Chemical Engineering June 2020 - 48

Separator
Rectifier
Condenser
Steam
Spent acid
Total
HCl
FeCl2
H20
Total
HCI
FeCl2
H20
208.00 kg/h
12.50
31.25
164.25
Metal conc.
76.50 kg/h
0.75
31.25
44.50
Figure 5. The vacuum evaporation
process shown here processes
5 m.t./d of spent HCl
plants choose to dispose, treat or
recover their acid rather than sacrifice
plant floorspace, time and energy
to squeeze value from the acid.
Management is often faced with the
dilemma of allowing " the tail to wag
the dog " - sacrificing speed of production
over waste minimization.
recovery of the acid addresses that
conundrum. Most of the commonly
used and proven technologies are
described below.
Vacuum evaporation. hydrochloric
acid is a binary solution of hcl
gas in water used extensively in
steel
Total
HCl
FeCl2
H20
Total
HCl
FeCl2
H20
Vacuum
Distillate
67.50 kg/h
0.50
0.00
67.00
Recovered
acid
64.00 kg/h
11.25
0.00
52.75
pickling and mining. When
the pickling action slows, the inhibited
solution of acid, water and
metal salt enters a vacuum evaporator
(figure 5) and is heated above
the boiling point of hcl (108.6°c)
and water. to maintain sensible
heat and ensure a more efficient
heat cycle, the forced circulation of
the mother liquor generates a mild
backpressure. With the rest of the
process operating under vacuum,
the pressurized and heated solution
is sprayed into a separator,
where both the water and hydrochloric
acid co-flash to vapor. the
remaining metal chloride salt solution
concentrates as more spent
acid enters the evaporator and
the hcl/h2o solution is vaporized.
When the metal salt's specific gravity
reaches the target, the metal
chloride concentrate is pumped to
a storage tank for transport to an
end user.
the hcl/h2o vapor travels
through a rectifier filled with acidproof
packing. a spray of water
48
descending over the top of the
packing mixes with hcl vapor as it
descends. the droplets increase in
acid concentration as they descend
and approach the 20% hcl azeotrope.
the remaining water vapor
passes upward through the packing
and into a water-cooled condenser.
the vapor condenses into reusable
process water. the rectification
process results follow the Mccabethiele
equation.
for efficiency, evaporative recovery
operates under partial vacuum.
this depresses the boiling point of
hcl from 108.6°c (227°f) to less
than 80°c (175°f). vacuum processing
lowers
the
energy
consumption,
cost of construction and
potential for chemical excursions.
all three major streams from the
vacuum evaporator are re-used or
repurposed. the final acid concentration
ranges between 15 and
20% - well within most pickling
and leaching parameters. if acid
concentrations are required above
the 20% azeotrope, the addition of
Spent acid
Total
HCl
FeCl2
H20
208.00 kg/h
12.50
31.25
164.25
Deionized water
Total
HCl
FeCl2
H20
112.50 kg/h
0.00
0.00
112.50
Figure 6. This acid sorption process processes 5 m.t./d of spent HCl
ChemiCal engineering www.Chemengonline.Com June 2020
Resin
column
Total
HCl
FeCl2
H20
Filter
Total
HCl
FeCl2
H20
Cooling
tower
a dual-pressure distillation section
would be required. in steel pickling
or leaching, the iron chloride concentrate
captured has commercial
value in water treatment, odor
suppression and as a precursor to
the production of ferric chloride.
Processing one ton of spent acid
requires approximately 22 therms
of natural gas to produce the required
steam.
Acid sorption (retardation). this
technology is a sorption process by
which acid bonds to a special resin
inside a packed " short bed " column,
while allowing the metal chloride
and water to pass through. When
the resins have reached maximum
acid loading, the column is eluted
using water to recover the absorbed
acid on a batch basis. the process
flow diagram (figure 6) follows a
near spent hcl solution from a steel
pickling plant. the recovered acid
stream contains residual iron and
water. the waste metal concentrate
is nearly devoid of acid but requires
treatment as a liquid waste. the
sorption process allows continuous
use of the acid with low levels
of metal contamination. efforts over
the past decade to minimize metal
carryover into the recovered, clean
acid have shown improvement. Selective
capture and reuse of different
fractions of the eluate tend to lower
the waste and enrich the recovered
acid product.
Diffusion dialysis. this membrane
process (figure 7) operates under
the diffusion principle, where high
concentrations of molecules spontaneously
travel to areas of lower
concentration. eventually the conClean
acid
133.30 kg/h
12.00
17.30
104.00
Metal conc.
187.20 kg/h
0.50
13.95
172.75
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Chemical Engineering June 2020

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