Process Description
Caustic NaCN solution is produced with hydrocyanic acid
which is fed into the crystallizer cycle and with sodium hydroxide
solution (50 wt. %) which is added via the crystallizer. The
reaction takes place within the crystallizer cycle (pos. 5103)
The heat required to evaporate the water of the caustic NaCN
solution is added to the crystallizer cycle at the steam-heated
reboiler (pos. 5102). Before the vapor leaves the crystallizer
at its top, it is washed with fresh sodium hydorxide solution
and is almost entirely free of hydrocyanic acid. Liquid droplets
are carried along with the vapor condensers (pos. 5201 and 0502).
The condensation is achieved with vapor condensate which is
recycled and cooled on plate heat exchangers (pos. 5211 or 5212).
One of the heat exchanger is cooled with cooling tower water
and the other with chilled water. Since the pressure of the
cooling tower water and chilled water is higher than the pressure
of the vapor condensate, the condensate cannot get into the
cooling media.
The residual uncondensed vapor is drawn off with the inert gases
by a three-stage "steam jet injector". The driving
steam is also precipitated with cooled vapor condensate in injection
condenser.
NH3 is formed by saponification of NaCN in the crystallizer
cycle. Part of the ammonia is directly dissolved by vapor condensate
and discharged to the waste water pretreatment unit. The other
part is absorbed by cold vapor condensate in the inert gas washer.
The remaining low quantity of hydrocyanic acid is chemically
absorbed with diluted sodium hydroxite solution in the exit
air washer. The entrained droplets are washed out with vapor
condensate in the rewasher.
Following these gas washers, the uncondensable gases, such as
air drawn in through leaks, are discharged to atmosphere. Their
CN contents is below 5 mg/m3.
The resulting vapor condensate and driving steam are fed into
the "vapor condensate receptacle" and discharged to
the retreatment unit. This process us level-controlled.
The feed pump pumps part of the sodium cyanide suspension from
the crystallizer cycle to the drum filter. The NaCN crystal
are filtered out in the drum filter. The filter cake is predried
with hot air and is then blown to the "double paddle conveyer"
with purge air from the "seal air blower". The "double
paddle coveyer" transports the filter cake counter currently
to hot air into the venturi pipe of the pneumatic conveying
dryer.
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