Removal of nitrocresols from dinitrotoluene waste streams using fentons reagent

This invention relates to a process for removing trinitrocresols and picric acid contaminants from a wastewater stream generated in the production of nitroaromatics, particularly dinitrotoluene, by the mixed acid technique. The process involves contacting the crude dinitrotoluene generated by the mixed acid technique with an alkaline medium to generate an alkaline wash water containing water soluble nitrocresols and picric acid therein. This wastewater then, is separated from the organic component or may be recycled for contact with further quantities of crude dinitrotoluene product from the reactor. When the concentration of the water soluble salts of trinitrocresols and picric acid is of sufficient concentration, the wash water is treated with aqueous acid in sufficient proportion to reduce the pH to a level from 3-4. After pH adjustment, the medium is contacted with hydrogen peroxide and a ferrous ion under conditions to effect oxidation of a substantial portion of the trinitrocresol to carboxylic acid, nitric acid and carbon dioxide.

TECHNICAL FIELD 
This invention relates to an improved process for removing nitrocresols and 
organic water insoluble components from a nitroaromatic reaction product 
stream without generating an environmentally unacceptable aqueous 
discharge stream. 
BACKGROUND OF THE INVENTION 
Commercially, nitroaromatics, and particularly dinitrotoluene, are produced 
by the mixed acid nitration of toluene, the mixed acid being a mixture of 
concentrated sulfuric and concentrated nitric acid. In the production of 
dinitrotoluene process, for example, toluene is first nitrated to form 
mononitrotoluene and then separated from the spent acid aqueous phase. The 
crude mononitrotoluene is then dinitrated with fresh acid in a second 
nitration stage. As is known the dinitrotoluene product recovered from the 
dinitration reactor contains impurities, primarily nitrophenolics, such as 
nitrocresol and picric acid. 
Traditionally, it has been common practice to remove the nitrophenolic 
materials from the organic dinitrotoluene phase because it has has been 
believed they adversely affect the performance of hydrogenation catalysts 
in the reduction of dinitrotoluene to form toluenediamine. Removal of 
nitrophenolic material from the dinitrotoluene reaction product has been 
achieved by contacting that product with alkaline materials to convert the 
nitrophenolic materials with the crude dinitrotoluene reaction product to 
water soluble salts. The water soluble salts then are discharged. 
Recent environmental regulations have placed severe restrictions on the 
discharge of aqueous stress containing alkali metal salts of nitrophenolic 
materials. As is known these materials are not readily subject to 
biodegradation and then there is an unknown factor regarding the toxicity 
of the materials in the amounts that would normally be discharged to the 
environment. Therefore it is desired that techniques be developed to 
remove nitrophenolic materials from a dinitrotoluene reaction product 
without creating an environmentally unacceptable aqueous discharge stream. 
U.S. Pat. No. 4,482,769, although not prior art to this application, 
discloses a process for separating trinitroorthocresol from a reaction 
product while leaving dinitroorthocresol in the dinitrotoluene product. 
The process involves selectively precipitating the dinitroorthocresol from 
an aqueous stream by contacting with alkaline material. 
Patents which show the removal of nitrophenolic material from crude 
dinitrotoluene streams by the addition of alkaline material are British 
Pat. No. 1,031,450; and U.S. Pat. Nos. 4,224,249; 4,361,712 and 4,230,567. 
Only the '567 patent addresses the problem of disposal of the wastewater 
streams containing alkali metal salts of nitrophenolic material. As 
acknowledged in that patent, direct incineration of the wastewater stream 
is considered to be energy intensive and is unacceptable for that reason. 
The approach taken in the '567 patent involves a degradation process as 
opposed to a combustion process. 
Phenolic materials have presented problems when present in minimal waste 
streams and have been removed by various treatments. The use of Fenton's 
reagent was suggested as a means for oxidizing phenol and substituted 
phenols to hydroquinone and muconic acid. Phenols containing metal 
directing groups such as chloro, carboxyl and nitro groups have also been 
oxidized through the use of Fenton's reagent. Eisenhowever, Oxidation of 
Phenolic Wastes, 36 J. Water Pollution Control Federation, 1116 (1964). 
SUMMARY OF THE INVENTION 
This invention relates an improvement in a process for removing nitrocresol 
material produced in the nitration of aromatic compounds by the mixed acid 
technique. The improvement resides in contacting the resultant crude 
nitroaromatic product with an alkaline material to convert 
trinitrophenolic material therein to a water soluble salt, and thereby 
form a purified nitroaromatic organic/water-insoluble product and an 
aqueous by-product phase containing the alkali metal salt of 
trinitrophenolic materials; separating the aqueous phase from the organic 
phase; contacting the aqueous phase containing water soluble 
trinitrocresolic material with an acid, said acid being added in 
sufficient proportion to reduce the pH of the stream to below about 4 in 
said aqueous phase; contacting the aqueous phase with hydrogen peroxide 
and ferrous ion in sufficient proportion and under conditions to oxidize 
said trinitrocresolic material to nitric acid, carbon dioxide and 
carboxylic acid. 
Advantages of the process include: 
an ability to remove trinitrocresol contaminants by-products generated in 
nitroaromatic production without creating an environmentally unacceptable 
waste stream; 
an ability to remove contaminants in a nonenergy intensive manner; 
an ability to render such composition innocuous; and 
an ability to achieve optimum oxidation rates through the use of a 
continuous process. 
DETAILED DESCRIPTION OF THE INVENTION 
In the commercial manufacture of nitroaromatics, particularly 
dinitrotoluene, an aromatic compound is nitrated under liquid phase 
conditions using a mixture of concentrated nitric acid and sulfuric acid. 
In the production of nitroaromatics and particularly dinitrated products, 
e.g., dinitrobenzene or dinitrotoluene, some by-product nitrophenolic 
material is produced. This nitrophenolic material usually is in the form 
of nitrocresols, either dinitro or trinitrocresol, and picric acid. It is 
this by-product which must be removed from the crude reaction product from 
the nitration reactors without creating an environmentally unacceptable 
stream. Removal of this material is necessary from the nitroaromatic 
product as many believe the presence of nitrophenolic materials interferes 
with the catalyst in subsequent reduction of the nitro group. 
In nitration processes the reaction product is removed from the nitration 
zone and passed to a separator where the organic phase is separated from 
the aqueous phase. According to the process herein, the crude 
nitroaromatic composition is contacted with a dilute aqueous 
alkaline-containing solution to convert nitrocresols and picric acid to 
water soluble salts thereby generating an organic phase and an aqueous 
phase. Conventionally aqueous alkaline material suited for converting the 
nitrophenolic material to water soluble salts include sodium carbonate, 
ammonium hydroxide, sodium hydroxide, sodium bicarbonate, potassium 
hydroxide, and other alkaline materials. Solution concentrations for 
achieving conversion to water soluble salts generally are from about 0.1 
to 50% by weight, and generally from about 1 to 10% by weight. 
Contacting of the crude organic product with an aqueous alkaline solution 
is at a temperature from about 25.degree. to 80.degree. C. typically at 
atmospheric pressure to about 50 psig. Normally contacting is done at or 
about 70.degree. C. and atmospheric pressure as this appears to be the 
most convenient way of converting the nitrophenolic materials to water 
soluble salts. Neither temperature nor pressure is critical to the 
conversion step. 
Once the crude nitroaromatic composition has been treated with aqueous 
alkaline material, an organic layer and aqueous layer are formed. The 
aqueous layer is separated from the organic layer by decanting leaving a 
top aqueous layer containing water soluble salts of nitrophenolic material 
e.g., water soluble salts of dinitrocresol and trinitrocresol. To maximize 
the effectiveness of the alkaline treatment, the aqueous alkaline mixture, 
after separation from the treated organic phase, is often recycled for 
contact with additional quantities of crude nitroaromatic product to 
enhance or increase the concentration of the water soluble salts in the 
aqueous phase and decrease the amount of unreacted aqueous alkaline 
material in that phase. Generally, the aqueous alkaline phase obtained on 
separation from the organic layer after nitration is recycled until the 
concentration of alkali metal salts of nitrophenolic material ranges from 
about 0.5 to 1.5% by weight usually 0.9-1.2% by weight. 
When the concentration of water soluble nitrophenolic salt in the aqueous 
medium reaches from desired concentration, at least a portion of the 
aqueous phase is separated for further treatment and disposal of the 
water-soluble salts of nitrophenolic material. In contrast to the prior 
art, the aqueous phase containing water-soluble nitrophenolic salt is 
first treated with an acidic material under conditions sufficient to 
reduce the pH of the aqueous phase to below about 4.5 for contact with an 
oxidizing agent, preferably about 2-4. This reduction can be accomplished 
by the addition of an inorganic acid such as nitric acid or sulfuric acid. 
Since both of these acids are available as spent acids from the nitration 
process, nitric acid and sulfuric acid are preferred. Addition of the acid 
is done at temperatures from about 25.degree.-80.degree. C. and 
atmospheric pressure to about 50 psig, typically 25.degree.-30.degree. C. 
and atmospheric pressure. Although some oxidation of phenolic materials 
can be effected at a pH as high as 5, trinitrocresols do not oxidize under 
such conditions. Continuous oxidation is preferred since a constant pH may 
be maintained leading to optimum oxidation rates and optimum usage of 
peroxide. This occurs at a pH of about 3. A batch oxidation is less 
preferred because of the generation of a pH profile leading to slower 
rates of oxidation. 
To effect oxidation of the trinitrocresol, a reagent referred to as 
Fenton's reagent comprising hydrogen peroxide and a ferrous iron source is 
added to the wastewater. At elevated temperatures, e.g. from 70.degree. to 
90.degree. C., the trinitrocresols and picric acid are oxidized to nitric 
acid, carbon dioxide, and carboxylic acids within a reaction time of about 
one-half to one hour. Often these may be ring ruptured compounds which as 
soluble in acidic streams present as waste acid in a nitroaromatic plant. 
Complete mineralization of trinitrocresol to carbon dioxide and nitric 
acid requires 17 moles of hydrogen peroxide per mole of trinitrocresol. 
Sometimes not all of the trinitrocresol need be oxidized in order to 
provide for a stream combinable with other waste to produce an 
environmentally acceptable stream, therefore, lesser quantities of 
hydrogen peroxide may be used. Typically, for purposes of this invention 
from about 7 to 12 moles hydrogen peroxide are added per mole of 
trinitrocresol. As stated, sufficient hydrogen peroxide is added to the 
system to reduce the trinitrocresol content to about 150 to 600 or 
approximately 5 to 20% of that originally in the stream. This level is 
sufficiently low that the trinitrocresol or phenolic material when added 
to plant effluent does not precipitate on addition to an acidic waste 
stream. A weight ratio of peroxide to total nitrocresol material used is 
from 1.1 to 3.0 or preferably 1.3 to 1.8. In other words, the oxidation of 
the trinitrocresols to a level well below their solubility in the 
wastewater at temperatures of 30.degree. C., and preferably as low as 
20.degree. C. In other words, if the wastewater stream were cooled to a 
temperature of about 20.degree. C., no precipitation of trinitrocresol 
would occur and thereby create a hazard in wastewater disposal. This level 
is typically from 5-20% of the level in the original waste. 
The ferrous ion used to catalyze the oxidation of nitrophenolic material is 
provided by a ferrous salt, typically ferrous sulphate. This is added in 
an amount to provide from 2.5-5.times.10.sup.- M, and preferably 
3-4.times.10.sup.-3 molar concentration. 
The following examples are provided to illustrate various embodiments of 
the invention and are not intended to restrict the scope thereof.

EXAMPLE 1 
Into a 304 ml glass stirred tank reactor is charged 250 ml of 
dinitrotoluene alkaline wash water obtained on treatment of the reaction 
product from a dinitrotoluene plant. The vessel contents are agitated by 
means of two stainless steel impellers and the vessel raised to 70.degree. 
C. by intermittent cooling coils. Three feed pumps are then activated with 
the composition of each of the feeds to the vessel and rates as follows. 
Alkaline wash water containing water soluble salts of trinitrocresol 
3.24.times.10.sup.-3 moles ferrous sulfate heptahydrate per liter wash 
water--6.7 g/min 
Sulfuric acid wash water 6% H.sub.2 SO.sub.4 and 6% NHO.sub.3 --0.14 g/min 
(as controlled by pH controller--set pH=3) 
Aqueous hydrogen peroxide--8.06% by weight at 0.25 g/min., the weight ratio 
H.sub.2 O.sub.2 /trinitrocresol equaled 1.11:1 grams hydrogen peroxide per 
gram of trinitrocresol. 
The reactor was permitted to achieve steady state (3.6 hours) at a 
pH=3.+-.0.03 and samples were then removed for analysis for dinitroluene 
by gas chromotography and for dinitrocresols and trinitrocresols by high 
performance liquid chromotography (HPLC). The results of these analyses 
are shown in Table 1. 
The wastewater treated with Fenton's agent was acidified to a pH=1 with 
concentrated sulfuric acid and no precipitate was observed. This shows 
sufficient oxidation of the trinitrocresol had taken place to produce an 
environmentally acceptable stream which would not precipitate on contact 
with waste acid water. 
TABLE 1 
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Removal of Organics in Dinitrotoluene Alkaline 
Wash Water with Fenton's Reagent 
Alkaline Reactor % 
Wash Water Effluent Removal 
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Dinitrotoluenes 
1210 ppm 802 ppm 34 
Dinitrocresols 
136 ppm 43 ppm 68 
Trinitrocresols 
2602 ppm 274 ppm 89 
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EXAMPLE 2 
The procedure of Example 1 was repeated in a 218 ml CSTR reactor with the 
feed rates set as follows. 
Alkaline wash water--4.20 g/min containing 3.54.times.10.sup.-3 moles 
ferrous sulfate heptahydrate/liter of waste water. 
Acid wash water--0.090 g/min (as controlled by pH controller set at 
pH=3.30). 
Aqueous hydrogen peroxide--0.25 g/min at 9.18% by weight. 
The steady state reactor effluent was sampled and analyzed as in Example 1. 
The results are displayed in Table 2. 
TABLE 2 
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Removal of Organics in Dinitrotoluene Alkaline 
Wash Water with Fenton's Reagent 
Alkaline Reactor % 
Wash Water Effluent Removal 
______________________________________ 
Dinitrotoluenes 
1420 ppm 742 ppm 48 
Dinitrocresols 
254 ppm 19 ppm 93 
Trinitrocresols 
3818 ppm 156 ppm 96 
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