). The flow was at the rate of 6 - 8 ml per minute. The resin was eluted with 15% HBr. The cobalt and manganese content in the elute on AAS analysis was as follows:
Cobalt = 700 ppm
Manganese = 700 ppm
The total recovery of cobalt and manganese from the precipitate and filtrate was 99.6% and 98.5% respectively.
Examples 2 to 6
The procedure of Example 1 was followed by varying the precipitation time and temperate, precipitating agent and/or chelating ion exchange resin as per the following Table 2.
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Table 2

The average recoveries of cobalt and manganese from the
precipitate and filtrate in Examples 2 to 6 were 99.4% and 98.3% respectively.
The solution of tartarate precipitates of Examples 1 to 3 and the elutes of Examples 1 to 6 were directly recycled to the oxidation reactor for reuse.
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The solutions obtained by acidification of the sulfite and oxalate precipitates were treated with sulfuric acid followed by filtration. The resulting filtrate was treated with sodium bicarbonate. The precipitate obtained was filtered and dissolved in acetic acid and recycled to the oxidation reactor.
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We claim:
1) A process for the selective recovery of cobalt and
manganese catalysts from purge stream in the manufacture of
polycarboxylic acids comprising:
a) precipitating cobalt and manganese from the purge stream with a selective precipitating agent at 40 - 100°C, the molar ratio of the precipitating agent to the cobalt and manganese in the purge stream being 0.7 -0.95: 1, followed by filtration of the purge stream to separate the precipitate from the filtrate.
b) regenerating the cobalt and manganese from the precipitate by acidification; and
c) recovering cobalt and manganese from the filtrate by running the filtrate through an ion exchange bed comprising a chelating ion exchange resin followed by eluting the ion exchange bed with a mineral acid.
2) A process as claimed in claim 1, wherein the purge stream
is obtained from terephthalic acid manufacture.
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3) A process as claimed in claim 1 or 2, wherein the precipitating agent is tartaric acid, oxalic acid or sodium sulfite.
4) A process as claimed in any one of claims 1 to 3, wherein the molar ratio of the precipitating agent to the cobalt and manganese is 0.9 : 1.
5) A process as claimed in any one of claims 1 to 4, wherein the precipitation is carried out at 60 - 90°C.
6) A process as claimed in any one of claims 1 to 5, wherein the acidification of the precipitate is carried out with hydrobromic acid.
7) A process as claimed in any one of claims 1 to 6, wherein the chelating ion exchange resin is of imidophosphonate.
8) A process as claimed in any one of claims 1 to 7, wherein the mineral acid for elution is hydrobromic acid.
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9) A process for selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids, substantially as herein described particularly with reference to Examples 1 to 6.
10) A device for carrying out the process of selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids ascertained in claim 1 comprising a precipitation vessel (2) provided with heating means (2a) and connected to a filtration vessel (5) which in turn is connected to a dissolution vessel (6) and an ion exchange column (7) comprising an ion exchange bed of chelating ion exchange resin (8).
11) A device as claimed in claim 10, wherein the heating means comprises a steam jacket provided over the precipitation vessel.
12) A device as claimed in claim 10 or 11, wherein the filtration vessel comprises a filter bed or centrifuge.
13) A device as claimed in any one of claims 10 to 12, wherein the chelating ion exchange resin is of imidophosphonate.
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14) A device for carrying out the process of selective recovery of cobalt and manganese catalysts from purge stream in the manufacture of polycarboxylic acids substantially as herein described particularly with reference to Fig 1 of the accompanying drawings.
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Dated this 10th day of October 2001