Note: Descriptions are shown in the official language in which they were submitted.
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Removal of arsenic from flue-gas
The present invention concerns a gas cleaning process, specially adapted for
the removal of
traces of arsenic oxides in exhaust gases, in particular in off-gases from
metallurgical smelting
processes or coal burning processes.
Arsenic is present in many minerals, concentrates, and recycled metal-bearing
materials.
Arsenic and many arsenic compounds are also relatively volatile at high
temperature.
Consequently, most metallurgical operations produce arsenic bearing gases, in
particular when
pyrometallurgical processes are applied. Examples are burning of coal, or the
recovery of
metals such as copper and lead, using smelting processes. Emissions from
furnaces and
converters can cause health problems in the work place and/or result in
elevated levels of toxic
pollutants such as lead and arsenic in the immediate vicinity of the smelter.
According to known processes, the major part of the arsenic in gas streams can
be recovered
by condensation, filtration, or adsorption on active carbon.
Condensation and filtration allow for arsenic abatement down to about 0.2 to
0.8 mg/Nm3 in the
gas phase. Hydrated lime Ca(OH)2 can be injected in the gas, thereby not only
serving as
condensation surface, but also adsorbing the arsenic by forming a Ca ¨ As
precipitate. A further
reduction of arsenic down to 0.05 mg/Nm3 is then typically obtainable.
A more complete elimination of arsenic is however desired in view of the eco-
toxicity of this
metal and of its compounds. Moreover, typical industrial operations involve
the continuous
release of huge volumes of gases, thus exacerbating the environmental issue.
A known process for the further reduction of arsenic is passing the gas
through a bed of active
carbon.
It has been recognized that the effectiveness of arsenic adsorption on active
carbon decreases
with increasing temperature. The gas stream has therefore to be cooled down to
well below
100 C. Unfortunately, the adsorption kinetics at this temperature are rather
slow. A sufficient
contacting time between active carbon and gas can only be achieved by using a
voluminous
bed, which therefore needs to contain a large quantity of active carbon. This
results in bulky and
expensive equipment. The active carbon is moreover not effectively utilized,
as it never gets
saturated in arsenic during its normal operational life.
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It is the aim of the present divulgation to propose a scheme to solve the
above problems, in
particular to accelerate the adsorption kinetics of arsenic compared to active
carbon, while
allowing for the abatement of arsenic down to less than 0.01 mg/Nm3. This
scheme makes use
of a bed of SILP (Supported Ionic Liquid Phase), i.e. a porous carrier
typically prepared by
soaking a carrier phase in a selected ionic liquid.
Processes for the capture of metals or their oxides using supported ionic
liquid phases have
been described before. They are however not optimized for the elimination of
arsenic.
.. US20140001100 discloses a process for the capture of elemental mercury from
a hydrocarbon
fluid using ionic liquids. Suitable ionic liquids comprise an organic cation,
a metal cation, and an
anion. The ionic liquid is believed to perform a dual function. First, the
metal cation part of the
ionic liquid oxidizes the mercury. The oxidized mercury, being destabilized in
its organic
environment, is then efficiently captured in the ionic liquid.
U520070123660 similarly concerns a process for the capture of gaseous forms of
elemental or
oxidized mercury, but also of lead, zinc and cadmium. Use is made of a
combination of a ligand
and of an ionic liquid. Oxidizing agents are added when elemental species need
to be captured.
A process is hereby divulged for the removal of arsenic oxides in process
exhaust gases,
comprising the step of passing the exhaust gases through a supported ionic
liquid phase bed,
characterized in that the ionic liquid comprises one or more cations from the
list consisting of
substituted phosphonium, ammonium, imidazolium, pyrrolidinium, and pyridinium,
and one or
more anions from the list consisting of chloride, bromide, and carboxylate.
By process exhaust gases are meant gases from metallurgical smelting processes
or from other
burning processes.
Preferably, the substituted phosphonium cation is according to formula [Pm no
p]+, and the
substituted ammonium cation is according to formula [Nm no p]+, wherein the
substituents are
hydrocarbon chains containing m, n, o, and p carbon atoms each, with the
proviso that
m+n+o+p > 10 when the anion is a halide, and m+n+o+p <30 when the anion is a
carboxylate.
The hydrocarbon chains substituents of the cation are preferably unbranched
and saturated.
The anions are preferably unbranched, unsaturated monocarboxylates, containing
1 to 8 carbon
atoms.
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The most preferred ionic liquid is [P66614] Cl. This product is commercially
available as
CYPHOS IL 101.
The process is most suitable for removing arsenic oxides comprising As203
and/or As205.
Preferably, the supported ionic liquid phase comprises a support phase from
the list consisting
of alumina, silica, and activated carbon. A support phase having a BET of more
than 50 m2/g is
desired. A weight ratio of support phase to ionic liquid weight between 3: 1
and 50: 1 is most
suitable.
The advantages are of the disclosed process and corresponding equipment are:
- the volume of the adsorption bed can be reduced;
- the cleaning apparatus itself can be more compact;
- the pressure drop across the adsorption bed can be reduced.
The investment can therefore be lower than when using active carbon, and the
running costs
decreased.
Such a SILP may also adsorbs elements other than arsenic which may also be
present in the
gas phase, such as Zn, Hg, Cd, Pb, Sb, and Se, dependent upon the precise
ionic liquid
selected. For example, the ionic liquid identified as trihexyl-tetradecyl-
phosphonium chloride
[P6 6 6 14] Cl lends itself well for the capture of As, but also of Pb, Cu,
Cd, Se and Zn. There is
also clear evidence for the uptake of Sb and Se when using 1-butyl-3-
methylimidazolium
acetate [C4C1im] [C1CO2]. These ionic liquids were tested using an active
carbon substrate.
The supporting substrate should be highly porous and should be wetted by the
envisaged ionic
liquid. Typical candidates are silica, alumina, titanium oxide, zirconium
oxides, activated carbon,
porous polymers, zeolites, and metal-organic frameworks.
When targeting the adsorption of arsenic, ionic liquids susceptible to
dissolve significant
amounts of it are clearly preferred.
When the arsenic-contaminated exhaust gases originate from a metallurgical
smelting process,
the spent SILP can be directly recycled to that process. A capture mechanism
ahead of the
SILP adsorption step is then needed to avoid the accumulation of the metals
captured by the
SILP. The recycled SILP could even be considered as a valuable reaction agent.
This would be
the case, e.g. when dealing with an active carbon substrate and a
pyrometallurgical process
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needing a reducing agent. Similarly, silica or alumina substrates could
usefully be recycled to a
process needing fluxing for the formation of a slag.
In a first example, the increase in capacity of the SILP is demonstrated.
For the preparation of activated carbon-based SILP, activated carbon WS 490
from Chemviron
Carbon is used. One part by weight of the ionic liquid is dissolved in nine
volume parts of
methanol. The solution is added to nine part by weight of activated carbon and
left overnight to
ensure complete adsorption. The solvent is removed in three steps: 1.5 h at 45
C and
300 mbar, 1.5 h at 65 C and 300 mbar, and 1.5 h at 65 C and 250 mbar.
Using this method, batches of SILP are produced using ionic liquids [P66614]
Cl and [C4C1irn]
[C1CO2].
The BET is measured to characterize the specific surface of the obtained SILP
material. From
this analysis, the pore volume and the pore size is determined using BJH
analysis. These
determinations are performed using nitrogen for the untreated activated carbon
(AC) as well as
for the above-prepared SILP samples. This is reported in Table 1.
Table 1: BET results for untreated activated carbon (AC) and for SILPs
Material Pore volume (cm3g-1) Pore area (m2g-
1)
AC (uncoated) 0.78 1191.8
AC with 10 wt.`)/0 [P6 6 6 14] Cl 0.63 1021.4
AC with 10 wt.% [C4C1im] 0.68 1088.7
[C1 CO2]
The pore size of all three materials are also recorded. In all three materials
pore diameters
smaller than 40 A are dominant. This demonstrates the persistence of the pore
structure after
coating of the activated carbon with the ionic liquids. However, the fraction
of these small pores
is slightly reduced after coating. It is therefore assumed that the ionic
liquid covers the inner
pores of the activated carbon.
Ionic liquids are selected according to their capacity to dissolve As203. This
list is reported in
Table 2, along with the saturation limit as function of temperature.
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Table 2: As203 solubility in selected ionic liquids
Ionic liquid 50 C 70 C 80 C
wt.% mol% wt.% mol% wt.%
mol%
[P2 2 2 8] CI - - - - 4.94
6.55
[P4444] Cl - - - - 5.95
8.61
[P4446] Cl 1.13 1.82 3.50 5.58 5.43
8.56
[P4448] Cl -- -- - -- 3.86
6.65
[P66614] Cl 1.63 4.17 3.02 7.55 9.13
20.8
[P88810] Cl 0.68 1.85 - - 7.10
17.4
[P88818] Cl 2.31 2.31 - - 28.4
28.4
[N8881] Cl 1.54 3.08 - - 6.24
12.0
[P2 2 2 14] [HCO2] 8.08 9.77 11.5 24.12 14.8
29.0
[P4444] [HCO2] 5.88 11.9 14.9 16.71 18.3
20.4
[P2 2 2 8] [01002] 21.5 28.7 23.2 30.72 23.3
30.7
[P2 2 2 14] [01002] 7.03 12.5 10.6 18.38 13.9
25.4
[C2C1im] [01002] 18.2 16.1 25.7 22.94 26.8
26.8
[C4C1im] [01002] 12.2 12.2 28.4 28.48 34.7
35.6
[C8C1im] [01002] 11.9 14.8 19.6 23.87 19.6
23.9
It can be derived from Table 1 that the coating layer of the selected ionic
liquids is capable of
adsorbing about 10 kg of As203 per tonne of SILP. Assuming that the active
carbon substrate
5 will also contribute to the capacity of adsorption, the total capacity of
the SILP can be estimated
to be double the capacity of the active carbon alone. This increase of
capacity is a first
advantage of soaking the active carbon in a selected ionic liquid.
In a second example, the enhanced adsorption kinetics is shown.
In a first step, two adsorption columns are prepared, one filled with un-
soaked activated carbon
to be used as a reference, the filled other with activated carbon soaked in
[P66614] Cl as
described in Example 1. Each column comprises a small amount of glass wool at
the bottom,
followed by a steel mesh and 10 g of adsorption material. Two additional
layers of adsorption
material are added, each separated by a steel mesh. Each layer has an average
height of
1.63 cm. A steel mesh and glass wool is added on the top layer so as to
stabilize the adsorption
bed. The internal diameter of the column is about 4.2 cm.
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In a second step, As203 bearing gas is fed to the columns. To this end, a side
stream is
sampled from the off-gases produced by a lead blast furnace. After a first
dust filter, the gas is
divided into three parallel streams. One stream is directly passed through to
a cascade of
washing bottles for the analysis of the inlet concentrations. The analysis of
the As203 in the
washing bottles allows for the determination of the input concentration. The
other two are
passed through the respective adsorption columns. Each column outlet is
individually connected
to a separate cascade of washing bottles. Each cascade is followed by a drying
column and a
pump where the gas flow rate is adjusted to 3 L/min for each stream. The
temperature of the
gas entering the columns is about 140 C. The experiment is conducted for 48
h.
As summarized in Table 3, it is observed that the output arsenic concentration
is reduced by a
factor of 3 when ionic liquid soaked active carbon is utilized instead of un-
soaked active carbon.
As the operating conditions are identical, and as the levels are far below
saturation effects, it is
believed that the ionic liquid provides for accelerated adsorption kinetics.
This is a second
advantage of soaking the active carbon in a selected ionic liquid. This
advantage prevails even
when substrates other than activated carbon are used, such as silica or
alumina.
Table 3: Arsenic adsorption and yield
Column Input concentration Output concentration
Capture yield
mg/Nm3 mg/Nm3 (0/0)
None (pass through) 0.45 0.45 0.
AC (un-soaked) 0.45 0.0059 98.7
AC with 10 wt.% [P6 6 6 14] Cl 0.45 0.0013 99.7
