Chemical Engineering June 2020 - 46

90
80
70
60
50
40
30
20
10
Iron solubility in sulfuric acid
12% H2SO4
16% H2SO4
20% H2SO4
24% H2SO4
32
2
4
39.2
6
8
46.4
10
12
53.6
14
16
60.8
Temperature
Figure 2. The solubility of iron in sulfuric acid varies with concentration and temperature
the total power consumed in this
operation of the recovery system approaches
41 kW/ton of spent acid
processed. the use of a plate-andframe
(liquid/liquid) heat exchanger
reduces the crystallizer energy demand
by more than 30%.
Both batch and continuous
crystallization processes use fluid
transport, filtration, mixing, heat exchange,
refrigeration and centrifuge
(screen) separation to achieve the
recycle of sulfuric acid.
ferrous sulfate heptahydrate and
copper sulfate tetrahydrate crystals
recovered in the process are sold as
co-products. thousands of tons are
produced each year and used in agriculture,
wastewater treatment and
mining. the low operating cost of
an acid recovery crystallizer and the
value of the co-product make this
technology preferable to disposal
and neutralization.
HCl recycle: Pyrohydrolysis
differing from the other technologies
covered in this article, pyrohydrolysis
is a recycle (or regeneration) rather
than a recovery technology. originating
from the thermolytic technology
used in mining, pyrohydrolysis
has been used since the 1950s to
recycle spent hydrochloric acid from
large steel operations. Both fluidized
bed and spray roasting thermally
break the metal chloride bond and
create metal oxide and hcl products.
the metal oxides are collected
46
as beads or powder at the bottom
of the roaster. the resulting hcl gas
is mixed with water to form a near
azeotrope of hcl after absorption
and rectification.
12fecl2 + 3o2 → 8fecl3 + 2fe2o3
(oxidation of ferrous to ferric)
2fecl3 + 3h2o → 6hcl + fe2o3
(decomposition reaction)
(1)
(2)
a simplification of the half reactions
is given by equation (3):
fecl2 + h2o → feo + 2hcl
(△H requires energy)
(3)
in a pyrohydrolysis process, such
as that shown in figure 4, spent acid
travels through an evaporative step
Recovered
acid
Total
Fe
Zn
208.00 kg/h
6.70
0.00
Metal Conc 3.2%
Filter
Spent acid
Total
Fe
Zn
Chiller
10 kW
Recovered
acid pump
Return
acid
tank
Crystals
Centrifuge
FeSO4 * 7H2O
48.00 kg/h
1.0 ton/d
Figure 3. Spent sulfuric acid from pickling operations can be recycled using a crystallization process
ChemiCal engineering www.Chemengonline.Com June 2020
256.00 kg/h
15.40
0.00
Metal Conc 6%
Interchanger
Reactor
tank
Settler
tank
18
20
68
22
24
75.2
26
28
82.4
30 °C
°F
before entering the roaster. this preconcentrated
iron chloride solution
is then pressure-injected tangentially
into a ceramic-lined roaster operating
at 550-700°c. the first step of
the reaction is the oxidation of ferrous
chloride to ferric chloride in the
presence of oxygen in the combustion
gas. the ferrous chloride concentrate
oxidizes and decomposes
to a ferric oxide (fe2o3) solid that
is constantly removed from the bottom
of the roaster. the hcl gas and
water generated in the reaction exit
the roaster to be scrubbed and concentrated
to a near azeotropic hcl
concentration in the absorber/rectifier
column. as reported by a manufacturer,
pyrohydrolysis requires 50
therms of natural gas to process one
ton of spent acid. the process recycles
90+% of the original fresh acid
back into good-quality pickling acid.
the ferric oxide co-product of
some fluidized bed systems can be
re-melted into steel. if the contaminant
levels are low, the fe2o3 powder
from the spray-roasting method
can also be used in pigment, ferrite
magnets, cosmetics and polishing
applications. environmental monitoring
of the process includes accidental
emissions and release of iron
oxide dust, hcl vapor and chlorine.
HCl recovery
When a large portion of the hydrochloric
acid has been consumed in
a pickling or leaching process, the
bath is considered spent. if the meFe
(g/L)

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Chemical Engineering June 2020

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