WO2022034401A1

WO2022034401A1 - Recovery of metal catalysts from oxidizer purge streams

Info

Publication number: WO2022034401A1
Application number: PCT/IB2021/056614
Authority: WO
Inventors: Peter Anthony GANNON; Ashley James WELLS
Original assignee: Koch Technology Solutions UK Ltd; Koch Technology Solutions LLC
Current assignee: Koch Technology Solutions UK Ltd; Koch Technology Solutions LLC
Priority date: 2020-08-14
Filing date: 2021-07-21
Publication date: 2022-02-17
Anticipated expiration: 2023-02-14
Also published as: CN115867683A; EP4196618A1

Abstract

A process is described for recovering cobalt and/or manganese from a residue from a catalytic oxidation process for producing an aromatic polycarboxylic acid, wherein the residue comprises water-soluble cobalt and/or manganese compounds and an acidic organic component. The process comprises providing a first solution containing part of the residue including at least part of the water-soluble cobalt and/or manganese compounds. An alkali metal compound selected from an alkali metal carbonate and an alkali metal bicarbonate is added to the first solution to raise its pH to ≥5 to ≤7 and release CO2. The release rate of CO2 gas from the first solution is between 350 and 400 kg/hour. An alkali metal carbonate is then added to the first solution to further raise the pH thereof to ≥7, thereby precipitating cobalt and/or manganese carbonate-containing species.

Description

RECOVERY OF METAL CATALYSTS FROM OXIDIZER PURGE STREAMS

FIELD

[001] The present application relates to the recovery of metal catalyst from the oxidizer purge streams produced in the synthesis of aromatic polycarboxylic acids.

BACKGROUND

[002] Cobalt or manganese or a combination of cobalt and manganese, e.g. in the form of their acetates, together with a source of bromide ion, is known to provide effective catalysis for the liquid phase oxidation of aromatic polycarboxylic acid precursors, such as para-xylene, to produce the aromatic polycarboxylic acids, such as terephthalic acid. The liquid phase oxidation is carried out using a lower monocarboxylic aliphatic acid, such as acetic acid, as a solvent in which the catalyst system is dissolved.

[003] The aromatic polycarboxylic acid produced by the oxidation process is withdrawn from the reactor as a slurry of crystals in a mother liquor comprising mainly the aliphatic carboxylic acid together with an aqueous phase containing dissolved catalyst components and an organic phase containing some polycarboxylic acid and precursors thereof. Further precipitation of the aromatic polycarboxylic acid is usually obtained by means of a crystallisation process before separating the crystals from the mother liquor. The solids-liquid separation may be carried out by means of an integrated filtration and washing system, such as disclosed in EP-A-502628 and WO- A-93/24440, the entire disclosures of which are incorporated herein by reference.

[004] After separation of the aromatic acid product from the mother liquor, conventional practice is to recycle a major part of the mother liquor and its catalyst metal content to the oxidation reactor and to purge a minor part, typically 10 to 40%, to avoid undue build-up of primarily organic contaminants within the reaction system. The mother liquor purge is treated to recover the aliphatic carboxylic acid for recycle to the oxidation reaction, leaving a high melting point and viscous residue which contains, inter alia, metal and bromine catalyst components and organic acidic materials.

[005] It has long been recognized that efficient utilization of catalyst and process economics call for the further processing of such residues to allow recovery of catalyst metal for reuse in the catalytic liquid oxidation process. The literature is replete with methods for the recovery of the catalyst metals. One commonly used route for recovery involves contacting the residue with water so as to extract the desired metals. Usually, the residue is contacted with water in such a way that the metal catalyst components dissolve while the organic contaminants remain largely undissolved. Following separation of the resultant extract solution from the undissolved components, the solution is contacted with an alkali metal carbonate or bicarbonate to precipitate the catalyst metals as carbonates or bicarbonates so that they can then be recovered for further treatment, if necessary, and recycled to the oxidation reactor. Such an approach is disclosed in, for example, British Patent No. 1,413,829, where cobalt and manganese are recovered from the extract solution by raising its pH from an initial value of about 3.5 to a final value of 7 to 9, preferably 7 to 8.1, by adding sodium bicarbonate or, more preferably, sodium carbonate to the extract solution.

[006] There remains, however, significant interest in improving existing processes for the recovery of catalyst metals from the oxidizer purge streams produced in the synthesis of aromatic polycarboxylic acids and, in particular, in reducing alkali metal usage without sacrificing the efficiency of the metal recovery.

SUMMARY

[007] It has now been found that the total alkali use required for residue acid neutralization/metal precipitation may be reduced if the increase in pH is conducted in two or more discrete stages, preferably in separate vessels, with the first stage raising the pH to a value from 5 to 7 and the second stage raising the pH to a higher value of 7 or above. Particularly where an alkali metal carbonate is used for both stages, the two-stage operation permits the CO2 liberated by neutralisation of strong mineral or organic acids with alkali carbonate solution to degas from solution prior to further pH adjustment to alkaline conditions for precipitation of metal carbonate salts from solution.

[008] Thus, in one aspect, the present application provides a process for the recovery of cobalt and/or manganese from a residue from a catalytic oxidation process for the production of an aromatic polycarboxylic acid, wherein the residue comprises an inorganic component containing water-soluble cobalt and/or manganese catalyst compounds and an acidic organic component, the process comprising:

(a) producing a first solution containing part of the residue including at least part of the water-soluble cobalt and/or manganese catalyst compounds;

(b) adding at least one alkali metal compound selected from an alkali metal carbonate and an alkali metal bicarbonate to the first solution to raise the pH thereof to a first value from >5 to <7 and release CO2 from the first solution;

(c) adding an alkali metal carbonate to the first solution to further raise the pH thereof to a second value greater than the first value and >7, thereby precipitating cobalt and/or manganese as a carbonate-containing species and producing a second solution; and

(d) recovering the precipitated cobalt and/or manganese carbonate-containing species from the second solution.

Optionally, the process further comprises the step following step (b) of stopping the addition of the at least one alkali metal compound when a maximum release rate of CO2 gas released from the first solution has been reached.

BRIEF DESCRIPTION OF THE DRAWINGS

[009] Figure 1 is a graph plotting first stage pH value against the ratio of the total weight of sodium carbonate solution to the weight of residue extract solution in the two-stage metal recovery process of Example 1.

[0010] Figure 2 is a graph plotting first stage pH value against CO2 release rate in the two- stage metal recovery process of Example 1

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0011] An important commercial process for the production of aromatic polycarboxylic acids, such as terephthalic acid, involves the oxidation of polyalkyl aromatic compounds, such as paraxylene, in the presence of a catalyst system comprising cobalt or manganese or a combination of cobalt and manganese, together with a source of bromide ions. The oxidation is typically conducted in the liquid phase using a lower monocarboxylic aliphatic acid, such as acetic acid, as a solvent in which the catalyst system is dissolved. After recovery of the polycarboxylic acid product and removal of the acetic acid solvent from the oxidation zone effluent, a residue slurry remains which comprises an inorganic component containing one or more water-soluble cobalt and/or manganese catalyst compounds and an acidic organic component containing one or more mono-, di- and tricarboxylic acids. There are several ways by which the catalyst compounds can be extracted from this residue slurry. For example, the residue slurry may be contacted with an aqueous medium, such as water, typically at a temperature of 50°C to 100°C, such that the metal catalyst components dissolve to produce an aqueous extract which is used as the first solution in the present process, while the organic contaminants remain largely undissolved. In other embodiments, a combined organic/aqueous medium, such as a toluene/water mixture, can be used for the extraction, followed by filtration and decanting to generate the first solution used in the present process. Such a combined organic/aqueous medium extraction is disclosed in, for example, International Patent Publications Nos. WO2011/119395 A2 and WO2016/023958 and allows simultaneous extraction of both the organic and inorganic components of the residue slurry. In other embodiments, the residue slurry can be fed directly into the present process with the first solution being produced by dissolving the cobalt and/or manganese catalyst compounds in an aqueous solution of at least one alkali metal compound selected from an alkali metal carbonate and an alkali metal bicarbonate.

[0012] The aqueous extract or slurry is treated to precipitate out and recover the desired cobalt and/or manganese catalyst components so that these can be recycled to the oxidation reactor. The present application is directed to an improved process for recovering cobalt and/or manganese, generally as their carbonate salts, from such feed streams.

[0013] The present process is conducted in at least two distinct stages, typically a first or neutralization stage, where organic acids are neutralized by adding alkali and then a second or precipitation stage, each of which can be conducted in the same vessel or each can be conducted in a different vessel. The process can be operated on a batch basis or on a continuous basis. Generally, if the process is conducted on a continuous basis, the second or precipitation stage is carried out in a separate vessel from that used to perform the first or neutralization stage.

[0014] In the first or neutralization stage, a first solution is provided either from a prior extraction step as described above or is produced in situ by dissolution of the water-soluble cobalt and/or manganese catalyst compounds in the residue slurry by an aqueous solution of at least one alkali metal compound selected from an alkali metal carbonate and an alkali metal bicarbonate. A neutralizing agent comprising at least one alkali metal compound selected from an alkali metal carbonate and an alkali metal bicarbonate, optionally together with an alkali metal hydroxide in equal parts with the alkali metal carbonate is then added to the first solution to raise the pH thereof to a first value from >5 to <7, preferably from >5.5 to <6.5, neutralizing the organic acids and releasing CO2 from the first solution. The preferred alkaline species used as the neutralizing agent is an alkali metal carbonate, especially sodium carbonate, and the alkali addition can be conducted at any temperature from ambient (25 °C) to 100°C, preferably from 60°C to 90°C. Increasing the pH of the aqueous extract may be accompanied by the precipitation of so-called tramp metals, such as iron and chromium, which may also be present in small amounts in the first solution. Any precipitated tramp metals can readily be removed by filtration or centrifuging.

[0015] In embodiments, the first solution is treated during the neutralization step to facilitate release of CO2 from the first solution. Suitable treatments include agitating the first solution, such as by stirring, passing of a gas, such as nitrogen, through the first solution and/or reducing the pressure of the headspace above the first solution.

[0016] As the pH of the first solution increases during the neutralization step, the rate of CO2 release from the first solution per unit weight of the at least one alkali metal compound added increases eventually reaching a maximum before starting to decrease again. The rate of CO2 release per unit weight of added alkali metal compound is monitored and the addition of the alkali metal compound is ceased when the rate decreases after reaching a maximum. The release rate of CO2 gas from the first solution is between 350 and 400 kg/hour. The method chosen to monitor the rate of CO2 release may differ according to whether the process is batch or continuous. For batch processes, the point at which the maximum release rate of CO2 has been reached may be determined in real time by observing the point at which CO2 gas bubbles cease being evolved within the pH range of >5 to <7. For continuous processes, the point at which the maximum release rate of CO2 has been reached corresponds with the minimum total alkali addition which was found during experimentation and depicted in Figure 1. The maximum CO2 release rate is typically between 350 and 400 kg/hour. The minimum total alkali addition occurred for the two step neutralization when the first step took place within the pH range of >5 to <7. When starting with solid organics, continuous experimentation showed that temperature, agitation and residence time were process parameters involved in ensuring solid organics had dissolved, allowing them to react with the alkali and give a representative pH.

[0017] After the first neutralization step has been terminated, the first solution, optionally after transfer to a different vessel, is then subjected to the second or precipitation stage of the present process, in which the pH of the first solution is raised from the first value to a second pH value greater than the first value and >7, such as from >7 to 9.5, for example from >8.5 to 9.5 giving more cobalt and manganese recovery, by adding an alkali metal carbonate, preferably sodium carbonate, to the first solution. Again, the alkali addition can be conducted at any temperature from ambient (25°C) to 100°C, preferably from 60°C to 90°C. Raising the pH of the first solution to a value >7 by alkali metal carbonate addition results in precipitation of the cobalt and/or manganese from the solution as the associated carbonate-containing species. The precipitated cobalt and/or manganese carbonate-containing species can then be recovered from the solution by any known method, such as filtration or centrifuging.

[0018] Preferably, the second, precipitation step is conducted without the deliberate addition of alkali metal hydroxide beyond any added in the first, neutralization step, since the presence of hydroxide ions can result in the precipitation of cobalt and/or manganese as hydroxide-containing species that are more difficult to convert back to the acetate than the carbonate-containing species. [0019] The significance of the two-stage recovery process disclosed herein may be better understood by reference to the following reversible reactions can occur when sodium carbonate is used to raise the pH of aqueous acidic medium:

[0020] Sodium carbonate is essentially fully ionized in solution. At low pH values (i.e. high H+ concentrations), particularly pH values less than or equal to 6, all the equilibria are forced to the right hand side, resulting in high concentrations of carbonic acid and the release of gaseous CO2. At such low pH values, incremental addition of carbonate results in the (almost) stoichiometric loss of carbonate as CO2, and the stoichiometric removal of H+ ions as water. As more carbonate is added the pH increases and the dominant carbonate species shifts towards [HCO3]’ and an increasing proportion as [CO3]²’. The molar ratio of [H+ removed] to [carbonate added] shifts from 2: 1 to 1:1. At higher pH values, such as above 7, the concentration of H+ is low, and the equilibria lie to the left hand side, such that the amount of dissolved CO2 is low and no CO2 is evolved as gas. Under these conditions the carbonate exists as (predominantly) bicarbonate and carbonate.

[0021] In the two stage process employed herein, the release of CO2 is accompanied by removal H⁺ ions as water; whereas in the single stage system more carbonate addition is required as H⁺ ions are locked up as [HCO3]’. In the two stage process, the stoichiometric carbonate requirement is 1 mole per 2 mole H+ (plus 1 mole/mole total Co + Mn for metals precipitation). In a single stage system, the stoichiometric carbonate requirement is 1 mole/mole H+ (plus 1 mole/mole total Co + Mn). We can therefore expect that the carbonate usage for a single stage process will be of the order of twice that for a two stage process.

[0022] The invention will now be more particularly described with reference to the following non-limiting Example and the accompanying drawings.

Example

[0023] A series of tests were run on a first solution obtained from the residue from a commercial plant for producing terephthalic acid by the liquid phase oxidation of para-xylene in the presence of a Co/Mn/Br catalyst dissolved in acetic acid. The first solution contains part of the residue including at least part of the water-soluble cobalt and/or manganese catalyst compounds and has a composition as summarized in Table 1 below:

Table 1

[0024] In each test, the first solution was initially neutralized with a 10 wt% sodium carbonate solution to a first pH value from 3.5 to 9 and then to a second pH value of 9-9.5. The results are shown in Figures 1 and 2. In Figure 1, the vertical axis shows the total alkali addition (stage 1+2) in relation to the weight of residues neutralised in the first solution. It can be seen there is a low point when the first pH value is 5.5-6.5, giving the minimum alkali addition. In Figure 2, the rate of CO2 release during the first neutralization stage is plotted against the pH of the first solution and it will be seen that the rate of release increases to a maximum and then decreases when the pH reaches a value of around 6. The point of maximum CO2 release represents the preferred time to cease the first neutralization step for a batch process.

[0025] While the present invention has been described and illustrated by reference to particular embodiments, those of ordinary skill in the art will appreciate that the invention lends itself to variations not necessarily illustrated herein. For this reason, then, reference should be made solely to the appended claims for purposes of determining the true scope of the present invention.

Claims

9 CLAIMS

1. A process for the recovery of cobalt and/or manganese from a residue from a catalytic oxidation process for the production of an aromatic polycarboxylic acid, wherein the residue comprises an inorganic component containing water-soluble cobalt and/or manganese catalyst compounds and an acidic organic component, the process comprising:

(ed) recovering the precipitated cobalt and/or manganese carbonate-containing species from the second solution.

2. The process of claim 1 , further wherein a release rate of CO2 gas from the first solution is between 350 and 400 kg/hour.

3. The process of claim 1, further comprising the step following step (b) of stopping the addition of the at least one alkali metal compound when a maximum release rate of CO2 gas released from the first solution has been reached.

4. The process of claim 3, further wherein a maximum release rate of CO2 gas from the first solution is between 350 and 400 kg/hour. 5. The process of claim 1, wherein the at least one alkali metal compound added in (b) further contains an alkali metal hydroxide in equal parts with the alkali metal carbonate.

3. The process of any preceding claim, wherein raising the pH of the first solution in (c) is conducted without the deliberate addition of alkali metal hydroxide beyond any added in (b).

4. The process of any preceding claim, wherein the at least one alkali metal compound added in (b) contains sodium carbonate.

5. The process of any preceding claim, wherein the alkali metal carbonate employed in (c) is sodium carbonate.

6. The process of any preceding claim, wherein (b) and (c) are conducted in separate vessels.

7. The process of any preceding claim, wherein the first pH value is from >5 to <7.

8. The process of any preceding claim, wherein the second pH value is from >7 to 9.5.

9. The process of any preceding claim, wherein the adding step (b) is conducted at a temperature from 60 to 90 °C.

10. The process of any preceding claim, wherein the adding step (c) is conducted at a temperature from 60 to 90 °C.