Cyanide reduction in nitroaromatic process

The invention relates to a method of eliminating or substantially reducing the amount of cyanide formed in a polynitroaromatic, e.g. dinitrotoluene process. The invention comprises removing the nitrophenolic material from the mononitroaromatic formed in a first stage nitration of toluene prior to feeding the mononitroaromatic to the subsequent nitration zones.

Nitroaromatics, particularly dinitrotoluene, are widely used as 
intermediates in the manufacture of aromatic amines, e.g. toluene diamine 
which then can be converted to isocyanates for polyurethane manufacture. 
Commercially, dinitrotoluene, for example, is produced by the mixed acid 
nitration of toluene, the mixed acid being a mixture of concentrated 
sulfuric acid and nitric acid. In this process mononitrotoluene is formed 
in a first nitration stage and then separated from the aqueous phase. The 
crude mononitrotoluene is then dinitrated with fresh acid in a second 
nitration stage and the aqeuous phase is recycled to the mononitration 
stage. The dinitrotoluene then is recovered from the dinitration stage and 
the impurities removed. 
One of the conventional treatments for removing impurities, usually in the 
form of phenolic materials, e.g. nitrocresols, from dinitrotoluene prior 
to conversion to toluene diamine production, has been to wash the 
dinitrotoluene with an aqueous alkaline material, e.g. an alkali metal 
carbonate or alkali metal hydroxide. These alkaline materials convert the 
nitrocresols formed during the nitration reaction to water soluble salts 
which are largely dissolved in the alkaline phase. The remaining salts and 
alkali are then removed from the dinitrotoluene by washing the 
dinitrotoluene with water. 
It has been observed that during the nitration of aromatics, e.g. 
polynitroaromatics, a substantial amount of hydrogen cyanide or other 
cyanide containing compounds are formed. Even though the cyanide compounds 
are produced in a small amount, as compared to other by-products, the 
level of production is such that it is in excess of that normally 
permitted for environmentally acceptable waste streams. Techniques are 
available for removing the cyanide material from the waste streams, 
although most of these procedures merely shift the environmental problem 
from one area to another. 
SUMMARY OF THE INVENTION 
This invention relates to an improvement in a two step nitration process 
for producing polynitroaromatics, preferably dinitrotoluene from a 
mononuclear aromatic compound by the mixed acid technique. The improvement 
resides in the reduction of cyanide material in the process. Cyanide 
prevention is achieved by removing nitrophenolic material from the crude 
mononitroaromatic feed produced in the first nitration stage prior to 
effecting polynitration of the crude mononitroaromatic in the second and 
subsequent nitration stages. The level of nitrophenolic material in the 
crude mononitroaromatic generally should average less than 2000 parts per 
million by weight in order to achieve cyanide concentrations for 
environmentally acceptable waste streams. 
Several advantages are associated with the process of this invention: A 
first advantage includes an ability to reduce cyanide formation in the 
process and provide for environmentally acceptable streams. Secondly, 
yield loss due to nitrous acid formation via the apparent reaction of 
nitric acid with nitrophenolic material is reduced. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It has been found that if the nitrophenolic materials are removed or 
substantially reduced in the crude mononitroaromatic feed prior to 
nitrating the mononuclear nitroaromatic in the subsequent nitration zone, 
the cyanide and nitrous acid concentrations are reduced dramatically. 
(Nitrophenols are precursors to cyanide in the nitration process, and 
therefore, extraction of the nitrophenols from the mononitroaromatic prior 
to subsequent nitration results in a reduction of cyanide generation.) 
For purposes herein, the invention is described in a typical dinitrotoluene 
process with nitrocresols (cresols) being the nitrophenolic material. The 
removal or reduction of nitrocresols can be accomplished by several 
techniques all of which are known in the prior art. One of the most common 
ways is to convert the nitrocresols to water soluble salts. In this regard 
an alkali metal hydroxide, carbonate or bicarbonate is used to convert the 
cresols to water soluble salts. The cresols then are effectively removed 
with the aqueous phase following the first stage nitration. Conventional 
aqueous alkaline materials suited for practicing the invention include 
sodium carbonate, ammonium hydroxide, sodium hydroxide, sodium 
bicarbonate, potassium hydroxide, and other alkaline materials. Solution 
concentrations for achieving extraction are from about 0.1-50% by weight, 
and generally 1-10% by weight. 
Another technique that can be used for removing the cresols, but is not as 
common as the alkali metal hydroxide treatment, is the use of any basic 
ion exchange system. In this system the crude mononitrotoluene is washed 
with water and is then passed over an ion exchange resin in the basic 
form. Another modification is that this extraction process can be 
performed in a cyclic manner, i.e., the water washed mononitrotoluene is 
passed over the resin to remove cresols, the cresols are then flushed from 
the resin, and the resin regenerated for subsequent processing of 
mononitrotoluene. Examples of conventional resinous materials for making 
the ion exchange resins are urea-formaldehyde resins, and cross-linked 
copolymers of aromatic divinyl compounds, e.g. a styrene-divinylbenzene 
copolymer. Generally, the functionality present on the ion exchange resin 
is an amine group, or a quaternary ammonium group. 
Normally, it is important to reduce the cresol content in the crude 
mononitrotoluene to an average level below about 2000 parts per million by 
weight, and preferably below about 500 parts per million to provide an 
environmentally acceptable waste stream. In order to reduce the cresol 
content to a level such that an environmentally acceptable waste stream, 
i.e., one that is substantially free of cyanide, is obtained, at least a 
sufficient quantity of alkaline material must be present to convert the 
cresols into water soluble salts and preferably the concentration of 
alkaline material in the wash will be upward of 50 to 500% of the 
stoichiometric quantity required. Of course if less than stoichiometric 
quantities of alkaline material are used, reduction in cyanide and nitrous 
acid formation will be observed, but such reduction will be proportional 
to the cresol removed and may not be sufficient to meet stringent 
environmental regulations. 
Although not intending to be bound by theory, it is believed the mechanism 
for cyanide and nitrous acid formation is as follows: 
##STR1## 
Mononucleararomatics other than toluene which can be used in the practice 
of this invention include benzene, nitrobenzene, chloronitrobenzene xylene 
and the like.

The following example is provided to illustrate a preferred embodiment of 
the invention and is not intended to restrict the scope thereof. 
EXAMPLE 1 
Toluene was nitrated in conventional manner in a 400 ml. continuous stirred 
reactor at 50.degree. C. The feed to the first stage nitration reactor 
consisted of toluene, 43.2% aqueous nitric acid, and 98% sulfuric acid, 
the feed rate being 10.3 ml per minute for the toluene, 12.5 ml per minute 
for the nitric acid, and 17.2 ml per minute for the sulfuric acid. The 
reactor product overflowed into a separator at a rate equal to the feed 
rate. 
The organic phase in the separator averaged about 11% by weight unreacted 
toluene, 1.2% by weight dinitrotoluene, 87% mononitrotoluene and about 
0.3-0.6% nitrocresols. The organic phase then was washed with an equal 
volume of water and twice with excess alkaline material consisting of an 
equal weight portion of aqueous 0.1 N sodium hydroxide solution. After 
washing with the sodium hydroxide solutions, the organic phase was washed 
again with water to remove all traces of alkali. 
The alkali treated crude mononitrotoluene substantially free of 
nitrocresols was then fed into a 400 ml second step nitration zone at a 
rate of 12.15 ml per minute along with 8.41 ml per minute aqueous 70% 
nitric acid and 18.08 ml per minute 98% sulfuric acid. The dinitration 
zone was maintained at 70.degree. C. Product was continuously removed from 
the dinitration zone and passed to a separator where it was separated into 
an organic phase and aqueous phase. The organic phase was washed with an 
equal volume of water for about 5 minutes and the water analyzed. The 
concentration of cyanide in the form of HCN in the wash water was 2.7 
parts per million by weight. The spent acid contained 895 parts per 
million by weight nitrous acid. 
A control experiment was performed in accordance with the above dinitration 
procedure except that the crude mononitrotoluene containing the 0.3-0.6% 
nitrocresols obtained from the first stage nitration was fed to the second 
stage dinitration without being washed with the aqueous sodium hydroxide 
solution. The HCN content of the wash water contained in the organic phase 
fom the dinitrator was 86 parts per million and the spent acid phase 
contained 2970 parts per million nitrous acid. 
These runs show that the reduction or removal of cresols from the crude 
mononitrotoluene feed to the dinitration zone is effective for reducing 
both cyanide and nitrous acid concentration in the waste effluent and 
spent acid from the dinitration zone. Another significant advantage of the 
processes utilizing the alkali wash of crude mononitrotoluene is that the 
dinitrotoluene product may have significantly lower quantities of cresols 
present and may not require alkali treatment. If, however, an alkali wash 
is utilized for the dinitrotoluene product, particularly where the 
dinitrotoluene is used for toluene diamine manufacture, the alkali wash 
solution for washing the dinitrotoluene may be recycled to the crude 
mononitrotoluene wash cycle to maintain waste water effluent at a low 
level.