Description METHOD FOR RECOVERING CATALYST METAL FROM WASTE OF TEREPHTHALIC ACID PRODUCTION Technical Field
[1] This invention relates to a method for recovering catalyst metal from waste of terephthalic acid production, and more specifically to a method for rapidly and environmentally-friendly recovering valuable catalyst metal such as Co, Mn or so on from waste of terephthalic acid production.
[2] Background Art
[3] Terephthalic acid, the raw material used in the polymerization of polyester, is produced by oxidizing para-xylene with oxygen or oxygen-containing gas in the presence of lower aliphatic monocarboxylic acid solvent such as acetic acid and catalyst including metal such as Co, Mn or so on, and by filtering or centrifugally separating the oxidized product. Acetic acid in the mother liquor, from which te rephthalic acid is filtered off, can be easily recovered by the method such as distillation. However, the valuable metal catalyst exists in mixed state with the byproducts and organic impurities such as alkylbenzene, para-xylene, 4-carboxylbenzaldehjde, benzoic acid and so on. In order to recover the catalyst metals, the methods of using sodium hjdroxide(NaOH), sodium carbonate(Na CO ), 2 3 carbonate salt or bicarbonate salt are generally used. However, these conventional methods of recovering catalyst metal have drawbacks in that the equipments are complicated, the recovery conditions are severe, and the recovery efficiency of catalyst metals by filtration is low. Especially, in case of using NaOH, the reaction solution of the recovery reaction is strongly alkaline, and pH thereof is 12 to 14. Even in case of using Na CO , pH of the reaction solution is 8 to 10. Therefore, it is not easy to 2 3 dispse the reaction solution after the metal recovery, and the reaction solution may induce the environmental contamination. In order to overcome the drawbacks, the method for recovering catalyst metal with carbonic acid gas, carbonate salt or bicarbonate salt scavenged from a scrubber is used. However, the method has a drawback in that the equipments become more complicated. [4]
Disclosure of Invention Technical Problem
[5] Therefore, it is an object of the present invention to provide a method for rapidly and environmentally-favorably recovering catalyst metal from waste of terephthalic acid production. It is other object of the present invention to provide a method for recovering catalyst metal from waste of terephthalic acid production, in which the equipments are simple, and the disposal of the reaction solution is easy.
[6] Technical Solution
[7] To accomplish these objects, the present invention provides a method for recovering catalyst metal from waste of terephthalic acid production, which comprises the steps of: adjusting pH of the waste of terephthalic acid production to 2.5 to 7; adding NaHCO slurry, which is prepared by mixing NaHCO and water, into a pre- 3 3 cipitation reactor, into which the waste of terephthalic acid production is added; heating a mixture of the waste of terephthalic acid production and NaHCO slurry; and 3 separating a metal precipitate from the reaction mixture. [8] Brief Description of the Drawings
[9] A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawing, wherein Fig. 1 is a diagram for illustrating the method for recovering catalyst metals according to an embodiment of the present invention.
[10] Mode for the Invention
[11] As shown in Fig. 1, in order to recover catalyst metal from the waste of terephthalic acid production according to an embodiment of the present invention, pH of the waste of terephthalic acid production, which contains catalyst metal such as Co, Mn or so on, other metals such as Fe, and various organic impurities, is adjusted to 2.5 to 7, and preferably 4 to 6. R-eferable waste of terephthalic acid production is a waste sludge obtained by extracting acetic acid and terephthalic acid from the reaction solution of terephthalic acid production, and then adding water into the reaction solution. The pH- adjusting step is to facilitate the transfer of the waste into a precipitation reactor 10 by pump and pipe line. When pH of the waste is less than 2.5, there is a problem in that
organic impurities in the waste are not properly dissolved or dispersed, and when pH of the waste is more than 7, there is a problem in that Co or Mn is liable to be reacted to form Co or Mn precipitate. The pH-adjusting step can be carried out by adding basic aqueous solution such as NaOH aqueous solution into the waste. I r example, pH of the waste can be adjusted to 4 to 5 by adding 48 weight% NaOH aqueous solution into the waste, and then pH of the waste is adjusted to 5 to 6 by adding 5 weight% NaOH aqueous solution into the waste, successively. The pH-adjusted terephthalic acid production waste is transferred into the precipitation reactor 10.
[12]
[13] Then, NaHCO slurry, which is prepared by mixing NaHCO and water, is added 3 3 into the precipitation reactor 10, into which the pH-adjusted terephthalic acid production waste is also added. The amount of water mixed with NaHCO is 3 preferably 1 to 3 times by weight with respect to that of NaHCO , and more preferably 3 1.5 to 2.5 times. The solubility of NaHCO in water is generally 7 to 8 at room 3 temperature, and increases to 13 as the temperature increases. However, in the present invention, excess NaHCO is used considering the amount of NaHCO consumed in 3 3 the following chemical reaction and precipitation step. If the amount of water is less than 1 time with respect to the amount of NaHCO , NaHCO can not be adequately 3 3 dissolved or dispersed in water, and if the amount of water is more than 3 times with respect to the amount of NaHCO , there is a problem in that the productivity can be de- 3 teriorated due to the excess water. In addition, the amount of NaHCO is preferably 2 3 to 2.2 times with respect to the amount of Co and Mn in the terephthalic acid production waste. If the amount of NaHCO is less than 2 times, there is a problem in 3 that the unreacted Co and Mn can be remained in the reaction solution, and if the amount of NaHCO is more than 2.2 times, there is a problem in that excess NaHCO 3 3 is not reacted and remained in the reaction solution, and Na concentration can be undesirably increased in the reaction solution.
[14]
[15] Then, the mixture of NaHCO slurry and the waste of terephthalic acid production is heated to the temperature of preferably 40 to 90°C, more preferably 60 to 85°C, and most preferably about 80°C. If the temperature of the mixture is less than 40°C, the excess NaHCO can not be dissolved sufficiently and carbonate group does not suf- 3 ficiently activated, and if the temperature is more than 90°C, there is a problem in that NaHCO can be evaporated without reacting with Co and Mn. The heating step can be 3 carried out after mixing the waste and the slurry. Alternatively, the waste and the
slurry are heated separately, and the heated waste and the heated slurry can be mixed. By maintaining the mixture of NaHCO slurry and the waste at the temperature of 40 3 to 90°C, the chemical reaction of organic impurities and the precipitation of catalyst metal is carried out simultaneously. Therefore, the present invention directly utilizes 2- CO /HCO , which is produced by heating the NaHCO slurry, for the above- 3 3 3 mentioned chemical reaction and precipitation, rather than using carbonic acid gas of scrubber for the chemical reaction of organic impurities. In the chemical reaction, the organic impurities in terephthalic acid production waste are converted to organic 2- carboxylic acid due to CO /HCO produced by decompsition of NaHCO , and the 3 3 3 produced organic carboxylic acid reacts with sodium(Na) in the precipitation reactor 10 to be dissolved in the reaction solution. Meanwhile, the valuable catalyst metal such as Co, Mn or so on is precipitated by the reaction with the carbonate group. Thus, when the reaction is complete, the organic impurities dissolve in the reaction solution, and exist in a liquid phase, and catalyst metal is precipitated in the form of more stable salt. The produced metal precipitate exists in the form of very stable salt, and can be stably stayed in the precipitation reactor 10 for a long time. Therefore, the large quantity of waste can be reacted in the precipitation reactor 10. The reaction solution is generally neutral, and specifically pH thereof is 6 to 8, for example pH thereof is 7.5. Accordingly, the reaction conditions are mild, and the disposal of the reaction solution, from which the metal precipitate is recovered, is easy and environmentally favorable.
[16]
[17] Then, the pure metal precipitate such as Co or Mn precipitate is separated from the reaction solution by means of, for example, filter 20 (See Fig. 1) or centrifugal separator. In the method for recovering catalyst metal according to the present 2- invention, CO /HCO has higher activity than the carbonate salt used in the con- 3 3 ventional metal recovery method. Therefore, the metal precipitate can be recovered more easily. The method of the present invention is applicable not only for the recovery of Co or Mn, but also for the recovery of other metal which can react with carbonate ion.
[18]
[19] Hereinafter, the preferable examples are provided for better understanding of the present invention. However, the present invention is not limited to the following examples.
[20]
[21] [Example 1]
[22] 48 weight% aqueous NaOH solution was added to waste of terephthalic acid production to adjust pH of the waste to 4, and 500g of the pH-adjusted waste was transferred into a precipitation reactor. NaHCO slurry, which was prepared by mixing 3 75g of NaHCO and 150g of water, was added into the precipitation reactor, and the 3 mixture was reacted and remained for 45 minutes. The temperature of NaHCO slurry, 3 and the temperature of the precipitation reactor were maintained at 75°C. After completion of the reaction, the reaction solution was filtered to recover the pure metal precipitate.
[23] [24] [Example 2] [25] Except for adjusting pH of the waste to 6, and using 70g of NaHCO and 140g of water, the metal precipitate was recovered in the same manner as described in Example 1.
[26] [27] [Comparative Example 1] [28] Except for using 70g of Na CO and 300g of water instead of 75g of NaHCO and 2 3 3 150g of water, the metal precipitate was recovered in the same manner as described in Example 1.
[29] [30] [Comparative Example 2] [31] Except for using 40g of NaOH and 80g of water instead of 75g of NaHCO and 3 150g of water, the metal precipitate was recovered in the same manner as described in Example 1,
[32] [33] The reaction conditions of Examples 1-2 and Comparative Examples 1-2, pH of the reaction solution and filterability of the reaction solution are set forth in the following Table 1.
[34] Table 1
[35] As shown in Table 1, the amount of alkalinizing agent used in Example 1 was more than that used in Example 2. This is due to the fact that the amount of alkalinizing agent, which is required in the precipitation reaction, increases when pH of the waste is low. In Table 1, pH of the reaction solution after coirnietion of reaction is the result of actual reaction and was not artificially controlled. In the above Examrles and Comparative examrles, the amount of the alkalinizing agent was twice by weight as much as the amount required in the precipitation reaction, which is resulted from the fact that the alkalinizing agent was used to remove the organic impurities with the increase of pH. Rom Table 1, it is clear that the reaction solution of Exarrnies is neutral, pH of which is about 7.5, and has good filterability, while those of Comparative examrles is strongly alkaline, and doesn't have good filterability.
[36]
[37] The amounts of Co, Mn, Fe and Na in the metal precipitate, the amount of the organic impurities in the metal precipitate, Co recovery efficiency, and Mn recovery efficiency are measured using chelatometry and elemental analyzer, and the results are set forth in the following Table 2. [38] Table 2
[39] From Table 2, it is clear that the recovery efficiency of catalyst metal is desirable, an dd ppuurree c catalyst metals can be obtained in Examries utilizing NaHCO for metal pre- 3 cipitation.
[40] [41] The metal salt, filterability thereof, pH of the reaction solution and metal oxidation state during the reaction are set forth in the following Table 3.
[42] Table 3
[43] From Table 3, it is clear that the more favorable metal salt can be obtained in mild conditions when NaHCO is used for the metal precipitation. 3
[44] [45] While the present invention has been described with respect to certain preferred em bodiments and examples only, other modifications and variations may be made without departing from the spirit and scope of the present invention as set forth in the following claims.