Note: Descriptions are shown in the official language in which they were submitted.
CA 02957733 2017-02-09
Integrated Hydrometallurgical Process
This invention relates to an integrated hydrometallurgical method to process
metal-
containing feedstock.
State of the Art
Since there are numerous waste products that contain re-usable valuable
substances
(so-called recyclable materials), the processing of metal-containing feedstock
is an
important process. For this purpose, these recyclable materials need to be
recovered
from the feedstock. This can be achieved by acidic or basic leaching.
The recovering of aluminum oxide is carried out according to the Bayer process
on
an industrial scale. This is a basic process, in which the aluminum-containing
feedstock of bauxite is reacted with a sodium hydroxide solution. During the
process,
aluminum dissolves as Na[Al(OH)4] and iron precipitates as Fe203. The
insoluble
residue is disposed of as so-called red mud. Important resources are thus
lost, and
moreover the disposal poses an environmental problem.
During the acidic leaching of metal-containing feedstock, the recovering is
often
incomplete due to an inappropriate acid concentration or temperature. It is
not
possible to recover all recyclable materials from the metal-containing
feedstock,
because they either do not precipitate separately from other substances or do
not
dissolve.
The goal of the invention is to provide a process that enables recyclable
materials to
be recovered from the feedstock.
According to the invention, this is achieved by providing an integrated
hydrometallurgical process for the processing of metal-containing feedstock,
comprising the following steps:
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a) dissolving metal-containing feedstock in aqueous hydrochloric acid, the
concentration of the hydrochloric acid being 2-10 wt%;
b) separating the insoluble constituents;
c) introducing dry gaseous HCI into the reaction mixture at 40-60 C until an
HCI concentration of 20-28 wt% has been reached, whereby LiCI, NaCI, KCI and
AlC13.6H20 precipitate;
d) collecting the solid by filtration;
dl) cooling the filtrate of step d) to approximately room temperature;
d2) introducing dry gaseous HCI until an HCI concentration of 28-35 wt% is
reached, whereby MgC12 precipitates and Ca2+ and Fe3+ remain in solution;
d3) filtering the solution of step d2);
d4) dissolving the solid of step d3) in water and spray-roasting at
temperatures
from 400 to 900 C, whereby MgO + HCI (gaseous) + H20 (gaseous) are formed;
d5) introducing SO3 into the filtrate of step d3), whereby H2SO4 is formed,
whereby Fe2(SO4)3 and CaSO4 precipitate and HCI is driven off;
d6) filtering the solution of step d5);
d7) heating the filtration residue of d6) to 500-1100 C, whereby Fe2(SO4)3
decomposes to form Fe203 + 3 SO3, whereby 2 SO3 decomposes to form 2 SO2 +
02, and CaSO4 remains unchanged;
d8) adding aqueous hydrochloric acid to the product of d7), whereby Fe203
dissolves selectively and CaSO4 as an insoluble sulfate remains unchanged as a
solid;
d9) collecting CaSO4 by filtration;
d10) hydrolyzing the FeCl3 contained in the filtrate of step d9) at 160-200 C
according to the reaction equation of
2 FeCI3 + 3 H20 4.- Fe203 + 6 HCI,
whereby the equilibrium is shifted to the right by adding H20 and removing
HCI;
dl 1) collecting Fe203 by filtration and washing with water, whereby purified
Fe203 is obtained;
e) heating the solid of step d) to 200-300 C, whereby amorphous A1203 is
formed and the chlorides of the elements of sodium, potassium and lithium
remain
unchanged;
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f) cooling the solid mixture to approximately room temperature;
optionally g) separating LiCI by washing with ethanol, and recovering LiCI;
h) separating NaCI and KCI and, if step g) was not carried out, LiCI by
washing
with water, and subsequently recovering NaCI, KCI and LiCI;
optionally i) dissolving the amorphous A1203 using aqueous hydrochloric acid
with a concentration of 2-10 wt%, separating the insoluble constituents,
introducing
dry gaseous HCI into the reaction mixture at 20-30 C, preferably
approximately 25
C, until an HCI concentration of 20-28 wt% has been achieved, collecting the
solid
by filtration and washing with water;
j) heating the amorphous A1203 and/or the solid of step i) to 1200-1400 C,
whereby a-A1203 is formed.
Employing this process, the recyclable materials of CaSO4, Fe203, LiCI, NaCI,
KCI
and A1203 can be recovered from the metal-containing feedstock. In addition,
in the
course of this reaction, HCI, SO3 and H2SO4 are formed, which can also be re-
used.
The advantage of this process is its compact, integrated process flow and the
recovering of highly pure metals in their salt or oxide form. The aluminum
oxide has a
purity of > 99.999%.
Metal-containing feedstock should be understood as those materials that
contain the
elements that can be separated and recovered in the process of the invention
(e.g.
aluminum, sodium, potassium, lithium, iron, calcium and magnesium). For
example,
the following materials may be used as metal-containing feedstock in the
process of
the invention:
1) worn-out accumulators from electromobility applications, which consist of a
mixture
of the above-mentioned elements and other elements and compounds. As regards
energy storage, the state of the art in electromobility is represented by Li-
ion
accumulators. Apart from lithium, these Li-ion battery packs also contain iron
and
aluminum substances in their framings. As electromobility is increasing,
lithium will
become a highly important recyclable material and is quite expensive even
today.
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The process of the invention enables lithium, iron and aluminum as well as
other
elements to be separated in an efficient and easy manner.
2) aluminum-containing ores that cannot be used in the Bayer process to
recover
aluminum due to their chemical composition. In particular, the Si02 content
constitutes a reason why these cannot be used in the Bayer process. Si02 has
the
property of binding aluminum in the form of sodium aluminum silicate during
alkaline
fusion using NaOH. In the Bayer process, this alkali silicate cannot be fused
any
further, which is why the aluminum bound in the sodium aluminum silicate is
not
available for aluminum recovering. The higher the Si02 content is in an
aluminum-
containing ore, the higher are the amounts of sodium aluminum silicate and
thus the
aluminum losses, as, in the Bayer process, these are deposited together with
oxidic
iron compounds in the form of red mud. The process of the invention avoids
this
problem completely, as it employs a hydrochloric-acid-based process route,
during
which Si02 precipitates as an insoluble component as early as in step a).
Therefore,
this process is suitable for processing ore deposits that have so far not been
accessible for chemical treatment.
3) red mud waste, which is also mainly composed of the above-mentioned
elements.
As already described in point 2), red muds from the Bayer process are
deposited,
which causes serious environmental problems. Using the process of the
invention,
red mud waste can be used to recover recyclable aluminum and iron materials.
In an embodiment of the invention, the process according to step h) may
comprise a
step h1), during which SO3 is introduced into the aqueous solution obtained in
step
h), whereby Na2SO4 and K2SO4, and optionally Li2SO4, are obtained and HCI is
formed.
Na2SO4 and K2SO4 and hydrogen chloride HCI also constitute valuable re-usable
substances.
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In another embodiment of the invention, the SO3 formed during step d7) may be
re-
used in step d5) and/or h1). This re-use reduces the required amount of
externally
produced S03.
In yet another embodiment of the invention, the HCI obtained during step d4),
step
d5), step d10) and/or step h1) may be used in an aqueous solution in step a)
and/or
i) and/or, after drying, in step c) and/or d2). In this way, a closed loop of
HCI is
achieved; further addition of HCI is required only to the extent to which
process-
related losses of HCI gas occur.
In one variant of the present invention, Na2SO4 may be used to dry HCI. Dry
Na2SO4
can absorb 10 mol of H20. Before Na2SO4 can be used to dry HCI, it has to be
cleaned from crystal water at just above 100 C. In Na2SO4, crystal water is
released
from the crystal structure from 32 C upwards. After that, Na2SO4 can be used
to dry
HCI.
In another variant of the invention, the filtrate of step d6), which comprises
H2SO4,
may be used to dry HCI, optionally after concentration of the H2SO4. Sulfuric
acid is
known to be hygroscopic and can thus be employed to dry chemicals. HCI may
simply be dried by H2SO4 by passing it through H2SO4. If the resulting H2SO4
has not
reached a sufficient concentration to act as a desiccant, it has to be
subjected to prior
concentration, e.g. to 80 to 100%, which requires the H2SO4 to be heated to
approx.
250 to 350 C.
In one embodiment of the invention, the introduction of HCI into the aqueous
phase
in step c) and/or d2) can be carried out by dripping the aqueous solution
through an
atmosphere enriched with dry gaseous HCI. Thereby, the aqueous phase is
enriched
with HCI during a counter-current process. Thus, the transfer area between the
gaseous HCI and the aqueous solution is large, which enables the latter to be
enriched to the desired concentration.
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In another embodiment of the invention, SO3 may be formed by combusting sulfur
and/or sulfur containing fuels to SO2 and passing it through an oxidation
catalyst, and
the heat generated thereby may be used in the process. Combusting sulfur
and/or
sulfur containing fuels leads to, one the one hand, the generation of the
required
SO2, which is converted into SO3 using an oxidation catalyst and may be used
in the
process, and, on the other hand, the generation of heat, which may be used in
the
process, e.g. to directly adjust the temperatures required in the individual
steps or to
produce electrical energy, using, e.g., a steam turbine.
In yet another embodiment of the invention, the amount of sulfur may be
adjusted so
that it is stoichiometric to the amount of Na, K, Li, Fe and Ca contained in
the metal-
containing feedstock. Thereby, these metals are completely reacted to form
sulfates
and are removed from the process. Excess SO3 does not accumulate in the
process.
The "amount of sulfur" means both the amount of elementary sulfur combusted
and
the amount of sulfur in the sulfur containing fuel.
In one embodiment of the invention, the heating of step c) may be carried out
up to
approx. 50 C. Thereby, a better separation of LiCI, NaCI and KCI, and
AlC13.6H20
from the other recyclable materials is achieved. The solubilities of the
chlorides of Li,
Na, K and Al are almost independent from temperature, while the solubilities
of the
other elements increase with a temperature rise in the solution.
In another embodiment of the invention, the heating of step e) may be carried
out up
to approx. 300 C. Thereby, a better conversion of AlC13.6H20 into amorphous
A1203
is achieved.
In another embodiment of the invention, the heating of step j) may be carried
out up
to approx. 1200 C. This temperature is sufficient for a-A1203 to be formed.
In another embodiment of the invention, heating in step d) may be carried out
up to
500-800 C, preferably 600-700 C. In these temperature ranges, a more
efficient
formation of MgO + HCI (gaseous) can be observed.
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In another embodiment of the invention, heating in step d7) may be carried out
up to
700-1000 C, in particular 900-1000 C. In this temperature range, the
conversion of
Fe2(SO4)3 into Fe203 + 3 SO3, which in turn decomposes into 2 SO2 + 02, is
more
efficient. Again, the SO2 has to be converted into SO3 by passing it through
an
oxidation catalyst.
In another embodiment of the invention, the heating of step d10) may be
carried out
up to approx. 180 C. The hydrolysis is more complete.
In another embodiment of the invention, the concentration of the hydrochloric
acid in
step a) may amount to 4-8 wt%. This concentration results in a more efficient
dissolution of the metal-containing contents of the feedstock.
In another embodiment of the invention, the concentration of the hydrochloric
acid in
step c) may amount to 22-26 wt%. In this concentration range, LiCI, NaCI and
KCI as
well as AlC13.6H20 precipitate selectively.
In another embodiment of the invention, the concentration of the hydrochloric
acid in
step i) may amount to 4-8 wt%. This concentration range enables amorphous
A1203
to be dissolved completely.
In another embodiment of the invention, the concentration of the hydrochloric
acid in
step d2) may amount to 30-33 wt%. In this concentration range, the complete
precipitation of MgC12 can be observed.
In view of the above process of the invention and the embodiments, a person
skilled
in the art will understand that an "integrated process" means closed process
loops
and material flows (HCI gas + dehydration agent = Na2SO4, K2SO4, H2504) and
the
simultaneous supply of energy to implement an overall process that is as
economic
and ecological as possible. In particular, the following points present
examples of the
integrity of a process:
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1) Generation of electrical energy. Electrical energy is generated using a
steam
turbine. The thermal energy required to drive the steam turbine is generated
by
combusting fossil fuels (natural gas, coal, fuel oil etc.) together with
sulfur containing
energy sources. Thereby, a sulfur containing (S02-containing) hot gas is
generated,
which is passed through a heat recovery generator to generate steam. The steam
generated thereby drives a steam turbine for electricity generation.
2) Use of oxidic sulfur components. The sulfur oxide components in the flue
gas are
a decisive element for the process to be successful. Sulfuric acid is required
in
subsequent process stages. An intermediate catalytic oxidation stage is
carried out to
produce H2SO4 from SO2. In this stage, SO2 is oxidized to form S03. The SO3-
containing gas is required in two subsequent process stages. On the one hand,
the
NaCI and KCI deposited in the first crystallization stage are to be reacted
with the
previously produced sulfuric acid to form Na2SO4 and K2SO4, while at the same
time
(g) HCI is to be released and recovered. The sulfated of Na and K that form
are used
for the overall process in a subsequent process stage. Both sulfates are
hygroscopic
and are thus employed to dehydrate wet HCI gas in a highly energy-efficient
manner.
The dehydration of the resulting HCI gas flow is essential for the overall
process, as
the crystallization of aluminum and Mg can take place only by using desiccated
(g)
HCI. On the other hand, S03-containing gas is fed into a reactor to
precipitate ferric
and calcium sulfate from an extremely strong hydrochloric acid solution. Apart
from
the precipitation of sulfates, two further chemical reactions take place in
this process
stage. By introducing (g) SO3, sulfuric acid is formed. The formation of
sulfuric acid is
in accordance with the following reaction formula:
SO3 + H20 => H2SO4.
The formation of sulfuric acid occurs as long as SO3 is introduced and as
water is
available. One side effect of the formation of H2SO4 consists in the release
of HCI
gas from the solution. The HCI gas produced in this reactor stage is collected
and
energy-efficiently dehydrated using Na2SO4/K2SO4, which is obtained by sulfate
formation as described above.
Likewise, the sulfuric acid formed is used to dehydrate recycled HCI gas.
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The dried HCI gas flows (desiccant Na2SO4, K2SO4, H2SO4) are re-used in the
overall
process for crystallization/precipitation operations of Al + Mg. In this way,
a closed
HCI gas loop is maintained.
3) Selective roasting of ferric sulfates as a preliminary stage for Ca/Fe
separation. As
initially described, steam is generated for electrical energy generation in a
combustion stage, during which the crucially important SO2 is formed from
sulfur
containing components. Furthermore, the thermal energy generated in this
combustion stage is to be used for the so-called "selective roasting". In this
process
stage, only the ferric sulfate fraction is to be converted into Fe203 at 1000
C, while
CaSai remains stable at this temperature. This pre-treatment enables iron and
calcium to be separated by selectively dissolving iron oxide in a downstream
leaching
stage using HCI. Gypsum is separated in the form of a solid residue.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 shows a schematic flow chart of the process of the present invention.
EXAMPLE
In one example of this invention, metal-containing feedstock is dissolved in
aqueous
hydrochloric acid, with the concentration of the hydrochloric acid amounting
to 2-10
wt%. Preferably, the hydrochloric acid concentration amounts to 4-8 wt%.
The following materials may be used as metal-containing feedstock:
1) worn-out accumulators from electromobility applications, which consist of a
mixture
of the above-mentioned elements and other elements and compounds. As regards
energy storage, the state of the art in electromobility is represented by Li-
ion
accumulators. Apart from lithium, these Li-ion battery packs also contain iron
and
aluminum substances in their framings. As electromobility is increasing,
lithium will
become a highly important recyclable material and is quite expensive even
today.
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The process of the invention enables lithium, iron and aluminum as well as
other
elements to be separated in an efficient and easy manner.
2) aluminum-containing ores that cannot be used in the Bayer process to
recover
aluminum due to their chemical composition. In particular, the Si02 content
constitutes a reason why these cannot be used in the Bayer process. Si02 has
the
property of binding aluminum in the form of sodium aluminum silicate during
alkaline
fusion using NaOH. In the Bayer process, this alkali silicate cannot be fused
any
further, which is why the aluminum bound in the sodium aluminum silicate is
not
available for aluminum recovering. The higher the Si02 content is in an
aluminum-
containing ore, the higher are the amounts of sodium aluminum silicate and
thus the
aluminum losses, as, in the Bayer process, these are deposited together with
oxidic
iron compounds in the form of red mud. The process of the invention avoids
this
problem completely, as it employs a hydrochloric-acid-based process route,
during
which Si02 precipitates as an insoluble component as early as in step a).
Therefore,
this process is suitable for processing ore deposits that have so far not been
accessible for chemical treatment.
3) red mud waste, which is also mainly composed of the above-mentioned
elements.
As already described in point 2), red muds from the Bayer process are
deposited,
which causes serious environmental problems. Using the process of the
invention,
red mud waste can be used to recover recyclable aluminum and iron materials.
Subsequently, the insoluble constituents are separated, e.g. by filtration.
Dry
gaseous hydrogen chloride HCI is introduced until an HCI concentration of 20-
28 wt%
has been reached, with the temperature of the reaction mixture being 40-60 C,
whereby LiCI, NaCI and KCI as well as A1C13.6H20 precipitate. HCI obtained
later in
the course of the reaction may be re-used in this step. The introduction of
HCI into
the aqueous phase may be carried out by dripping (spraying) the aqueous
solution
(solution) through an atmosphere enriched with dry gaseous HCI. In this way,
the
aqueous phase (solution) is enriched with HCI in the counter-current.
Preferably, the
temperature during the introduction of HCI amounts to approximately 50 C and
the
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concentration of the free hydrochloric acid in the solution is 22-26 wt%. Due
to the
extremely high solubility of HCI gas in the aqueous phase (solution), the
solubility
limits of the individual chloride salts are lowered, which causes the
chlorides to
precipitate. The resulting solid is collected by filtration, and the filtrate
is cooled to
approximately room temperature. Subsequently, dry gaseous HCI is continued to
be
introduced until an HCI concentration of 28-35 wt% is reached, which causes
MgCl2
to precipitate and Ca2+ and Fe3+ to remain in solution. Hydrogen chloride
obtained
later in the course of the reaction may be re-used in this step. The
introduction of HCI
into the aqueous phase may be carried out by dripping the aqueous solution
through
an atmosphere enriched with dry gaseous HCI. In this way, the aqueous phase is
enriched with HCI in the counter-current. Preferably, the concentration of the
free
hydrochloric acid in the alkaline solution is 30-33 wt% in this step. This
solution is
filtered, and the solid is dissolved in water and is spray-roasted at
temperatures from
400 to 900 C, whereby MgO + HCI (gaseous) + H20 (gaseous) are formed. As
described, the HCI formed thereby may be introduced into the reaction process
further upstream and further downstream. Heating is preferably carried out up
to 500-
800 C, in particular 600-700 C. Subsequently, 803 is introduced into the
above
filtrate, whereby H2SO4 is formed, whereby Fe2(SO4)3 and Ca804 precipitate and
HCI
is driven off. This step is referred to as outcrossing of HCI. The SO3 is
formed by
combusting sulfur and/or sulfur containing fuels. Sulfur reacts to form SO2,
which is
subsequently passed through an oxidation catalyst, whereby SO3, which can be
used
in the process, is formed. As described, the HCI formed thereby may be
introduced
into the reaction process further upstream or further downstream. 803 formed
later in
the process may be re-used in this step. Now, the solution can be filtered.
The filtrate
comprises H2SO4 and may be used to dry HCI, optionally after concentration of
H2SO4. In this way, the desiccant required to dry HCI is produced in the
process
itself. The filtration residue is heated to 500-1100 C, whereby Fe2(SO4)3
decomposes to form Fe2O3 + 3 SO3, whereby, in turn, 2 SO3 decompose into 2 SO2
+ 02, and CaSO4 remains unchanged. Heating is preferably carried out up to 700-
1000 C, especially 900-1000 C. In particular, the phase diagram shows a
maximum
value for Fe2O3 and a minimum value of Fe2(SO4)3 and FeSO4 at 900-1000 C;
thus,
this is where the conversion of Fe2(SO4)3 into Fe203 is optimal and produces
minimal
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side products. The S02-containing flue gas is passed through an oxidation
catalyst to
oxidize SO2 to S03. SO3 may be used at the points at which SO3 is introduced,
as
described. Aqueous hydrochloric acid is added to the product, whereby Fe203
selectively dissolves and CaSO4 as an insoluble sulfate remains unchanged as a
solid. CaSO4 is collected by filtration. The purity of CaSO4 amounts to
approximately
90 wt%. Hydrolysis of the FeCI3 contained in the filtrate is carried out at
160-200 C
according to the reaction equation of
2 FeCl3 + 3 H20 Fe203 + 6 HCI,
whereby the equilibrium is shifted to the right by adding H20 and removing
HCI. As
described, the HCI formed thereby may be introduced into the reaction process
further upstream and further downstream. Preferably, hydrolysis is carried out
at
approximately 180 C. Fe203 is collected by filtration and washed with water,
whereby purified Fe203 is obtained. The purity of Fe203 amounts to
approximately 99
wt%. The solid obtained in the first crystallization stage by means of
introducing HCI
gas is heated to 200-300 C, whereby amorphous A1203 is formed and the
chlorides
of the elements of sodium, potassium and lithium remain unchanged. Heating is
preferably carried out up to approximately 300 C. Subsequently, the solid
mixture is
cooled to approximately room temperature. Optionally, LiCI is separated by
washing
with ethanol and obtained. The purity of LiCI amounts to approximately 99.5
wt%.
Due to the high covalent fraction in the bond of LiCI, LiCI dissolves in
ethanol. NaCl
and KCI and, if the step of separating of LiCI by means of ethanol was not
carried
out, LiCI are separated by washing with water and are subsequently recovered.
In
this case, NaCI, KCI and LiCI are in the form of a mixture. SO3 may be
introduced
into the obtained aqueous solution, whereby Na2SO4 and K2SO4 and, optionally,
Li2SO4 are obtained and HCI is formed. SO3 formed earlier in the process may
be re-
used in this step, as described. Na2SO4 may be used to dry HCI. Dry Na2SO4 can
absorb 10 mol of H20. Before Na2SO4 is used to dry HCI, it has to be cleaned
from
crystal water at just above 100 C. In Na2SO4, crystal water is released from
the
crystal structure from 32 C upwards. Then, Na2SO4 may be used to dry HCI. As
described, the HCI formed thereby may be introduced into the reaction process
further upstream and further downstream. Optionally, the amorphous A1203 is
dissolved using aqueous hydrochloric acid with a concentration of 2-10 wt%,
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preferably 4-8 wt%, and is again separated from the insoluble constituents;
then, dry
gaseous HCI is introduced at a temperature of 20-30 C, preferably
approximately 25
C, until an HCI concentration of 20-28 wt%, preferably 22-26 wt%, is reached,
and
the solid is collected by filtration and washed with water. The amorphous
A1203
and/or the solid collected by filtration in the previous step is heated to
1200-1400 C,
preferably approximately 1200 C, whereby a-A1203 is formed. The a-A1203 has a
purity of 99.999%.
As it is an integral part of this hydrometallurgical process, the provision of
energy for
the overall process is an essential factor. One conventional method for
generating
electrical energy consists in operating steam turbine systems. Sulfur and
natural gas
may be employed as fuels to generate the required thermal energy. The main
energy
source is usually natural gas. The amount of natural gas to be combusted
depends
on the thermal and electrical energy required to carry out the suggested
process and
may thus be used in a variable manner. Liquid sulfur should be considered as a
fuel
additive, which is, on the one hand, combusted with natural gas as an
additional
source of energy. On the other hand, sulfur should not only be regarded as an
additional source of energy but is also needed as a precursor for the
production of
S02/503. Sulfur oxides are, as described, vitally important for the overall
process for
crossing out the HCI line in order to precipitate Fe2(SO4)3 and CaSO4 and for
producing Na2SO4, which is in turn used as a desiccant for wet HCI gas. The
amount
of co-combusted sulfur depends on the mass flows of sodium, potassium,
lithium,
iron and calcium. These substances have to be reacted in quantity to sulfates.
By
taking into account that the selective roasting of ferric sulfate serves as a
second
source of S02/S03, the mass balance of the amount of sulfur to be co-combusted
has to be adjusted on a case-by-case basis.
The thermal energy is mainly to be used in electricity generation using a
steam
turbine, which enables all electrical loads to be supplied with energy. Part
of the
thermal energy may be used in the regeneration of the desiccants of Na2SO4, 1-
12SO4,
in FeCl3 hydrolysis and in vaporization steps.
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As an alternative to energy generation by combusting natural gas and sulfur,
the
condensation enthalpy of sulfuric acid may be recovered from the S03-
containing gas
flows after the oxidation catalyst stage. The condensation enthalpy may be
recovered
by means of a gas/gas heat exchanger. The recovered energy may be re-used in
low-energy processes like, for example, the dehydration of Na2SO4 (¨ 100 C).
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