WO2010071599A1

WO2010071599A1 - Process for the treatment of waste water generated in an aromatic acid production process

Info

Publication number: WO2010071599A1
Application number: PCT/SG2008/000486
Authority: WO
Inventors: Ooi Lin Lum; Wen Yue Ge; Tian He Guan; Wei Qi; Ling Gang Kong; Xiao Yan Wu
Original assignee: Hydrochem S Pte Ltd
Current assignee: Hydrochem S Pte Ltd
Priority date: 2008-12-17
Filing date: 2008-12-17
Publication date: 2010-06-24
Anticipated expiration: 2011-06-17
Also published as: TW201026613A

Abstract

There is disclosed a treatment process for removing insoluble aromatic acids, heavy metal oxidation catalysts, metal impurities, from waste water generated in an aromatic acid production process. The treatment process comprises the steps of: (a) filtering the waste water to recover the insoluble aromatic acids; (b) passing the filtrate of step (a) through an ion exchange resin to selectively remove at least one heavy metal impurity; and (c) passing the effluent of step (b) through an ion exchange resin to selectively remove said oxidation catalysts.

Description

PROCESS FOR THE TREATMENT OF WASTE WATER GENERATED IN AN AROMATIC ACID PRODUCTION PROCESS

Technical Field

This invention relates to the treatment of waste water generated from an aromatic acid production processes and to the recovery of useful components contained within the waste water.

Background

Aromatic acids are commercially important chemicals for the production of plastic materials. One particularly important aromatic acid is purified terephthalic acid (PTA) , which has seen increasing demand in recent years due to its use as a raw material in the production of various applications such as in coatings, composite materials based on unsaturated polyester resins, hot-melt adhesives and production in the production of polyester fibres.

The industrial production of PTA has been driven by the increasing demand for polyester fibres, which is produced from polyethylene terephthalate (PET) . PTA is a raw chemical compounds used in the production of PET. PET is used to make fabrics for apparel and home furnishings such as bed sheets, bedspreads, curtains and draperies. Polyester fibres can also be spun together with natural fibres, such as cotton, to produce a cloth with improved properties such as wrinkle resistance.

As a result, processes for producing and purifying PTA are of particular interest to the polyester industry. One well-known PTA production method involves the oxidation of paraxylene in the presence of oxidation catalysts such as cobalt (Co) and Manganese (Mn) to produce PTA. Thereafter, the PTA is purified by dissolution in a high temperature aqueous solution, followed by treatment with a hydrogenation catalyst and subsequently re-crystallized by cooling down the solution. Such a purification process generates large amounts of waste water and contained within this waste water are compounds such as, dissolved organic substances, heavy metal impurities and oxidation catalyst metals. It is also expected that some PTA will be entrained in the waste water and the recovery of this entrained PTA is understandably desired. More importantly, however, is the recovery of the expensive and recyclable catalyst metals dissolved in the waste water. Accordingly, there have been methods proposed hitherto pertaining to the recovery of PTA, water and oxidation catalysts from the PTA waste water.

In one known method, the PTA waste water stream from the PTA purification process is routed to a filter to recover the insoluble PTA. The filtrate is thereafter passed through an ion exchange resin (IER) to adsorb thereon the catalysts and any other metal impurities present in the waste water. Finally the effluent from the IER is sent to a reverse osmosis (RO) unit to recover water.

The IER is regenerated using ^"a strong acid as the regenerant. The regenerant solution, containing a plethora of tramp metals and catalyst metals, is treated with alkaline solutions such as sodium hydroxide and sodium carbonate (Na₂CO₃) , and in particular sodium hydroxide (NaOH) , in order to precipitate the metals as hydroxides, which can thereafter be separated from the regenerant solution. The pH of the regenerant solution is first adjusted to the range of 4 to 5, whereby some metals are precipitated as hydroxides and removed as sludge. More NaOH is added to further increase the pH of the regenerant solution to 8.5 to 9.5. It is at this pH whereby the bulk of the catalyst metals are precipitated as hydroxides and removed as sludge. The sludge can be removed using frame and plate filters. The catalyst containing sludge is re-dissolved using an appropriate solvent (usually acetic acid) and the dissolved solution is passed through an IER for purification before the catalyst solution is returned to the PTA oxidation process .

The waste water stream that has passed through the IER column further undergoes alkaline addition to increase its pH to a value of 5 to 7, thereby maintaining the solubility of the organic substances. The waste water is thereafter passed through a two-stage reverse osmosis unit to remove the organic salts, organic compounds and other trace amounts of metal ions. The recycled water is the routed back for re-use in the PTA production process.

The known method is disadvantageous in that it requires a two-stage addition of alkaline solution (e.g. Na₂CO₃) . A first alkaline addition to precipitate heavy metal impurities and a second alkaline addition to precipitate the catalyst metals. The precipitated catalyst metals are thereafter filtered from the waste water, washed with water, re-dissolved with acid and undergoes a further purification step before substantially pure catalysts can be recovered.

Accordingly, this known process is disadvantageous in the following ways:

(1) Means of removal of impurities via precipitation results in a non-ideal extent of impurity removal due to the low levels of selectivity in the precipitation; (2) A two-stage alkaline addition method results in the loss of useful catalyst metals as will be further explained below;

(3) Low selectivity further results in the recovered catalyst having metal impurities entrained therein, thereby suffering from low catalyst purity; and

(4) A two-stage alkaline addition method also results in the use of large amounts of alkaline such as NaOH / NaCO₃, which renders the process uneconomical.

The reasons for the low selectivity are twofold. Firstly, the separation of heavy metal impurities and useful metal catalysts are achieved through a sequential pH change. Whilst it is desired that the unwanted metal impurities are precipitated during the initial pH adjustment to about 4 or 5, it should be appreciated that such precipitation will be substantially inefficient, with an appreciable amount of metal impurities remaining behind in the regenerant solution or vice versa, wherein some of the useful catalysts are prematurely precipitated and removed together with the unwanted tramp metal sludge. This is because precise control of pH is difficult to implement ^' in large-scale industrial processes .

Secondly, the metal catalysts, namely Co and Mn, are precipitated as sludge during the subsequent addition of NaOH to adjust the pH of the solution to about 9. Such a method suffers from a lack of selectivity as the sludge removed will typically contain hydroxides of other metals, as well as that of Co and Mn. Consequently, this lack of selectivity in the catalyst recovery step results in the recovered catalyst possessing a low level of purity. This is particularly detrimental as the recovered catalyst is to be recycled back into the PTA purification process. Heavy metal impurities that may be present in the recycle catalyst stream can further lead to catalyst fouling and reduce the overall yield of PTA production.

Also disadvantageous about this known process is the need to add alkaline to the waste water stream prior to the passing through the RO unit. Typically, about 1000 milligrams of NaOH needs to be added to every litre of waste water in order to sufficiently modify the pH. Again, this results in high operating costs associated with the high NaOH consumption. Furthermore, due to the increased amounts of Na⁺ ions in the waste water stream, a two stage RO unit is consequently required to adequately remove the metal ions and the organic salts/compounds. This further adds to capital investment and operating costs. Additionally, there are stringent requirements in PTA production in relation to the levels of Na+ ions present in the recovered water. In one embodiment, there is a requirement for the concentration of Na⁺ ions present in the recovered water to be less than 50 milligrams per litre, so that they can suitably be recycled back to the PTA purification process.

In another known process for treating PTA waste water, metal impurities are precipitated and removed in a first alkaline addition. The waste water subsequently passes through a chelating resin for adsorption of the catalyst metals. Using inorganic acids, e.g. HCl, the resin is regenerated and alkaline solution is further added to the regenerant to precipitate and recover the catalyst metals. The waste water stream, having passed through the chelating resin, undergoes alkaline addition to increase its pH to about 6 - 7. A two stage RO unit is thereafter used to recover water from the waste water stream. The disadvantages of this process is similar to that disclosed above, namely, high consumption of alkaline, a need to install a two-stage RO unit, thereby incurring high capital investment and operating costs associated with the above.

Accordingly, there is a need to provide a process for the treatment of waste water generated in an aromatic acid production process that overcomes, or at least ameliorates, one or more of the disadvantages described above .

There is also a need for a process for the treatment of PTA waste water that does not require an adjustment of the pH to highly alkaline conditions. There is a further need for a process for the treatment of PTA waste water that consumes less NaOH and does not require a two-stage reverse osmosis unit. There is also a need for a process for treating PTA waste water that is highly selective in nature and results in improved purity of the recovered catalyst .

Summary

In a first aspect, there is provided a treatment process for removing insoluble aromatic acids, oxidation catalysts, and metal impurities, from waste water generated in an aromatic acid production process, the treatment process comprising the steps of:

(a) filtering the waste water to recover the insoluble aromatic acids;

(b) passing the filtrate of step (a) through an ion exchange resin to selectively remove at least one metal impurities; and

(c) passing the effluent of step (b) through an ion exchange resin capable of adsorbing said oxidation catalysts. The at least one said metal impurities may be selected from the group comprising of chromium, nickel and iron.

Advantageously, by selectively removing some metal impurities before step (c) , the removed metal impurities will not be adsorbed onto the resin in step (c) . This allows better adsorption of the oxidation catalysts on the resin in step (c) . Accordingly, the recovered oxidation catalysts may be substantially free of metal impurities .

In one embodiment, there is provided a treatment process for removing insoluble aromatic acids, oxidation catalysts, and metal impurities, from waste water that has a less than 5 weight percent monocarboxylic acids, more preferably less than 1 weight percent monocarboxylic acids, yet more preferably less than 0.05 weight percent monocarboxylic acids, generated in an aromatic acid production process, the treatment process comprising the steps of:

(a) filtering the waste water to recover the insoluble aromatic acids;

(c) passing the effluent of step (b) through an ion exchange resin capable of adsorbing said oxidation catalysts .

Optionally, the process of the first aspect comprises the step of:

(d) passing the effluent of step (c) through a reverse osmosis system to remove the organic salts and organic compounds.

Also optionally, the process of the first aspect may further comprise the step of heating the filtrate passing out of step (a) to about at least 50⁰C to about at least 60°C. In one embodiment, the filtrate is heated to a temperature of 60⁰C.

Advantageously, between step (c) and optional step (d) , it is not necessary to add alkaline solution to the waste water.

Advantageously, steps (a) , (b) , (c) and optionally step (d) are undertaken to respectively substantially remove said insoluble aromatic acids, metal impurities, said oxidation catalysts and optionally, said organic salts and organic compounds.

In a second aspect, there is provided a process for the recovery of heavy metal oxidation catalyst from waste water of an aromatic acid production process containing insoluble aromatic acids., heavy metal oxidation catalysts, and metal impurities, the process comprising the steps of:

(e) filtering the waste water to recover the insoluble aromatic acids;

(f) passing the filtrate of step (e) through an ion exchange resin capable of selectively removing at least one or more of said metal impurities selected from the group comprising of chromium, nickel and iron;

(g) passing the effluent of step (f) through an ion exchange resin to adsorb said oxidation catalysts on the resin;

(h) desorbing the resin, once removed from the waste water, into an aqueous regenerant solution; and

The method may further comprise the step of: (i) passing the effluent from step (g) through a reverse osmosis unit to remove any dissolved organic salts and compounds. Advantageously, the passing step (i) produces industrial grade water.

In a third aspect, there is provided a process for the treatment of waste water containing aromatic acids, the process comprising the step of passing the waste water through a reverse osmosis system at a pH less than about 5. Advantageously, the disclosed process does not require the addition of an alkaline solution to alter the pH of the waste water.

In a fourth aspect, there is provided a treatment process for removing insoluble aromatic acids, heavy metal oxidation catalysts, metal impurities, from waste water generated in an aromatic acid production process, the treatment process comprising the steps of: passing the waste water through a filtering means to remove entrained solids; passing the filtrate through an ion exchange resin to remove metals and recover oxidation catalysts therefrom; heating the effluent stream from said ion exchange resin to at least 60 ^CC; and passing the heated effluent stream through a reverse osmosis unit to thereby recover water.

In the heating step of the process of the fourth aspect, the effluent stream may be heated from a temperature of at least 60⁰C to at least 90⁰C. Advantageously, the heating step allows the effluent stream to maintain at saturation state or a less than saturated state, thereby preventing crystallization of any dissolved organic salts or organic compounds, which is otherwise undesirable for a subsequent reverse osmosis step.

Definitions

The following words and terms used herein shall have the meaning indicated: The term "aromatic acid" as used herein refers to a compound having an acid group attached to an aromatic ring carbon. An exemplary aromatic acid is terephthalic acid or pure terephthalic acid (PTA) .

The term "oxidation catalyst" as used herein refers to heavy metals such as cobalt metal ions and manganese metal ions, which can be used in the oxidative catalysis of paraxylene to terephthalic acid.

"As used herein, the term "heavy metals" refers to metals which are typically encountered in industrial waste water having an atomic mass number greater than 24." Exemplary heavy metals include arsenic, calcium, chromium, copper, lead, magnesium, mercury, silver, and zinc.

The term "organic substances" as used in the context of this specification refers to any substances that are comprised of hydrocarbon compounds present or generated during production of aromatic acids, such as terephthalic acid. Exemplary organic^' substances include partially oxidized intermediates formed during PTA synthesis such as paratoluic acid and 4-carboxybenzaldehyde, etc.

The term "ion exchange resin", abbreviated as "IER", as used herein refers to synthetic polymers, containing positively charged or negatively charged active sites, which are able to bind to an oppositely charged ion from a surrounding solution.

The term "industrial grade water" in the context of this specification refers to water that is substantially free of metal ions and organic substances.

The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention. Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value .

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within rhat range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Disclosure of Optional Embodiments

Exemplary, non-limiting embodiments of a process for treating PTA waste water and oxidation catalysts recovery will now be disclosed. In one embodiment, the waste water entering filtration step (a) is mainly comprised of water as its solvent, and may have less than 5 weight percent monocarboxylic acids, more preferably less than 1 weight percent monocarboxylic acids, yet more preferably less than 0.05 weight percent monocarboxylic acids, . In one embodiment, the waste water has less than 0.05 percent by weight of monocarboxylic acids.

In the filtering step of the first aspect, any filtration design commonly known to one skilled in the art may be used. Exemplary filters can include stainless steel membranes, ceramic membranes, polymer membranes, plate and frame filters, bag filters, etc

In the first aspect, the selective removal of metal impurities in step (b) may comprise the removal of nickel (Ni), Chromium (Cr) and incidental iron (Fe) by adsorption onto the ion exchange resin (IER) . The IER employed here can be a weak acid resin or a chelating resin capable of adsorbing ions of Cr, Ni and Fe. In one embodiment, the IER used in step (b) is a weak acid resin. An exemplary resin capable of selective removal of metal impurities such as. Ni ions from waste water is a weakly acidic cation exchange resin like DOWEX MAC-3™ from The Dow Chemical Company, United States of America.

The adsorption of Cr, Ni and Fe is highly selective, and this step advantageously serves to concentrate the oxidation catalysts in the waste water stream. In one working embodiment, a waste water stream having an initial Ni content of about 0.16 parts per million (ppm) , may contain less than 0.002 ppm of Ni after passing out from the IER.

The oxidation catalysts used in the generation of aromatic acids, such as PTA, are typically cobalt (Co) and/or Manganese (Mn) . Accordingly, the removal of oxidation catalysts from the waste water may comprise passing the waste water through a weak acid resin or a chelating resin that is capable of forming a chelate with Co and Mn metal ions. In one embodiment, the IER employed here is a chelating resin.

The process of the first aspect may comprise, after step (C), a polishing step of:

(Cl) passing the waste water through another IER to further adsorb and remove any remaining metal impurities. Exemplary metal impurities that are typically removed in the polishing step include calcium (Ca) and magnesium (Mg) . The resin of the IER used in the polishing step (cl) may be a strong acid or a chelating resin. In one embodiment, the resin of step (cl) is a strong acid resin.

Advantageously, the prior removal of Cr, Ni and some Fe by the weak acid resin or chelating resin allows the subsequent removal of Co and Mn in the chelating resin to be more selective and efficient. Also advantageously, the separation of tramp metal impurities in the polishing step (Cl) from the useful metal catalysts is effected through an innovative arrangement of IERs, thereby negating a need to adjust the pH of the waste water. This advantageously overcomes the problems of low purity and yield associated with catalyst recover via precipitation by alkaline. Also advantageously, this obviates the need for using large volumes of alkaline solution.

In the regeneration of resins, the disclosed method also negates the need for multiple filtration/sedimentation units to be installed for the purposes of removing the precipitated tramp metals during regeneration step, thus leading to substantial capital savings . Even more advantageously, the inventors have surprisingly found that the selective removal of Co and Mn is enhanced and made more selective by the prior removal of Ni, Cr and Fe. The highly selective adsorption of Co and Mn onto the chelating resin allows for a high percentage recovery of the oxidation catalyst metals from the waste water. In one embodiment, the waste water effluent stream passing out from the chelating resin contains less than 0.02 parts per million (ppm) of cobalt and manganese.

As a result of this highly selective recovery, the recovered catalysts also possess a relatively higher degree of purity as compared to the catalysts recovered from existing processes, which allows the recovered oxidation catalysts to be recycled for use in further oxidative catalysis for PTA production.

The chelating resin may be regenerated using a strong inorganic acid such as hydrochloric (HCl), acetic acid or hydrobromic acid (HBr) . The regenerant stream, containing the recovered catalysts and the- acid, may be recycled directly back to PTA production process. Optionally, the regenerant stream may be passed into a treatment unit to further purify and concentrate the recovered catalysts for recycle. In one embodiment, the regenerant stream is passed back to the PTA production process directly, without the need for post-rreatment .

In one embodiment, the recovered catalyst may comply with the China Industrial Quality Index for the liquid composite catalyst of Co-Mn-Br as provided in Table 1 below. Table 1

The waste water being eluted from the IER column in the polishing step (Cl) will typically contain dissolved organic salts and compounds which are not removed by the resin beds. Accordingly, after the polishing step (Cl) and prior to the passing step (d) wherein the effluent is passed through the reverse osmosis unit to remove these soluble organic salts and compounds, the process may further comprise the step of: heating the waste water effluent to a temperature at least about 60⁰C, more preferably at least about 7O⁰C. In one embodiment, the heating step is between about 60⁰C to about 90⁰C, more preferably between about 60°C to about 8O⁰C. The high temperature of the effluent stream increases its saturation capacity and prevents the crystallization of the dissolved organic substances. Known prior art processes prevent crystallisation through the addition of NaOH, which as mentioned increases the sodium content of the waste water and is not desirable for water recovery. The disclosed method does not employ NaOH for this purpose. Advantageously, this allows the present method to subsequently use a single stage reverse osmosis unit to recover water. Furthermore, the Na ion content present in the recovered water is also markedly reduced and complies with industrial requirements.

The reverse osmosis membrane used in step (d) of the disclosed process can be a cellulosic type membrane, an aromatic polyamide type membrane or a thin film composite (TFC) type having a polyamide surface.

The ion exchange resins used in the disclosed process can adopt various configurations known in the art. Exemplary resin configurations include fixed bed resins, moving bed resins, pulse bed resins and simulated moving bed resins. In one embodiment, the resin beds used are moving bed resins. Moving bed resins allow the adsorption and desorption to occur simultaneously at different sections of the resin bed. Advantageously, the resin can be regenerated continuously without being taken off-line and causing disruption to the disclosed process.

Brief Description Of Drawings

The accompanying drawings illustrate a disclosed embodiment and serve to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Figure 1 shows a rough schematic of the process flow diagram of the treatment of PTA waste water.

Figure 2 shows a detailed schematic of the process flow diagram of the treatment of PTA waste water and the steps for catalyst recovery thereof. Detailed Description of Drawings

Now referring to Fig. 1, there is shown a process flow diagram for the treatment of PTA waste water. The process comprises passing the waste water 12 through a filter 20, an ion exchange system 40 and a reverse osmosis unit 60.

Any insoluble PTA 13 is recovered by the filter as retentate and recycled back to the PTA production process 140. The permeate 14, now substantially free of insoluble aromatic acids, enters the ion exchange system 40. The ion exchange column 40 comprises at least two resin beds disposed therein. A weak acid resin bed 40a is provided to adsorb metal ions such as Fe, Cr and Ni from the permeate 14. A regenerant stream 28 is simultaneously passed into weak resin bed 40a to elute the adsorbed metal ions into a regenerant stream 32. The waste water stream, now substantially free of the metal ions, exits the weak resin bed 40a as effluent waste water 16. The effluent 16 is subsequently passed into chelating resin bed 40b whereby catalyst metals, cobalt and manganese, are adsorbed thereon. A regenerant stream 34 is simultaneously passed into chelating resin bed 40b to elute the catalyst metals into a regenerant stream 36. The recovered catalyst 58 can be recycled directly back into the PTA production process 140 for further oxidative catalysis reactions.

The waste water exits the ion exchange system 40 as effluent stream 22, which is substantially free of metals and oxidation catalysts but contains appreciable amounts of dissolved organic salts and compounds. A reverse osmosis unit 60 is provided to remove the dissolved organic salts and organic compounds as retentate stream 25 and recover industrial grade water 44. Now referring to Fig. 2, there is shown a detailed process flow diagram 10 for PTA waste water treatment and catalyst recovery.

Typically, the PTA waste water generated from the PTA purification process is at a temperature of about 100⁰C - 130⁰C and a pH of between 1.8 to about 3.5. The PTA waste water stream 12 is first -passed through a stainless steel membrane filter 20 to recover any insoluble PTA 13 entrained in the waste water. Whilst only a stainless steel membrane is disclosed here, it should be clear that- any alternative means of filtration practicable by one skilled in the art could be substituted here. The recovered PTA 13 is recycled directly back to the aromatic acid production process. The filtrate stream 14 contains trace amounts of metal impurities such as Mg, Ca, Ni, Fe, and Cr in their respective ionic forms and also oxidation catalysts Co and Mn, also in ionic form. In order to mediate the waste water and at the same time recover the useful catalysts, filtrate 14 undergoes heating via a heat exchanger 11 to increase its temperature by about 10 to 20 °C, before it is passed through an ion exchange resin (IER) system 40. The IER system 40 comprises a weak acid ^■resin bed 40a, a chelating resin bed 40b and a strong acid resin bed 40c. Each of these resin beds can adopt either fixed bed, simulated moving bed or moving bed configuration. In a preferred embodiment of the invention, the resin beds are of a moving bed design.

Corrosive metal impurities such as Ni, Cr and Fe are selectively adsorbed onto the weak acid resin bed 40a. The effluent stream 16 that passes our thereafter is substantially free of Ni, Cr and Fe. Once the resin capacity is sufficiently exhausted, regenerant 28 is introduced into the weak acid resin bed 40a to elute the adsorbed cations, Ni, Cr and Fe, from the weak acid resin bed 40a, regenerating the resin bed 40a in the process. The regenerant 28 used here is a 4-8% hydrochloric acid (HCl) . The regenerant stream 32, containing the HCl regenerant and the eluted metals Ni, Cr and Fe, is discharged to a waste water treatment unit 70 for further treatment and disposal.

Advantageously, the separation of metal impurities like Ni from the useful oxidation catalysts does not require the addition of large amounts of expensive base like NaOH. Also advantageously, as this selective removal of metal impurities in a first resin bed is a highly selective process, it is able to remove substantially almost all of the corrosive metal impurities from the waste water. More importantly, the weak acid resin employed in this step does not adsorb the useful Co and Mn ions, thereby concentrating the catalyst metals in the effluent stream that exits the resin bed 40a. This is helpful in the subsequent recovery of the catalysts .

In a moving bed configuration, a portion of the weak acid resin bed 40a is constantly being regenerated as a separate portion continues to adsorb the corrosive metal ions. Advantageously, adsorption and desorption processes occur simultaneously and negates the need to take the resin bed system 40 offline for resin regeneration. This is turns reduces disruptions to the overall process and improves the ion exchange efficiency.

The effluent stream 16 is then passed through a chelating resin bed 40b. This step allows for the selective removal of the oxidation catalyst metals, Co and Mn, from the waste water. The expended resin is regenerated with a 4% HCl stream 34, thereby forming a reqenerant stream 36, containing the regenerant 34 and oxidation catalysts Co and Mn. The regenerant stream 36 is then recycled directly back to the PTA production process 80. The waste water, now substantially free of metal impurities Ni, Cr and Fe and catalyst metals Co and Mn, exit the chelating resin bed 40b as effluent stream 18. The remaining metals that are present within the mother liquor include metals like Ca, Mg and Na.

The effluent stream 18 then passes through a strong acid resin bed 40c to capture all the remaining metals subsisting in the waste water. The expended strong acid resin 40c is regenerated with a 4-8% HCl stream 38 and the purge stream 42 is similarly discharged to a waste water treatment unit 90 for further treatment and disposal. The effluent stream 22 exiting from strong acid resin bed 40c is substantially free of metal impurities and oxidation catalysts. In order to treat the water such that it may be suitable for industrial usage, further treatment steps are undertaken to remove any dissolved organic impurities that may still be present within the effluent stream 22. A reverse osmosis (RO) membrane unit 60 is employed here to remove any dissolved organic salts and compounds. In order to keep the waste water below a state of saturation, thus preventing any crystallization of the organic substances which may otherwise be detrimental to the reverse osmosis process, a heat exchanger 50 is installed therebetween the resin column 40 and the RO unit 60. Typically the effluent stream 22 is heated up to a temperature of 60 ⁰C to 90⁰C to increase the saturation point of the mother liquor.

Advantageously, by increasing the temperature of the effluent stream 22, the saturation extent of the waste water is increased, thereby preventing organic salts and compounds from crystallizing. Conventionally, NaOH is added into the waste water to suppress such crystallization. This method entails significant economic drawbacks as the costs of NaOH remain a huge proportion of the total operating costs. By circumventing the need for base addition, the present method boasts of considerable economic advantages.

The heated stream 24 is filtered through the RO membrane 60 which separates the bulk of the organic salts and compounds from the waste water, forming industrial grade water that is substantially free of organic salts. The retentate stream 25 containing the removed organic substances is routed to a waste water treatment unit (not shown) . Trace amounts of organic impurities may still be present after filtration through the RO membrane 60. A resin bed 100 is provided after the RO unit to further "polish" the RO filtrate by removing any trace organic substances. Water stream 44 thus formed is of sufficiently high purity and is suitable for industrial applications. This recovered water 44 can also be recycled directly back for use in the PTA production process .

Applications

The disclosed method may be used to recover useful oxidation catalysts from the waste water generated by PTA production, without necessitating the use of large amounts of expensive NaOH. The disclosed method selectively separates Ni, Cr and Fe from the bulk of the metals in a first selective IER. Advantageously, this allows the oxidation catalysts to be concentrated in the mother liquor and also reduces the possibility of metal impurities binding to a second IER designated for removal of Co and Mn.

The disclosed method also employs selective IER to separate the useful metal catalysts from the metal impurities. Advantageously, this method yields a higher purity catalyst recovery as opposed to separation via differential pH precipitation. Accordingly, the disclosed method may recover substantially all of the catalyst metals entrained in the PTA waste water. The recovered catalyst also complies with the industrial standards as set out in Table 1. Advantageously, the recovered catalyst is suitable to be directly recycled back into the PTA production process.

The temperature of the effluent stream exiting the IER column is preheated to about 60⁰C to about 90 °C prior to entry into the reverse osmosis unit. The high temperature advantageously increases the saturation capacity of the effluent waste water and prevents any dissolved organic substances from crystallizing, thereby hindering the osmosis process. As NaOH is not used in suppressing the crystallization of the organic substances, the sodium content of the waste water is kept low. Accordingly, the disclosed method is able to recover water from PTA waste water with the use of a single stage reverse osmosis unit rather than a double stage RO unit. Furthermore, this also serves to lower the overall NaOH demanded by the process, again leading to substantial cost savings.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing^" disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. A treatment process for removing insoluble aromatic acids, heavy metal oxidation catalysts, metal impurities, from waste water generated in an aromatic acid production process, the treatment process comprising the steps of:

(a) filtering the waste water to recover the insoluble aromatic acids;

(b) passing the filtrate of step (a) through an ion exchange resin to selectively remove at least one heavy metal impurity; and

(c) passing the effluent of step (b) through an ion exchange resin to selectively remove said oxidation catalysts.

2. The process according to claim 1, the process further comprising the step of heating the filtrate passing out from step (a) to at least 50⁰C prior to passing through the ion exchange resin.

3. The process according to claim 1, the process further comprising the step of heating the filtrate passing out from step (a) to at least 60⁰C prior to passing through the ion exchange resin.

4. The process according to claim 1, the process further comprising the step of:

(d) passing the effluent stream from step (c) through reverse osmosis to remove salts and organic compounds.

5. The process according to claim 1, wherein said ion exchange resin of step (b) is selected from a group comprising of weak acid resins and chelating resins.

6. The process according to claim 5, wherein said heavy metal impurity is selected from the group consisting of chromium, nickel and iron.

7. The process according to claim 1, wherein said effluent of step (b) is substantially free of chromium, nickel and iron.

8. The process according to claim 1, wherein said ion exchange resin of step (c) is selected from a group comprising weak acid resins and chelating resins.

9. The process according to claim 1, wherein said ion exchange resin of step (c) is a chelating resin.

10. The process according to claim 1, wherein said oxidation catalysts is selected from the group consisting of cobalt, manganese and mixtures thereof.

11. The process according to claim 1, wherein said process further comprises the step of:

(cl) passing the effluent of step (c) through an ion exchange resin to remove any remaining metal impurities .

12. The process according to claim 1, wherein said effluent stream from step (c) is heated to a temperature of 6O⁰C to 90 °C prior to the reverse osmosis step (d) .

13. The process according to claim 1, wherein said effluent of step (b) contains less than 0.002 parts per million of nickel.

14. The process according to claim 1, wherein said effluent of step (C) contains less than 0.02 parts per million of cobalt.

15. The process according to claim 11, wherein said ion exchange resin of step (cl) is selected from the group consisting of strong acid resins and chelating resins.

16. A process according for the recovery of heavy metal oxidation catalysts from the waste water of an aromatic acid production process, said waste water containing insoluble aromatic acids, heavy metal oxidation catalysts, metal impurities, said process comprising the steps of:

(e) filtering the waste water to recover the insoluble aromatic acids;

(f) passing the filtrate of step (e) through an ion exchange resin capable of selectively removing at least one heavy metal impurities;

(h) desorbing the resin, once removed from the waste water, into an aqueous regenerant solution.

17. The process according to claim 16, wherein said heavy metal impurities are selected from the group consisting of-, chromium, nickel and iron.

18. The process according to claim 16, comprising the step of recycling said regenerant stream back to said aromatic acid production process.

19. The process according to claim 15, wherein said process further comprises the step:

(i) passing the effluent of step (g) through a reverse osmosis unit to remove organic salts and organic substances .

20. A process according to claim 19, wherein said waste water of step (i) has a pH of less than 5.

21. A treatment process for removing insoluble aromatic acids, heavy metal oxidation catalysts, metal impurities, from waste water generated in an aromatic acid production process, the treatment process comprising the steps of: passing the waste water through a filtering means to remove entrained solids; passing the filtrate through an ion exchange resin to remove metals and recover oxidation catalysts therefrom; heating the effluent stream from said ion exchange resin to at least 60 ⁰C; and passing the heated effluent stream through a reverse osmosis unit to thereby recover water.