Process for working up effluents containing nitro-hydroxy-aromatic compounds

The invention concerns a process for working up an effluent containing nitro-hydroxy-aromatic compounds which comprises heating said effluent to 150.degree. C. to 500.degree. C. with the exclusion of air and oxygen and under elevated pressure. The process results in a decrease of nitro-hydroxy-aromatic compound concentration in the effluent to below 20 ppm. and the resulting material can then be fed to a microbiological sewage plant for further working up, without the microorganisms therein being killed by the bactericidally active nitro-hydroxy-aromatic compounds.

The invention relates to a process for working up effluents containing 
nitro-hydroxy-aromatic compounds, such as are obtained, for example, 
during the nitration of aromatic compounds or during the preparation of 
nitro-hydroxy-aromatic compounds. 
It is known that effluents containing organic compounds can be purified and 
worked up by extraction or by wet oxidation (wet combustion) (for the wet 
combustion of organic effluents, see E. Schmitt, Berichte der 
Abwassertechnischen Vereinigung e.V. (Reports of the Industrial Effluents 
Association e.V), volume 8, Verlag R. Oldenburg, Munich 1957). The 
extraction process is expensive in terms of apparatus and thus less 
economical. The decomposition of the nitro-hydroxy-aromatic compounds in 
the effluents is only partly achieved during wet oxidation. The 
concentration, in the effluents of the wet oxidation, of the 
bactericidally active nitro-hydroxy-aromatic compounds which remain is too 
high for subsequent biological working up. 
A process has now been found for working up effluents containing 
nitro-hydroxy-aromatic compounds, which is characterised in that the 
effluents containing nitro-hydroxy-aromatic compounds are heated to 
temperatures in the range from 150.degree. C. to 500.degree. C. with 
exclusion of air and oxygen and under elevated pressure. 
The process according to the invention makes it possible to decrease the 
content of nitro-hydroxy-aromatic compounds in the effluents, such as are 
obtained, for example, during the nitration of aromatic compounds and 
during the preparation of nitro-hydroxy-aromatic compounds, to values 
below 20 ppm. The effluents can thus be fed to a biological sewage plant 
for further working up, without the micro-organisms there being killed by 
the bactericidally active nitro-hydroxy-aromatic compounds. 
It is advantageous for the process according to the invention if the 
effluents contain the nitro-hydroxy-aromatic componds in the form of their 
water-soluble salts. 
The structure and composition of the nitro-hydroxy-aromatic compounds 
present in the effluents depend on the particular aromatic compounds 
employed for the nitration. General application of the process according 
to the invention for working up effluents containing 
nitro-hydroxy-aromatic compounds is not restricted by the structure and 
composition of the nitro-hydroxy-aromatic compounds in the effluents. 
The following nitro-hydroxy-aromatic compounds, which can also be present 
in the form of their water-soluble salts, may be mentioned as examples: 
mono-, di- and tri-nitrophenol, mono-, di- and tri-nitrocresol, mono-, di- 
and tri-nitroresorcinol and mono-, di- and tri-nitroxylenol. 
Possible salt-forming agents are all the metals which are capable of 
forming water-soluble salts with the nitro-hydroxy-aromatic compounds. 
Preferred metals which may be mentioned are the alkali metals, for example 
lithium, sodium, potassium and rubidium. 
In order to ensure that the process according to the invention proceeds 
smoothly, the amount of nitro-hydroxy-aromatic compounds, or salts 
thereof, present in the effluents should be such that complete solution of 
the nitro-hydroxy-aromatic compounds, or salts thereof, in the effluents 
is still possible. The amount of nitro-hydroxy-aromatic compounds in the 
effluents can easily be determined analytically, for example by titration 
with potassium permanganate or by HPLC (High Pressure Liquid 
Chromatography). 
The working up of the effluents containing nitro-hydroxy-aromatic compounds 
can be carried out in a pressure vessel and/or tube reactor at 
temperatures in the range from about 150.degree. to about 500.degree. C., 
preferably at 200.degree. to 350.degree. C., particular care being taken 
that air and oxygen are excluded. It can be appropriate to flush the 
apparatus with an inert gas beforehand. 
It is also possible to heat the effluents under an inert gas atmosphere or 
under an initial pressure of inert gas of, for example, 0.1 to 100 bars. 
Examples of possible inert gases are nitrogen and/or argon. 
When the effluents are heated, pressures in the range from about 50 to 
about 350 bars are establishedl, depending on the temperature and, if 
appropriate, the initial pressure of inert gas. 
The nitro-hydroxy-aromatic compounds present in the effluents are 
decomposed on heating the effluents under pressure. The concentration of 
the nitro-hydroxy-aromatic compounds in the effluents can be reduced to 
values under 20 ppm, preferably under 10 ppm, by the process according to 
the invention. 
About 5 to 120 minutes, in most cases 15 to 30 minutes, are required for 
decomposing the nitro-hydroxy-aromatic compounds, depending on the 
reaction conditions and the nitro-hydroxy-aromatic compounds present. 
The process according to the invention can be carried out as follows: an 
aqueous waste liquor, from the washing process carried out after the 
nitration of benzene, which has a basic reaction and contains about 3,100 
ppm of the sodium salt of mono-, di- and tri-nitrophenol, in addition to 
0.25% by weight of free sodium hydroxide, 0.5% by weight of sodium 
sulphate, 1.8% by weight of sodium nitrate, 0.04% by weight of sodium 
nitrite and 0.05% by weight of sodium carbonate, is brought to a reaction 
temperature of 300.degree. C., whilst stirring, in a pressure vessel 
and/or the reactor which has been flushed with nitrogen and is under an 
initial nitrogen pressure of 30 bars, a pressure of 120 bars being 
established. After a residence time of 15 minutes at this temperature--not 
including the heating and cooling time--the apparatus is cooled to room 
temperature, via a heat exchanger, and the liquor is removed. 
Analysis after this treatment gave the following results: 3.5 ppm of the 
sodium salts of mono-, di- and tri-phenols, 0.1% by weight of free sodium 
hydroxide, 0.5% by weight of sodium sulphate, 0.5% by weight of sodium 
nitrite, 2.0% by weight of sodium nitrate and 0.14% by weight of sodium 
carbonate. The waste liquor was fed undiluted to a biological sewage plant 
and could be worked up there without any trouble. 
The process according to the invention is distinguished by the following 
advantages, compared with other processes for working up effluents from, 
for example, nitration processes or the preparation of 
nitro-hydroxy-aromatic compounds: the process according to the invention 
is simpler in terms of apparatus, requires less energy and is thus 
considerably cheaper than the customary extraction processes. The 
effluents do not need to be freed from extraction agents and the 
extraction agents do not have to be worked up. No substances or 
concentrated extracts which are unacceptable from a safety point of view 
and which would subsequently have to be destroyed separately are thereby 
obtained. 
Furthermore, the process according to the invention is cheaper than the 
process of direct combustion of effluents, which consumes a very large 
amount of energy. 
In comparison to the wet oxidation, which does not achieve decomposition of 
the nitro-hydroxy-aromatic compounds to values below 20 ppm, the values 
obtained in the process according to the invention can, surprisingly, 
within the same or shorter reaction times, be brought far below 20 ppm, by 
which means biological after-treatment of the effluents is first possible 
at all. 
The effluents pre-treated by the processes according to the invention can 
be fed directly, without dilution, to a biological sewage plant. 
A further advantage of the process according to the invention is that 
deposition of dirt or cracked products on the walls of the heat exchanger 
or reactor is avoided.

The examples which follow serve to illustrate the process according to the 
invention: 
EXAMPLE 1 (COMISON EXPERIMENT) 
1,200 ml of an aqueous-alkaline waste liquor, from a benzene nitration 
process, containing 3,100 ppm of the sodium salts of mono-, di- and 
tri-nitrophenol, 0.25% by weight of sodium hydroxide, 0.5% by weight of 
sodium sulphate, 1.8% by weight of sodium nitrate, 0.04% by weight of 
sodium nitrite and 0.05% by weight of sodium carbonate are filled into a 
two liter autoclave with a stirrer and a means for measuring the pressure 
and the temperature. Air is then forced in up to a pressure of 15 bars. 
The autoclave is then heated to 300.degree. C. and left at the reaction 
temperature for 15 minutes. A pressure of 90 bars results. After cooling, 
the waste liquor is removed from the autoclave. Analysis gives the 
following values: 580 ppm of the sodium salts of mono-, di- and 
tri-nitro-phenol, 0.17% by weight of sodium hydroxide, 0.5% by weight of 
sodium sulphate, 1.9% by weight of sodium nitrate, 0.05% by weight of 
sodium nitrite and 0.12% by weight of sodium carbonate. 
It was found that this waste liquor cannot be worked up biologically. 
EXAMPLE 2 
The waste liquor employed in Example 1 was treated in a two liter autoclave 
analogously to Example 1, but the autoclave was flushed with nitrogen 
beforehand and, instead of air, nitrogen is forced in up to a pressure of 
30 bars. The autoclave is then heated again to 300.degree. C., whilst 
stirring, and kept at the reaction temperature for 15 minutes. During the 
procedure, the pressure increases to 114 bars. After cooling and removing 
the liquor, these were analysed. The following values were obtained: 2.6 
ppm of the sodium salts of mono-, di- and tri-nitrophenol, 0.12% by weight 
of sodium hydroxide, 0.5% by weight of sodium sulphate, 2.1% by weight of 
sodium nitrate, 0.05% by weight of sodium nitrite and 0.2% by weight of 
sodium carbonate. 
It was found that the waste liquor can readily be worked up biologically 
without trouble and without further dilution. 
EXAMPLE 3 
A waste liquor containing 4,300 ppm of the sodium salts of mono-, di- and 
tri-nitrophenol, 0.5% by weight of sodium hydroxide, 0.5% by weight of 
sodium sulphate, 1.7% by weight of sodium nitrate, 0.04% by weight of 
sodium nitrite and 0.05% by weight of sodium carbonate is treated in an 
autoclave exactly as described in Example 2. Subsequent analysis of the 
waste liquor gives the following results: 1.8 ppm of the sodim salts of 
mono-, di- and tri-nitrophenol, 0.30% by weight of sodium hydroxide, 0.5% 
by weight of sodium sulphate, 2.2% by weight of sodium nitrate, 0.08% by 
weight of sodium nitrite and 0.3% by weight of sodium carbonate. 
The waste liquor can be used in the undiluted form for biological working 
up. 
EXAMPLE 4 
A waste liquor, from a toluene nitration process, containing 1,980 ppm of 
the sodium salts of mono-, di- and tri-nitrocresol, 0.4% by weight of 
sodium hydroxide, 0.4% by weight of sodium sulphate, 1.7% by weight of 
sodium nitrate, 0.03% by weight of sodium nitrite and 0.06% by weight of 
sodium carbonate is heated to 300.degree. C., again as described in 
Example 2, under nitrogen and in an autoclave, a pressure of 109 bars 
being established. After a residence time of 15 minutes, the autoclave is 
cooled and the waste liquor removed from the autoclave is investigated 
analytically: 0.4 ppm of the sodium salts of mono-, di- and 
tri-nitro-cresol, 0.35% by weight of sodium hydroxide, 0.4% by weight of 
sodium sulphate, 1.9% by weight of sodium nitrate, 0.03% by weight of 
sodium nitrite and 0.16% by weight of sodium carbonate. 
This waste liquor can also be worked up in the undiluted form in a 
biological sewage plant. 
EXAMPLE 5 
A waste liquor, from a xylenol-nitration process, containing 2.600 ppm of 
the sodium salts of mono-, di- and tri-nitro xylenol and mono-, di- and 
tri-nitro-cresol, 0.45% by weight of sodium hydroxide, 0.5% by weight of 
sodium sulfate, 1.6% by weight of sodium nitrate, 0.03% by weight of 
sodium nitrite and 0.35% by weight of sodium carbonate is heated to 
290.degree. C., as described in Example 2, under nitrogene and in an 
autoclave, a pressure of 101 bars being established. After a resistance 
time of 15 minutes, the autoclave is cooled and the waste liquor removed 
from the autoclave is investigated analytically: 0.3 ppm of the sodium 
salts of mono-, di- and tri-nitro-xylenol and mono-, di- and 
tri-nitro-cresol, 0.3% by weight of sodium hydroxide, 0.5% by weight of 
sodium sulfate, 1.7% by weight of sodium nitrate, 0.03% by weight of 
sodium nitrite and 0.45% by weight of sodium carbonate. The waste liquor 
can be used in the undiluted form for biological working up. 
EXAMPLE 6 
A waste liquor, from a xylenol-nitration process, containing 3.100 ppm of 
the sodium salts of mono-, di- and tri-nitro-xylenol, mono-, di- and 
tri-nitro-cresol and mono-, di- and trinitro-resorcinol, 0.5% by weight of 
sodium hydroxide, 0.6% by weight of sodium sulfate, 1.9% by weight of 
sodium nitrate, 0.04% by weight of sodium nitrite and 0.3% by weight of 
sodium carbonate is heated to 310.degree. C., as described in Example 2, 
under nitrogene and in an autoclave, a pressure of 116 bars being 
established. After a resistance time of 15 minutes, the autoclave is 
cooled and the waste liquor removed from the autoclave is investigated 
analytically: 0.8 ppm of the sodium salts of mono-, di- and 
tri-nitro-xylenol, mono-, di- and tri-nitro-cresol and mono-, di- and 
tri-nitro-resorcinol, 0.35% by weight of sodium hydroxide, 0.6% by weight 
of sodium sulfate, 2.1% by weight of sodium nitrate, 0.04% by weight of 
sodium nitrite and 0.45% by weight of sodium carbonate. 
This waste liquor can be worked up in the undiluted form in anbiological 
sewage plant.