Waterwright
Saving water makes cents.
Sodium
azide is used as the source of nitrogen in the detonation of automobile air
bags.
It
is found in the wash water and laundry water of the air bag propellant plants.
Since it is very toxic it must be removed from the water before
discharging to the sewer.
Sodium
azide can be treated to less than 1 ppm using sodium nitrite.
The treatment involves converting the sodium azide to hydrogen azide so
the reaction with the sodium nitrite can proceed.
This process must be done carefully since the hydrogen azide is a
poisonous gas and is explosive. The
treatment vessel should be scrubbed and explosion proofed to class 1 div. 2
standards.
The
treatment cycle requires the analysis of the incoming wastewater to determine
the azide concentration. Using the
azide concentration the, nitrite dosage is determined and the nitrite is added
to the reactor. The azide is
transferred to the reactor maintaining a pH of
above 8. Once the transfer
is complete the pH is slowly lowered under agitation with sulfuric acid.
As the pH drops below 5, nitrogen gas is released.
The pH is lowered slowly until it reaches 2.8.
By this time, the azide should be completely reacted.
The water can be tested with ferric chloride.
If a red ferric azide solution forms there is still residual azide and
more nitrite must be added. If too
much nitrite is added and the pH is dropped too fast NO2 gas is released and can
be seen as a brown plume at the scrubber discharge. Running the reaction slowly, using the correct amount of
sodium nitrite, and running sodium hypochlorite in the scrubber can eliminate
the plume.
N3-
+ H+
è. HN3
HN3
+ H2O + 2 e- è 3/2 N2
+ 2OH-
NO2-
+ 2H2O è ½ N2 + 4 OH- + 2e-
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