Process for recovering sodium nitrite

The present invention provides a process for recovering a metal nitrite, e.g., sodium nitrite, from a reaction mixture formed from an aromatic displacement reaction such as the synthesis of an aromatic bis(ether phthalimide) in an organic non-polar solvent. The process comprises forming a reaction mixture from the synthesis of an aromatic bis(ether phthalimide) comprising a recoverable amount of metal nitrite; treating the reaction mixture with an amount of water effective to produce an aqueous solution phase of metal nitrite and an organic non-polar phase; and separating the aqueous solution phase of metal nitrite from the organic non-polar phase.

CROSS-REFERENCE TO RELATED APPLICATIONS
 Not applicable.
 FEDERALLY SPONSORED RESEARCH
 Not applicable.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a process for recovering sodium nitrite, potassium
 nitrite, lithium nitrite, or nitrite mixtures containing at least one of
 the foregoing from the reaction mixture resulting from the synthesis of a
 polyimide such as, for example, an aromatic bis(ether phthalimide).
 2. Brief Description of the Related Art
 Sodium nitrite may be produced as a reaction by-product in aromatic
 displacement reactions such as the synthesis of aromatic bis(ether
 phthalimide) compounds. Synthesis of these compounds has been described in
 U.S. Pat. No. 5,068,353 to Dellacoletta. Of particular interest is the
 product of sodium nitrite as the reaction by-product of the synthesis of
 bisimide having the formula (I).
 ##STR1##
 Several techniques have been used to recover bisimide from a reaction
 mixture including solid-liquid separation techniques such as filtering at
 a temperature at which the bisimide is substantially completely soluble
 while alkali metal salt impurities are substantially insoluble. (See U.S.
 Pat. No. 5,068,353.)
 Bisimide has also been recovered by extractive purification employing a
 conventional caustic wash as the extractant. In this process, bisimide in
 the toluene reaction solvent is extracted with a sodium hydroxide wash to
 remove the sodium nitrite, unreacted starting materials, catalyst and
 other reaction by-products. A disadvantage to this method is that the
 sodium hydroxide also hydrolyzes some of the bisimide product, converting
 it to aqueous, soluble amide-acid sodium salts. (See U.S. Pat. No.
 5,068,353.)
 The sodium hydroxide wash, containing the bulk of the sodium nitrite
 present in the reaction mixture, is typically disposed of by concentrating
 and burning in an incinerator or is disposed of through biotreatment. The
 organic materials in the wastewater (i.e., the sodium hydroxide wash) are
 destroyed and the sodium nitrite is converted to nitrogen and sodium
 carbonate in the burning process. A disadvantage to this process is the
 violent, uncontrollable nature of the reaction due to the high amounts of
 organic impurities present in the sodium hydroxide wash. As the sodium
 nitrite by-product can itself be marketed, it would be advantageous to
 recover the sodium nitrite from the reaction mixture in sufficient purity
 to be marketable.
 What is needed in the art is a method for recovering useable sodium nitrite
 from the reaction mixture formed from the synthesis of aromatic bis(ether
 phthalimides).
 SUMMARY OF THE INVENTION
 The process of the present invention comprises forming a reaction mixture
 comprising one or more products including a recoverable amount of a metal
 nitrite, preferably at least one of sodium nitrite, lithium nitrite, and
 potassium nitrite, in a non-polar solvent; treating the reaction mixture
 with a polar solvent in an amount effective to produce two phases
 comprising an aqueous solution phase of the metal nitrite and an organic
 non-polar phase; and separating the aqueous solution phase from the
 organic non-polar phase.
 Various features and advantages of the present invention will be
 appreciated and understood by those skilled in the art from the following
 detailed description.
 DETAILED DESCRIPTION OF THE INVENTION
 The present invention provides a process for recovering lithium nitrite,
 potassium nitrite, sodium nitrite, or mixtures containing at least of the
 foregoing from a reaction mixture formed from the synthesis of a
 polyimide. For ease of discussion, the present application will discuss
 the process of the present invention as it applies to the recovery of
 sodium nitrite from a reaction mixture formed from the synthesis of
 bisimide, wherein the reaction mixture comprises a recoverable amount of
 sodium nitrite, bisimide, unreacted material, and by-products and
 impurities, dissolved in a non-polar organic solvent. The reaction mixture
 is thus referred to as non-polar.
 For example, bisimide is synthesized by reacting
 4-nitro-N-methylphthalimide of the formula (II):
 ##STR2##
 with bisphenol A disodium salt of the formula (III)
 ##STR3##
 in a non-polar solvent, such as refluxing toluene, in the presence of a
 phase transfer catalyst.
 Suitable non-polar organic solvents useful in the present invention include
 solvents in which the major reaction product, bis(ether phthalimide), is
 soluble and sodium nitrite is insoluble. Some possible solvents include,
 but are not limited to, toluene, xylene, trimethylbenzene,
 dichlorobenzene, chlorobenzene, anisole, and higher hydrocarbon solvents
 including dodecane.
 Meanwhile, preferred phase transfer catalysts include quaternary ammonium
 salts and phosphonium salts including bis(tri-n-butyl)-1,6-hexylene
 diammonium dibromide, tetrapropylammonium bromide, tetrabutylammonium
 chloride, tetrabutylammonium fluoride, tetrabutylammonium acetate,
 tetrabutylphosphonium bromide, tetraphenylphosphonium bromide and
 hexaethylguanidinium chloride, among others, with hexaethylguanidinium
 chloride catalyst of the formula (IV):
 ##STR4##
 particularly preferred.
 The resulting reaction mixture comprises, for example, the major reaction
 product, bisimide, along with sodium nitrite, the minor reaction product
 monoimide of the formula (V):
 ##STR5##
 unreacted starting material, catalyst, such as hexaethylguanidinium
 chloride, and by-products having the formulas (VI):
 ##STR6##
 and H.sub.2 NEt.sub.2 (diethylamine), wherein "Et" is an ethyl group, along
 with unidentified organic impurities.
 In order to separate the sodium nitrite from the reaction mixture, a polar
 solvent, typically water, is added to the reaction mixture in an amount
 effective to dissolve the sodium nitrite, resulting in two immisible
 phases: for example, an aqueous phase comprising an aqueous solution of
 sodium nitrite and a small amount of organic materials, typically less
 than about 3 percent by weight ("wt %") organic materials based upon a
 combined weight of organic materials, catalyst, and sodium nitrite; and an
 organic non-polar phase comprising bisimide, catalyst, reaction
 by-products and impurities, which are not appreciably soluble in water,
 dissolved in a non-polar solvent. The amount of water added is tailored to
 produce the desired percent solution of sodium nitrite according to
 customer requirements or the subsequent purification technique to be used.
 Typically, up to about 45 wt % sodium nitrite solution based on the total
 weight of aqueous solution, is desirable for most purification techniques,
 with about 10 wt % to about 42 wt % sodium nitrite preferred, and about 37
 wt % to about 42 wt % especially preferred. The pH of the water utilized
 is general about neutral with deionized water often preferred.
 Dissolution of the sodium nitrite in water can optionally be facilitated
 via the addition of heat. A dissolution temperature can readily be
 selected empirically by one of skill in the art based on the choice of
 solvent, with such temperature typically between about 25.degree. C. and
 about 110.degree. C. For example, for reactions in which the non-polar
 organic solvent is toluene, a temperature of about 70.degree. C. to about
 85.degree. C. is preferred.
 Once the two phases have reached equilibrium, the aqueous solution of
 sodium nitrite is readily separated from the non-polar reaction mixture,
 typically by drawing out the sodium nitrite solution through the bottom of
 the reaction vessel. The separated aqueous sodium nitrite solution is
 herein referred to as the "water wash" to distinguish from the
 conventional caustic wash. At this point, the sodium nitrite is
 sufficiently free of organic material such that it may be marketed as is
 or subjected to further purification techniques.
 The determination of whether further purification techniques will be
 employed is based upon the desired purity of the sodium nitrite, and the
 yield of the reaction mixture employed. High yield reaction mixtures are
 required to achieve a clean separation between the organic non-polar phase
 and the aqueous phase. If the reaction yield is not sufficiently high,
 bisphenol A disodium salt will be present in the aqueous solution in
 higher quantities than those found in high yield mixtures. Furthermore,
 solid bisphenol A or its sodium salt will also be present, floating in the
 aqueous sodium nitrite solution. Consequently, the bisimide yield based on
 4-nitro-N-methylphthalimide and bisphenol A disodium salt is preferably
 above about 90%, more preferably above about 97.5%, and most preferably
 above 98%. For reactions providing less than or equal to 90% yield of
 bisimide, filtration is employed to remove precipitated bisphenol A
 disodium salt or bisphenol A from the aqueous phase.
 The purification process may be any suitable purification technique or
 combination of techniques, including passing the aqueous sodium nitrite
 solution through a filter such as a semi-permeable membrane, subjecting
 the recovered aqueous sodium nitrite solution to carbon treatment, resin
 treatment, melt treatment, biotreatment, high temperature and high
 pressure treatment, or any other appropriate conventional technique or
 combination of techniques that removes residual organic material.
 For example, melt purification typically comprises isolating a sample of
 aqueous solution comprising a minimal amount of organic impurities,
 preferably below about 3 wt % based on the combined weight of organic
 materials and sodium nitrite; removing the water, and melting the sodium
 nitrite crystals in a slow and controllable purification at temperatures
 up to 500.degree. C., decomposing impurities in the dried sodium nitrite.
 The melt purification of the present invention is conducted without a
 resulting violent reaction, due to the low amounts of organic material
 present in the aqueous solution. In contrast, the conventional caustic
 wash reacts quickly and violently at a temperature of about 300.degree. C.
 and is extremely hazardous. This explosive reaction is believed to be due
 to the conventional caustic wash comprising higher quantities of organic
 matter, typically about 20 wt % based on the combined weight of organic
 materials and sodium nitrite.
 Alternatively, the water wash can be processed through commercially
 available membranes to filter out organic material. In membrane
 processing, organic material is selectively rejected from the membrane and
 is reconcentrated in the recirculating feed to the membrane. Filtering can
 more readily be accomplished with the process of the present invention
 than with the conventional caustic wash process due to the fact that the
 usual pH of a conventional caustic wash is about 13-14, which is not
 readily tolerated by the majority of available commercial membranes. Some
 possible membranes which can be employed with the present invention
 include a Desal membrane available from Desal Corporation, and a LCI
 membrane available from LCI Corporation, with the Desal membrane typically
 preferred due to a 400% increase in filtration rate.
 Yet another purification technique comprises purification by resin
 treatment, with the use of cross-linked resins, such as cross-linked
 polystyrene resins, particularly preferred because they are readily
 available, economical, and chemically stable allowing reuse.
 Alternatively, a process similar to that taught in U.S. Pat. No. 5,709,800
 can be employed, where the aqueous solution is subjected to a temperature
 and pressure sufficient to remove impurities from the aqueous sodium
 nitrite solution by converting the residual organic material in the
 solution to sodium carbonate and low molecular weight biodegradable
 materials. The preferred temperatures are those at which the reaction is
 rapid, while pressures are selected based upon the desired temperature.
 Suitable temperatures typically include about 300.degree. C. to about
 400.degree. C., more preferably about 350.degree. C. to about 380.degree.
 C. Suitable pressures include pressures of about 1,000 pounds per square
 inch (psi) to about 4,000 psi, more preferably about 2,000 psi to about
 3,500 psi.
 All references cited herein are hereby incorporated in their entireties.
 The following examples are provided to illustrate the process according to
 the present invention. It should be understood that the examples are given
 for the purpose of illustration and do not limit the invention.

EXAMPLE 1
 Two identical reaction mixtures were prepared by reacting about 287
 kilograms ("kg") (632 pounds "lbs") of bisphenol A disodium salt and about
 434 kg (958 lbs) of 4-nitro-N-methylphthalimide in about 1,919 kg (4,230
 lbs) of toluene in the presence of about 2.7 kg (6 lbs) of
 hexaethylguanidinium chloride catalyst to produce a reaction mixture
 comprising bisimide dissolved in toluene, other reaction products and
 impurities, and a recoverable amount of sodium nitrite. Sample 1 was
 treated according to the process of the present invention using about 208
 liters (55 gallons) of water to dissolve the sodium nitrite producing a 40
 wt % aqueous solution of sodium nitrite based on total weight of solution,
 i.e., water wash. Sample 2 was treated with about 643 liters ("L") (170
 gallons) of an aqueous solution of 1 wt % sodium hydroxide based on the
 total weight of solution. In each example, the contents of the vessel were
 agitated for about 5 minutes at about 82.degree. C. to dissolve the sodium
 nitrite. The reaction mixtures were allowed to settle for about 25 minutes
 before the water wash and conventional caustic wash were each drummed off
 through the appropriate valve.
 Table 1 provides a compositional comparison between the water wash and the
 conventional caustic wash.
 TABLE 1
 Sample Nos.
 Parameter 1 2
 Flow rate (gph) 156 417
 Ph 8 to 12 13 to 14
 Density 1.3 1.17
 sodium nitrite (wt %) 40 22
 organics (wt %) 0.8 to 1.0 2.0 to 2.8
 Total organic carbon (TOC) 5500 13000
 hydrolyzed bisimide (wt %) 0.2 1
 hydrolyzed monoimide (wt %) 0.01 0.26
 BPA salt (wt %) 0.01 0.3
 4-nitrophthalic acid (wt %) 0.1 0.26
 4-nitrophthalamide acids (wt %) 0.1 0.26
 HEG-Cl (wt %) 0.37 0.28
 sodium chloride (wt %) 0.43 0.18
 organic unknowns (wt %) 0.2 0.26
 gph is gallons per hour.
 As can be seen from Table 1, treatment of the reaction mixture according to
 the process of the present invention provides a sodium nitrite solution
 having 40 wt % sodium nitrite based on the total weight of solution
 compared with 22 wt % sodium nitrite obtained with the conventional
 caustic wash. The total organic carbon content (TOC) in the water wash is
 5500 ppm by weight compared with 13,000 ppm by weight of TOC for the
 conventional caustic wash.
 EXAMPLE 2
 Separate reaction mixtures were prepared for Samples 3-6 and Samples 7-10
 by reacting about 434 kg (958 lbs) of 4-nitro-N-methylphthalimide with
 about 287 kg (632 lbs) of bisphenol A disodium salt in about 1,919 kg
 (4,230 pounds) of toluene in the presence of about 2.7 kg (6 lbs) of
 hexaethylguanidinium chloride catalyst to produce a reaction mixture
 comprising bisimide dissolved in toluene, other reaction products and
 impurities, and a recoverable amount of sodium nitrite. For Samples 3-6,
 the reaction mixture was treated with 55 gallons of water according to the
 process of the present invention. For Samples 7-10, the reaction mixture
 was treated with about 643 L (170 gallons) conventional caustic wash
 comprising 1 wt % aqueous solution of sodium hydroxide. For all examples,
 the contents of the vessel were agitated for about 5 minutes at about
 82.degree. C. to dissolve the sodium nitrite. The reaction mixture was
 allowed to settle for about 25 minutes.
 Table 2 provides a compositional comparison between Samples 3-6 and Samples
 7-10.
 TABLE 2
 Sample Nos.
 Water Wash Process Caustic
 Wash Process
 Constituent 3 4 5 6 7 8
 9 10
 Bisimide Yield of 98.69 98.8 98.7 98.45 99.18 97.96
 98.66 98.69
 reaction mixture (mol %)
 (based on BPA)
 Bismide Yield of 99.64 99.5 98.3 99.38 99.02 99.46
 99.39 99.58
 reaction mixture (mol %)
 (based on NPI)
 Total organic carbon in 5,300 4,430 5,400 5,465 11,930 13,636
 11,377 13,100
 aqueous phase (ppm by wt.)
 Sodium nitrite in 42.6 37.8 39.4 41.1 23.6 20.6
 19.6 19.5
 aqueous phase (wt %)
 pH of aqueous phase 10 10 11 10 &gt;13 &gt;13
 &gt;13 &gt;13
 4-nitrophthalamide acid 1063 1254 2539 2193 1954 848
 385 950
 in aqueous phase
 (mg/L)
 4-nitrophthalic acid in 769 1160 1528 1412 1210 1128
 651 913
 aqueous phase (wt %)
 Bisphenol A Disodium 48 23 58 68 4400 10,252
 5,658 7,689
 salt in aqueous phase
 (mg/L)
 Hydrolyzed monoimide 86 130 50 -- 4516 2167
 2854 4465
 in aqueous phase
 (mg/L)
 Hydrolyzed bisimide in 2,072 1,557 771 429 11,930 13,635
 10,039 13,138
 aqueous phase (mg/L)
 As can be seen in Table 2, the water wash obtained according to the process
 of the present invention comprises a 37.8 wt % to 42.6 wt % sodium nitrite
 solution with lower levels of organic material than present in the
 conventional caustic wash.
 In addition to the sodium nitrite recovery process, the present inventive
 process gives an added benefit in that it also results in a reduction in
 the amount of bisimide product yield that is lost during the bisimide
 extraction process (i.e., the sodium nitrite recovery process). As can be
 seen in Table 1, the water wash comprises 0.2 wt % bisimide, as compared
 with 1 wt % bisimide in the conventional caustic wash.
 EXAMPLE 3
 Membrane treatment using a pilot unit was performed for Samples 11-14 (See
 Table 4). Typical operating conditions of the pilot unit were 400 psi
 pressure on the feed stream at a temperature of about 30.degree. C. The
 feed rate was 4 gallons per minute using a membrane that has 17.2 square
 feet of surface area contained in a standard cylindrical housing. This
 pilot unit was used to process Samples 11-14.
 In Samples 11 and 12, a 100-gallon sample of water wash was processed with
 membrane treatment using a Desal 5 membrane. The permeate flow on the
 Desal 5 membrane was about 400 milliliters per minute ("mL/min"). An
 advantage of the present invention is the ability to a more cost efficient
 membrane that permeates faster but only withstands a pH of about 8 to
 about 12. The lower pH of the water wash does away with the necessity of
 selecting a slower membrane for its ability to withstand the higher pH of
 conventional caustic wash.
 In Samples 13 and 14, a 100-gallon sample of water wash was processed with
 membrane treatment using an LCI membrane. The permeate flow on the LCI
 membrane was about 40 mL/min.
 Table 4 illustrates the further reduction in organic content achieved by
 membrane treatment of water wash as compared with membrane treatment of
 conventional caustic wash.
 TABLE 4
 Sample No./ TOC NaNO.sub.2 4NAA 4NPA BPA
 H-MI H-BI
 stream (ppm by wt) (mg/L) (mg/L) (mg/L) (mg/L)
 (mg/L) (mg/L) pH
 11.sup.a 4610 40.1 1230 917 98 68
 690 10
 11.sup.b 1635 40.2 654 270 10 --
 -- 10
 (Desal)
 11.sup.c 14,880 36.9 3,638 3,600 117
 771 4149 11
 12.sup.a 5,019 37.8 1,253 1,160 23
 130 1557 11
 12.sup.b 425 43.7 107 65 7 0
 16 11
 (0% filtered)
 (LCI)
 12.sup.b 562 37.9 158 99 7 0
 21 11
 (50% filtered)
 (LCI)
 12.sup.b 1,101 41.4 328 247 10 0
 85 11
 (85% filtered)
 (LCI)
 13.sup.d 17,218 16.6 1,045 1,258 10,435
 3,759 13,896 &gt;13
 13.sup.b 1,423 24.3 265 328 492 27
 110 &gt;13
 13.sup.c 118220 13.8 4,727 5,501 45,945
 19,693 71,609 &gt;13
 14.sup.d 13,985 19.5 950 913 7689
 4,466 13,138 &gt;13
 14.sup.b 3,143 22.2 462 554 1,237
 181 418 &gt;13
 14.sup.c 95,055 21.3 2,776 2,590 30,699
 21,484 63,161 &gt;13
 .sup.a Water First Wash
 .sup.b Permeate
 .sup.c Rejected Stream
 .sup.d Caustic Wash (LCI)
 An additional advantage of the present invention is that the water wash can
 be more readily processed than conventional caustic wash with temperature
 and pressure such as with the temperature and pressure processing of waste
 material taught in U.S. Pat. No. 5,709,800.
 High temperature and high pressure processing (i.e., temperatures of about
 200.degree. C. to about 500.degree. C. and pressures of about 10
 atmospheres to about 400 atmospheres) of the water wash results in the
 nitrite oxidizing the dissolved organic materials, destroying the
 4-nitro-N-methylphthalimide and bisphenol A disodium salt, the hydrolyzed
 bisimide and hydrolyzed monoimide, thus rendering the treated wash more
 amenable to further purification through biotreatment. This is due to the
 lesser amount of dissolved organic material present in the treated water
 wash.
 A comparison of the levels of TOC and sodium nitrite remaining after
 treatment to high temperature and pressure is found in Table 5.
 TABLE 5
 time temp. pressure NaNO.sub.2 TOC
 composition (min) (.degree. C.) (psi) (wt. %) (ppm by wt.)
 control -- -- -- 23.4 3500
 treated 55 380 3600 23.6 1959
 In a typical denitrification biotreatment process, the material to be
 treated is contacted with anaerobic bacteria, followed by contact with an
 aerobic biomass whereby the organic materials are mineralized. The aerobic
 biomass is then clarified, filtered, and discharged. In order to render a
 conventional caustic wash amenable to biotreatment, the pH must be
 adjusted from pH 13 to pH 9, causing precipitation of the bulk of the
 organic material, primarily bisphenol A disodium salt, hydrolyzed
 bisimide, and hydrolyzed monoimide. The material is then filtered and the
 filtrate treated with anaerobic denitrifying bacteria.
 The process of the present invention eliminates the need for the pH
 adjustment and filtration steps, lowering the capital investment and
 operating costs of a biotreatment process.
 The water wash of the present invention can additionally be more
 economically treated than conventional caustic wash with carbon to further
 reduce the amount of dissolved organic material.
 EXAMPLE 4
 In Samples 15-18, a 50 gram sample of water wash was processed with carbon
 treatment. In Examples 19-22, a 50 gram sample of membrane processed water
 wash was processed with carbon treatment. Table 6 shows the further
 reduction in total organic carbon achieved by the carbon treatment of
 water wash (Samples 15-18) and of permeate of water wash (Samples 19-22).
 TABLE 6
 Sample Nos.
 Composition 15 16 17 18 19 20 21 22
 Carbon 0 0.055 0.201 0.402 0 0.05 0.204 0.401
 (g)
 sample size (g) 50 50 50 50 50 50
 pH 10 10
 4NAA (mg/L) 1341 1191 1000 778 252 18 142 4
 4NPA (mg/L) 918 833 809 812 68 63 64 52
 BPA (mg/L) 59 29 26 24 12 9 10 8
 H-4MI 116 83 32 21 0 0 0 0
 (mg/L)
 H-BI 2610 2366 2018 1565 0 0 0 0
 (mg/L)
 TOC (ppm) 5450 5007 4677 3889 750 655 450 259
 The water wash may be further treated with resins, particularly
 cross-linked resins, to remove quaternary ammonium halides from the water
 wash. Although any resin capable of absorbing organic material from an
 aqueous solution can be employed, cross-linked polystyrene resins are
 particularly preferred. It is contemplated that water wash can be treated
 with resins more economically than conventional caustic wash by virtue of
 less organic material being present.
 EXAMPLE 5
 In Sample 23, a brine solution comprising 914 mL water, 247 grams of sodium
 nitrite (21% by weight NaNO.sub.2), 9.14 grams of a 50% sodium hydroxide
 solution, 9.08 grams hexaethylguanidinium bromide, and 700 .mu.L toluene
 was processed with resin treatment.
 An adsorption column loaded with 95 grams XAD-4 resin (available from
 Aldrich Chemical Company, Inc.) was provided. The column was maintained at
 20.degree. C. and washed with 1 mL of water. One liter of the brine
 solution was pumped through the column at 20 mL/min, and the effluent was
 analyzed to determine the presence of hexaethylguanidinium bromide.
 The column was regenerated by washing with water at 4 mL/min. The eluent
 produced ("water flush") was analyzed to determine the presence of
 hexaethylguanidinium bromide. It was noted that the first 70-80 mL of
 water flush eluent constituted brine from the loading cycle and represents
 the interstitial volume of the packed column. The results of the analyses
 of the brine solution eluent and water flush eluent are provided in Table
 7.
 TABLE 7
 HeGBr in Brine HeGBr in Water
 Brine (mL) Eluent (ppm) Water flush (mL) Flush Eluent (ppm)
 200 0.65 40 5267
 350 0.51 70 4990
 450 0.33 80 11150
 500 0.76 90 31725
 550 7.73 100 41274
 600 20.6 110 41582
 650 67.4 120 35113
 700 126 130 28399
 750 208 140 24980
 800 369 150 20113
 850 560 160 18850
 900 769 180 10045
 950 1040 200 11890
 1000 1133 220 9867
 240 8532
 270 7177
 300 6006
 360 4682
 400 4035
 450 3419
 500 3000
 EXAMPLE 6
 The following example demonstrates the use of the melt purification
 process.
 A 40 wt %, based on total weight of solution, sodium nitrite solution
 containing about 1 wt % dissolved organic material was provided. The
 organic material was identified as including hydrolyzed
 4-nitro-N-methylphthalimide, hydrolyzed bisimide, and hexaethylguanidinium
 chloride. The water was removed under vacuum at about 100.degree. C.
 producing an orange solid. The solid was melted with stirring at about
 400.degree. C. for about 1 hour, under 1 pound per square inch (psi) of
 nitrogen. The solid was then cooled, yielding sodium nitrite comprising
 less than 7.5 wt % sodium carbonate, 1.18 wt % sodium nitrate and trace
 amounts of sodium chloride. The melt purification process is accomplished
 without the accompanying explosive reaction that occurs when sodium
 nitrite is heated in the presence of relatively large amounts of organic
 materials due to the minimal amount of organic material present in the
 aqueous sodium nitrite solution of the present invention.
 Another advantageous feature of the present invention is the production of
 aqueous sodium nitrite solution that is more amenable than conventional
 caustic wash to purification processes due to the lower amounts of organic
 impurities present in the aqueous sodium nitrite solution.
 Another unexpected advantage of the process of the present invention is the
 reduced yield loss of bisimide with a water wash as opposed to a
 conventional caustic wash. For example, a reaction product mixture of 779
 grams had a bisimide loss of 524 mg using the water only first wash. For
 comparison, a reaction product mixture of 760 grams had a bisimide loss of
 1369 mg using the conventional caustic first wash. It should be clear that
 the present invention includes a method for reducing the yield loss of
 bisimide from a polyimide reaction mixture by washing with a polar
 solvent, preferably water, before washing with caustic.
 While the present invention has been described with reference to particular
 embodiments thereof, it will be understood that numerous modifications may
 be made by those skilled in the art without actually departing from the
 spirit and scope of the invention as defined in the appended claims.