Chemical Engineering June 2020 - 50

Clean acid
Total
HCl
FeCl2
H20
183.75 kg/h
11.40
7.50
164.85
Spent acid
Total
HCl
FeCl2
H20
208.00 kg/h
12.50
31.25
164.25
Filter
Total
HCl
FeCl2
H20
Diffusion dialysis
membrane
Deionized water
194.00 kg/h
0.00
0.00
194.00
Metal conc.
Figure 7. Diffusion dialysis can also be used
to process 5 m.t./d of spent HCl
centration approaches equilibrium.
dialysis deals with the rate of molecular
movement through a semipermeable
membrane. diffusion
dialysis uses an anion exchange
(positively charged) membrane as
the semi-permeable barrier. although
also positively charged,
acid acts more like a small protonated
water molecule and passes
through the membrane at an order-of-magnitude-higher
rate than
the much larger metal salts. clean
water (dialyzate) is fed on the permeate
side of the membrane to
absorb acid passing through the
semi-permeable
membrane
surface.
By passing in a countercurrent
manner, the acid concentration increases
as it passes across a long,
serpentine path toward saturation.
diffusion dialysis
is a continuous
process that depends on excellent
mass transfer and multi-layered
stacks of membranes. the membranes
do not completely reject the
metal ions but greatly retard their
concentration. the membranes
may be impacted by fouling and
concentration polarization. Prefiltration
and process control are very
important to process efficiency and
membrane longevity.
Acid replacement with oxalic acid.
recent attempts to recover hydrochloric
acid by reacting the spent
acid with oxalic acid have shown
some promise. although the use of
oxalic acid with iron-laden acid leach
in the mining and minerals industry is
not novel, use of the organic acid to
treat and recover spent hydrochloric
50
Total
HCl
FeCl2
H20
218.25 kg/h
1.10
23.75
193.40
acid is. the basic reaction seems to
be the following:
fecl2 + h2c2o4 →
fe(c2o4)(s) + 2hcl
(6)
this batch process adds a measured
amount of oxalic acid powder
to a mixing reactor filled with an aliquot
of the pickling bath. after a few
hours of reaction, the relatively insoluble
ferrous oxalate dihydrate forms
as a coordination polymer that can
be filtered from the slurry. the iron (ii)
oxalate forms a filter cake of small,
mustard-colored solids that are usually
landfilled. the filtered acid returns
to the pickling process.
Mixed acid (HF/HNO3) recovery
the two largest industries using
mixed hydrofluoric and nitric acid are
stainless-steel
ti + 6hf + 4hno3 →
h2tif6 + 4no2 (g) + 4h20
(7)
the most commonly used technologies
to recover hf/hno3 include:
acid sorption, vacuum evaporation,
pyrohydrolysis
and nanofiltration.
manufacturing and
titanium milling/etching. in the stainless-steel
descaling process, oxides
of iron, nickel and chromium (as well
as other alloying elements) must be
removed without causing underlying
attack on the base metal. the combination
of using oxidizing nitric acid
as a passivation agent to protect the
alloy surface, while at the same time
applying hydrofluoric acid to remove
scale from the surface, has worked
well for over fifty years.
in titanium-milling operations, nitric
acid dissolves some transient
impurities from titanium, while the
hydrofluoric acid dissolves the exposed
titanium [equation (7)]. the
nitric acid mitigates hydrogen embrittlement
while passivating the
this highly corrosive acid combination
presents a materials of construction
challenge for recovery/recycle
technologies. Since it etches
glass as well as dissolves most
metals, there are very few non-reactive
containment materials. thermoplastics
such as polyethylene,
polypropylene, Pvdf and teflon are
effective storage and containment
materials for both hydrofluoric acid
and the mixed acid hf/hno3. the
added cost of these materials, however,
is offset by the comparatively
high cost of the acids.
Acid sorption (retardation). this
resin method, previously discussed,
has proven effective in recovering
mixed acid for reuse. even though
the titanium, iron and other metal
byproducts of the process cannot
be economically recovered or
reused, the high value of the acid
justifies the capital and operating
expense in many cases. the acidic
waste is usually neutralized with lime
before the sludge is filter-pressed
for disposal.
Vacuum evaporation. forced-circulation
evaporation under vacuum
has proven capable of recovering
hf/hno3, while producing a titanium
fluoride salt with reclaim value.
the materials of construction, operating
temperature, and process
control differ somewhat from the
same technology used for recovering
hydrochloric acid. the process
uses the forced circulation co-flash
principle to evaporate hydrogen fluoride,
nitric acid, and water simultaneously.
the rectifier section scrubs
the acids, along with some water,
ChemiCal engineering www.Chemengonline.Com June 2020
titanium base metal. in both the titanium
milling and stainless-steel
cleaning and passivating processes,
both the acid composition ratio and
concentrations are important. the
initial hydrofluoric acid concentration
in a fresh solution rarely exceeds
10%.
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