U.S. patent application number 13/969009 was filed with the patent office on 2014-02-27 for device and method for the thermal treatment of fluorine-containing and noble metal-containing products.
This patent application is currently assigned to Heraeus Precious Metals GmbH & Co. KG. The applicant listed for this patent is Heraeus Precious Metals GmbH & Co. KG. Invention is credited to Horst Meyer, Jose Manuel Romero, Steffen Vo.
Application Number | 20140056786 13/969009 |
Document ID | / |
Family ID | 49028910 |
Filed Date | 2014-02-27 |
United States Patent
Application |
20140056786 |
Kind Code |
A1 |
Romero; Jose Manuel ; et
al. |
February 27, 2014 |
Device and Method for the Thermal Treatment of Fluorine-Containing
and Noble Metal-Containing Products
Abstract
The present invention relates to an ashing plant for enriching
noble metals from fluorine-containing materials, comprising a
thermal treatment chamber (1) having a refractory insulating lining
on the inside of the chamber and an exhaust gas cleaning system,
whereby the refractory insulating lining is resistant to
hydrofluoric acid and the exhaust gas cleaning system comprises at
least one thermal after-incineration chamber (2), at least one or
more acid scrubber(s) (3, 4) and at least one alkaline scrubber
(5).
Inventors: |
Romero; Jose Manuel; (Hanau,
DE) ; Meyer; Horst; (Altenstadt, DE) ; Vo ;
Steffen; (Limeshain, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Heraeus Precious Metals GmbH & Co. KG |
Hanau |
|
DE |
|
|
Assignee: |
Heraeus Precious Metals GmbH &
Co. KG
Hanau
DE
|
Family ID: |
49028910 |
Appl. No.: |
13/969009 |
Filed: |
August 16, 2013 |
Current U.S.
Class: |
423/1 ;
422/170 |
Current CPC
Class: |
Y02P 10/214 20151101;
H01M 8/008 20130101; C22B 11/026 20130101; C22B 7/001 20130101;
C22B 11/021 20130101; Y02W 30/86 20150501; Y02P 10/20 20151101;
Y02W 30/84 20150501 |
Class at
Publication: |
423/1 ;
422/170 |
International
Class: |
C22B 11/02 20060101
C22B011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 2012 |
DE |
102012016420.3 |
Claims
1. Ashing plant for enriching noble metals from fluorine-containing
materials, comprising a thermal treatment chamber (1) having a
refractory insulating lining on the inside of the thermal treatment
chamber (1), and an exhaust gas cleaning system, whereby the
insulating lining is resistant to hydrofluoric acid and the exhaust
gas cleaning system comprises at least one or more acid scrubber(s)
(3, 4) and at least one alkaline scrubber (5).
2. The ashing plant according to claim 1, wherein the exhaust gas
cleaning system further comprises at least one or more thermal
after-incineration chambers (2).
3. The ashing plant according to claim 2, wherein the inside of the
at least one thermal after-incineration chamber (2) is provided
with a hydrofluoric acid-resistant refractory insulating
lining.
4. The ashing plant according to claim 1 wherein the plant further
comprising an exhaust gas conduit (6) for guiding the exhaust gases
out of the thermal treatment chamber (1) and in that the inside of
said exhaust gas conduit (6) is provided with a hydrofluoric
acid-resistant refractory insulating lining.
5. The ashing plant according to claim 1 wherein the refractory
insulating lining has an aluminium oxide (Al.sub.2O.sub.3) content
of 85% by weight or more.
6. The ashing plant according to claim 1 wherein the thermal
treatment chamber (1), after-incineration chamber (2), and exhaust
gas conduit (6) comprise different insulating lining(s).
7. The ashing plant according to claim 1 wherein thermal treatment
chamber (1), after-incineration chamber (2) and/or exhaust gas
conduit (6) further comprise an external lining.
8. The ashing plant according to claim 7, wherein the external
lining is a mineral fibre.
9. The ashing plant according to claim 1 wherein the exhaust gas
cleaning system comprises at least one scrubber made of graphite
(3) with a double-walled design.
10. The ashing plant according to claim 9, wherein the
double-walled scrubber (3) comprises a water cooling system.
11. The ashing plant according to claim 1 wherein the exhaust gas
cleaning system comprises at least one single-walled scrubber (4)
made of graphite.
12. The ashing plant according claim 1 wherein the exhaust gas
cleaning system comprises a double-walled scrubber made of graphite
(3) and a single-walled scrubber made of graphite (4).
13. The ashing plant according to claim 1 wherein the alkaline
scrubber (5) comprises on its inside a coating made of a plastic
material, in particular made of polypropylene.
14. The ashing plant according to claim 1 further comprising a
control unit.
15. Method for enriching noble metals from fluorine-containing
materials, comprising a thermal treatment of the materials in a
thermal treatment chamber (1) having a hydrofluoric acid-resistant
refractory insulating lining, and a cleaning of the exhaust gases
generated during the thermal treatment, whereby the cleaning
comprises the following steps in the following order: a) if
applicable, thermal after-incineration in an after-incineration
chamber (2), b) scrubbing of the exhaust gases with water and/or an
acid, and c) scrubbing of the exhaust gases with a base.
16. The method according to claim 15 wherein step b) comprises the
following steps: b1) scrubbing and simultaneous cooling of the
exhaust gases in a double-walled graphite scrubber (3) followed by
b2) scrubbing of the exhaust gases in a single-walled graphite
scrubber (4).
17. The method according to claim 15, wherein the salt-enriched
scrubbing water obtained in step b) can be fed to a waste water
treatment plant periodically or continuously as side stream.
18. The method according to claim 15 wherein the exhaust gases are
washed in step c) with a base with a pH of at least 10 or more.
19. The method according to claim 15 wherein materials having a
fluorine content of up to 5% by weight are used as materials to be
subjected to the thermal treatment.
20. The method according to claim 15 wherein fluoro-organic
materials, PTFE films, fuel cells, catalysts and/or pastes are used
as materials to be subjected to the thermal treatment.
21. The method according to claim 15 wherein the method is
implemented in an ashing plant comprising a thermal treatment
chamber (1) having a refractory insulating lining on the inside of
the thermal treatment chamber (1), and an exhaust gas cleaning
system, whereby the insulating lining is resistant to hydrofluoric
acid and the exhaust gas cleaning system comprises at least one or
more acid scrubber(s) (3, 4) and at least one alkaline scrubber
(5).
22. (canceled)
23. (canceled)
24. (canceled)
Description
[0001] The present invention relates to a device and a method for
thermal treatment of noble metal-containing products which also
contain fluorine aside from noble metals.
[0002] Various methods have become established for recovering noble
metals from noble metal-containing products, such as, for example,
catalysts or fuel cells. In the hydrometallurgical method, the
noble metal-containing layer of catalysts is dissolved off the
ceramic support by means of strong acids or bases. Subsequently,
the noble metals are separated from the solution, for example
through precipitation reaction.
[0003] In the pyrometallurgical method, the separation of noble
metals proceeds through melting the noble metal-containing products
in a metallurgical process. The ceramic fraction is transferred in
a slag phase and tapped, the noble metals are alloyed into a
collector metal, which is then also tapped and processed
further.
[0004] The direct incineration of noble metal-containing sludges
and multi-element waste materials, as described in DE 31 34 733 C2
and WO 99/037823, is also known. The noble metal-containing ash
thus obtained is then leached in order to recover the noble
metals.
[0005] In thermal reprocessing, noble metal-containing products
that also contain fluorine aside from the noble metals have proven
to be a problem. Thermal treatment of these products produces
hydrogen fluoride gas, HF. Said gas reacts with water in the
ambient air to produce hydrofluoric acid.
[0006] Conventional thermal reprocessing plants, in particular
ashing plants, comprise a thermal treatment chamber and an exhaust
gas purification system. The thermal treatment chamber is a
component of a furnace and is provided with an insulating lining
made, for example, of fireclay bricks or a ramming mass. These
differ in composition. However, all insulating linings comprise,
inter alia, silicon dioxide SiO.sub.2 (glass) and calcium oxide
CaO. These components are attacked even by small amounts of the
hydrofluoric acid and hydrogen fluoride that are generated and thus
are dissolved out of the insulating lining which reduces the
service life of the furnace and thus of the plant.
[0007] Another problem related to hydrogen fluoride gas is the
condensation of hydrofluoric acid at the supporting external steel
shells. These are the shaping and mechanical load-bearing framework
of the components of an ashing plant. Hydrogen fluoride gas can
diffuse through pores in the insulating lining and, in the region
of the steel shells, react with water from the ambient air to
produce hydrofluoric acid. Condensation of hydrofluoric acid on the
steel shells causes them to corrode which can render the entire
plant unstable.
[0008] A method, in which the generation of HF is to be prevented,
is disclosed in EP 1 478 042 A1. In this method, components of fuel
elements and catalysts are mixed with inorganic additives. In the
subsequent thermal treatment process, the hydrogen fluorides and
other fluorine compounds are absorbed and chemically bound by the
additive. For this purpose, an up to 100-fold excess of the
additive is added to the hydrogen fluoride gas that is being
generated. However, it has been evident that the absorption at the
additive is insufficient or too slow in the case of materials
releasing hydrogen fluoride already at low temperatures allowing
some hydrogen fluoride gas to escape. Moreover, the additive
occupies a fraction of the volume of the incineration space such
that the quantity of material that can be processed is reduced.
[0009] It is therefore the object of the present invention to
provide a plant for thermal reprocessing of noble metal-containing
products that contain fluorine in addition to noble metals. The
plant according to the invention shall allow all
fluorine-containing products to be reprocessed, regardless of the
volatility of the materials contained therein.
[0010] Another object of the present invention is to provide a
method for enriching noble metals from fluorine-containing
materials.
[0011] A first embodiment meets the object on which the present
invention is based through an ashing plant for enriching noble
metals from fluorine-containing materials, comprising a thermal
treatment chamber (1) having a refractory insulating lining on the
inside of the thermal treatment chamber (1) and an exhaust gas
cleaning system,
whereby the refractory insulating lining is resistant to
hydrofluoric acid and the exhaust gas cleaning system comprises at
least one or more acid scrubber(s) (3, 4) and at least one alkaline
scrubber (5).
[0012] Another subject matter of the present invention is an ashing
plant for enriching noble metals from fluorine-containing
materials, comprising a thermal treatment chamber (1) having a
refractory insulating lining on the inside of the thermal treatment
chamber (1) and an exhaust gas cleaning system,
whereby the refractory insulating lining has an aluminium oxide
content of 85% by weight or more and the exhaust gas cleaning
system comprises at least one or more acid scrubber(s) (3, 4) and
at least one alkaline scrubber (5).
[0013] Moreover, the exhaust gas cleaning system according to the
invention preferably comprises at least one or more thermal
after-incineration chambers (2). Preferably, the exhaust gas
cleaning system comprises one after-incineration chamber (2).
[0014] FIG. 1 shows a schematic view of an ashing plant according
to the invention. The FIGURE shows a preferred embodiment that
comprises two acid scrubbers (3, 4).
[0015] The thermal treatment chamber (1) is a component of a
furnace, into which the materials to be processed are introduced.
For this purpose, the thermal treatment chamber (1) comprises an
opening for introduction of the corresponding materials. According
to the invention, the thermal treatment chamber (1) can be operated
with a sub-stoichiometric amount or with an excess of air. The
inside of the thermal treatment chamber (1) can comprise devices
for incineration of the fluorine-containing and noble
metal-containing materials. This concerns, for example, grates for
accommodation of troughs for incineration of solid materials.
Liquid materials can be introduced into the thermal treatment
chamber (1) and incinerated therein either batch-wise in troughs or
continuously by means of corresponding dosing facilities.
[0016] The temperature on the inside of the thermal treatment
chamber (1) usually is approx. 800.degree. C. In this context, the
refractory insulating lining is designed to be stable at this
continuous temperature. Moreover, it is also resistant to
temperature peaks of up to approx. 2,000.degree. C. It is feasible
according to the invention to heat the thermal treatment chamber
(1) directly or indirectly. All means of heating known according to
the prior art are feasible, for example gas and oil heating or
electrical heating.
[0017] According to the invention, an acid scrubber (3, 4) is a
scrubbing stage, in which exhaust gases from the thermal treatment
chamber (1) are washed with water or with water acidified by the
hydrogen fluoride gas to be washed out. According to the invention,
an alkaline scrubber (5) is a scrubbing stage, in which the exhaust
gases are washed with an alkaline agent.
[0018] Heating fluorine-containing materials in the thermal
treatment chamber (1) in the ashing plant according to the
invention produces exhaust gases that contain hydrogen fluoride
gas. Since the thermal treatment chamber (1) is lined with the
hydrofluoric acid-resistant insulating lining, the chamber is not
attacked by the exhaust gases. In the exhaust gas cleaning system
according to the invention, the exhaust gas is initially subjected
to thermal reprocessing in a thermal after-treatment chamber (2)
and then all hydrogen fluoride gas or hydrofluoric acid already
formed is removed in the acidic and alkaline scrubbing stages (3,
4, 5) such that the exhaust gases are then harmless and can be
guided to the outside, for example by means of a chimney.
[0019] The ashing plant according to the invention can provide
further cleaning stages or cleaning agents for exhaust gas cleaning
in order to remove, for example, soot, chlorine or nitrous gases
from the exhaust gases. Pertinent cleaning agents or cleaning
stages are described in the prior art.
[0020] According to the invention, the hydrofluoric acid-resistant
insulating lining is resistant both to hydrogen fluoride gas and to
hydrofluoric acid.
[0021] The noble metal-containing and fluorine-containing materials
are placed in the thermal treatment chamber (1). The exhaust gases
produced in the thermal treatment chamber (1) during thermal
treatment can first be guided into a thermal after-incineration
chamber (2). Preferably, said chamber is also provided with a
hydrofluoric acid-resistant refractory insulating lining. Moreover,
the ashing plant comprises an exhaust gas conduit (6) for guiding
the exhaust gases out of the thermal treatment chamber (1).
Preferably, the inside of said exhaust gas conduit (6) is also
provided with a hydrofluoric acid-resistant refractory insulating
lining.
[0022] Thermal treatment chamber (1), exhaust gas conduit (6), and
the after-incineration chamber (2), which is preferably present,
are the components of the ashing plant through which exhaust gas
flows before hydrogen fluoride gas is removed from the exhaust gas
in the acidic and alkaline scrubbers (3, 4, 5). Providing the
thermal treatment chamber (1) as well as the after-incineration
chamber (2), and the exhaust gas conduit (6) with a hydrofluoric
acid-resistant refractory insulating lining increases the service
life of the ashing plant according to the invention, since these
components cannot be attacked by the hydrofluoric acid or hydrogen
fluoride gas.
[0023] The refractory insulating lining of the present invention
can be a ramming mass. Said ramming mass preferably has an
aluminium oxide (Al.sub.2O.sub.3) content of 85% by weight or more,
in particular of 88% by weight or more. Said insulating lining is
stable at a working temperature and at a continuous temperature of
approx. 800.degree. C. However, it also withstands peak
temperatures of up to approx. 2,000.degree. C.
[0024] Ramming masses usually contain silicon dioxide (SiO.sub.2)
and/or calcium oxide (CaO) in addition to aluminium oxide. These
components are also present in conventional refractory insulating
linings. These are dissolved by hydrofluoric acid, which destroys
the insulating lining.
[0025] Surprisingly, it has been evident that an aluminium oxide
fraction of 85% by weight or more, in particular of 88% by weight
or more, being present in a ramming mass is sufficient to provide
hydrofluoric acid resistance. The ramming masses according to the
invention can also contain different fractions of calcium oxide and
silicon dioxide as further components in addition to aluminium
oxide. Despite the calcium oxide and/or silicon dioxide fraction of
the ramming mass being up to 15% by weight, in particular up to 12%
by weight, the ramming mass is not attacked by hydrofluoric acid.
In this context, the relative content of calcium oxide and/or
silicon oxide or their ratio with respect to each other is
irrelevant. Moreover, the corresponding ramming mass is easy to
process and easily adapts to the internal wall of the thermal
treatment chamber (1), exhaust gas conduit (6), and
after-incineration chamber (2).
[0026] In this context, it is feasible according to the invention
that the thermal treatment chamber (1), the after-incineration
chamber (2), and the exhaust gas conduit (6) comprise the same or
different insulating lining(s).
[0027] Preferably, the thermal treatment chamber (1), the
after-incineration chamber (2) and/or the exhaust gas conduit (6)
further comprise an external lining. The external lining can be
provided using materials that are known according to the prior art.
Preferably, the external lining is a mineral fibre. The external
insulation is surrounded by a steel plate. The steel plate fixes
the external insulation in place and serves for stabilisation and
shaping of the components of the ashing plant.
[0028] One problem during the incineration of fluorine-containing
products is the generation of hydrofluoric acid that causes
corrosion in the region of the supporting external steel shells. If
hydrogen fluoride gas HF diffuses through pores to reach the space
behind the temperature-resistant insulating lining in the reaction
space, it reaches the external steel constructs part of which are
load-bearing. Hydrogen fluoride gas can react with water from the
ambient air to form hydrofluoric acid in this location.
Hydrofluoric acid forms an azeotropic mixture with water at a
concentration of 38.2% HF, whereby the boiling temperature of the
azeotropic mixture is 112.degree. C. If hydrofluoric acid condenses
at the steel walls, it causes these to corrode.
[0029] The effect of having the external insulation is that the
temperature at the steel shell, i.e. at the external steel wall of
the plant components, does not drop below 120.degree. C. In this
context, the thickness of the external insulation is a function of
the temperature profile on the inside of the corresponding
component of the ashing plant. Condensation of hydrofluoric acid
does not take place at a temperature of 120.degree. C. Accordingly,
if hydrogen fluoride gas were to diffuse through the refractory
insulating lining towards the outside, no corrosion damage to
load-bearing steel constructs is to be expected.
[0030] According to the invention, the thickness of the insulation
on the thermal treatment chamber (1), exhaust gas conduit (6), and
after-incineration chamber (2) can differ. However, it is feasible
just as well that the insulation of all components is equal in
thickness. Accordingly, thermal treatment chamber (1), exhaust gas
conduit (6), and after-incineration chamber (2) can, for example,
comprise an external insulation made of a mineral fibre at a
thickness of approx. 10 cm.
[0031] In a preferred embodiment, the thermal treatment chamber (1)
comprises a refractory insulating lining on the inside, whereby the
thickness of the wall of the chamber plus the insulating lining is
approx. 30 cm, and an external insulation with a thickness of
approx. 10 cm.
[0032] According to the invention, the ashing plant preferably
comprises at least one scrubber made of graphite (3) with a
double-walled design. Said double-walled scrubber (3) preferably
comprises a cooling system, in particular a water cooling system.
The scrubber (3) can comprise a steel plate as external shell. The
double-walled scrubber (3) comprises valves for feeding and
discharging the coolant.
[0033] The coolant, for example water, flows between the external
graphite wall and the external shell. This leads to indirect
dissipation of heat from the exhaust gas via the scrubbing medium
and the graphite walls.
[0034] Water has proven to be particularly well-suited as coolant.
Water is inexpensive and easy to handle. Moreover, if there was any
exposure to the exhaust gases, there would be no hazard of
undesired chemical reactions occurring.
[0035] The thickness of the graphite walls preferably is in the
range of 3 cm to 4 cm. It has been evident that this thickness is
sufficient to maintain sufficient temperature and acid
stability.
[0036] Water flows into the double-walled scrubber (3) as scrubbing
agent for the exhaust gas. The water reacts with hydrogen fluoride
gas HF in the exhaust gas and removes it by scrubbing. The
scrubbing water containing the hydrogen fluoride gas in the form of
a salt is then collected and subjected to disposal.
[0037] Graphite is characterised not only by acid resistance, but
by high temperature resistance as well. The exhaust gases flow from
the thermal after-incineration chamber (2) or the thermal treatment
chamber (1) into an acid scrubber. The temperature of said exhaust
gases is up to 1,000.degree. C. It has been evident that the
temperature resistance of graphite is sufficient in this
context.
[0038] Preferably, the exhaust gas cleaning system according to the
invention further comprises at least one single-walled scrubber (4)
made of graphite. This scrubber also comprises a steel plate for
its external shell. Preferably, the thickness of the graphite wall
is in the range of 3 cm to 4 cm.
[0039] In a preferred embodiment, the exhaust gas cleaning system
comprises a double-walled scrubber made of graphite (3) and a
single-walled scrubber made of graphite (4). This embodiment is
shown in FIG. 1. Both acid scrubbers (3, 4) are cylindrical in
shape in this preferred embodiment and have an internal diameter of
more than 1 m. The exhaust gas contacts the scrubbing water over
the entire height of approx. 4 m or more and is cleaned in the
process. The scrubbing water usually flows into the scrubber from
above such that the exhaust gas is washed with water in the upper
region of the scrubber. In the process, it becomes enriched in
hydrogen fluoride gas to the effect that the exhaust gas is washed
with an acid, namely water containing a hydrofluoric acid fraction,
in the lower region. In this context, the terms, upper and lower,
refer to the spatial arrangement that is also shown in FIG. 1.
[0040] In the first double-walled scrubber (3), at least a majority
or all of the hydrogen fluoride gas is washed out of the exhaust
gas with water. Moreover, the exhaust gas is also being cooled. In
the second single-walled scrubber (4), hydrogen fluoride gas that
may still be present is bound and thus removed from the exhaust
gas. No further cooling of the exhaust gases is needed.
[0041] The ashing plant of the present invention further comprises
an alkaline scrubber (5). The exhaust gases are guided from the at
least one acid scrubber (3, 4) into said alkaline scrubber after
most or all of the hydrogen fluoride has been removed. The alkaline
scrubber (5) can comprise a coating on its inside that is resistant
to alkaline scrubbing water and any traces of hydrogen fluoride gas
that may still be present in the exhaust gas. Specifically, the
coating is stable when exposed to bases having a pH of at least 10
or more, in particular of at least 11 or more. Preferably, the
alkaline scrubber (5) comprises on its inside a coating made of a
plastic material, in particular made of polypropylene. The external
shell of the alkaline scrubber can consist of steel.
[0042] A plastic coating has proven to be easy to handle. The
lining is made to be homogeneous and is not associated with a risk
of cracks. Moreover, plastic materials, in particular
polypropylene, are resistant to alkaline scrubbing water as is used
in this scrubbing stage. If any residual HF were still to be
present at this stage, the coating would not be attacked by it.
[0043] The exhaust gas guided into the alkaline scrubber (5) is
largely free of hydrogen fluoride gas. However, it cannot be
excluded that some traces of hydrogen fluoride gas may still be
present. If the alkaline scrubber (5) were lined with the otherwise
common glass fibre-reinforced plastic materials (GFR), these
residual amounts of hydrogen fluoride gas would be sufficient to
attack and quickly etch away the glass fibres in the plastic
material. The internal lining would have to be replaced after just
a short time under these conditions.
[0044] The ashing plant according to the invention can further
comprise a control unit. The control unit controls the temperature
profiles needed during the thermal treatment depending on the
specific material and also controls the exhaust air line as a
function of negative pressure, temperature, and oxygen content of
the exhaust gas. As a matter of principle, both continuous and
discontinuous operation are feasible. A discontinuous operation is
preferred.
[0045] In a further embodiment, the present invention comprises a
method for enriching noble metals from fluorine-containing
materials, comprising a thermal treatment of the materials in a
thermal treatment chamber (1) having a hydrofluoric acid-resistant
refractory insulating lining and cleaning of the exhaust gases
generated during the thermal treatment, whereby the cleaning
comprises the following steps in the following order: [0046] a) if
applicable, thermal after-incineration in an after-incineration
chamber (2), [0047] b) scrubbing of the exhaust gases with water
and/or an acid, and [0048] c) scrubbing of the exhaust gases with a
base.
[0049] A noble metal-containing ash is generated during the thermal
treatment of the noble metal-containing and fluorine-containing
materials. Said ash is then reprocessed according to wet chemical
methods known according to the prior art in order to recover the
noble metals it contains. In the scope of the present invention,
noble metals are gold, silver, and the metals of the platinum
group.
[0050] The thermal treatment usually proceeds at a temperature of
up to 800.degree. C. Peak temperatures of up to approx.
2,000.degree. C. may occur briefly. Following the introduction of
the materials into the thermal treatment chamber (1), the
temperature is increased slowly up to a temperature of approx.
600.degree. C. to 800.degree. C. The specific temperature depends
on the materials to be processed.
[0051] The exhaust gases generated during the thermal treatment are
first subjected to thermal after-incineration in an
after-incineration chamber (2), if applicable. Subsequently, the
exhaust gases are washed with water or an acid in an acid scrubber.
According to the invention, water is used as the scrubbing agent.
Water washes hydrogen fluoride gas out of the exhaust gas. This
produces an acid, which washes out more hydrogen fluoride gas such
that, in the course of the entire scrubbing process, both water and
an acid wash the exhaust gases.
[0052] The scrubbing water can have room temperature in this
context. However, it is feasible just as well that the scrubbing
water has a temperature that is slightly higher than room
temperature. In this context, the temperature should not be more
than 10 to 20.degree. C. above ambient temperature.
[0053] The salt-enriched scrubbing water obtained in step b) of the
scrubbing process can be fed to a waste water treatment plant
periodically or continuously as side stream.
[0054] Preferably, scrubbing of the exhaust gases with water and/or
an acid comprises two steps, namely [0055] b1) scrubbing and
simultaneous cooling of the exhaust gases in a double-walled
graphite scrubber (3) followed by [0056] b2) scrubbing of the
exhaust gases in a single-walled graphite scrubber (4).
[0057] It is preferred to cool the exhaust gas in the double-walled
graphite scrubber (3) by means of a cooling system, in particular a
water cooling system. Provided water is used as coolant, the
temperature of the water preferably is approx. 60.degree. C.
[0058] Hydrogen fluoride gas is removed from the exhaust gas by
introducing water into the inside of the double-walled scrubber
(3).
[0059] In order to ensure that the hydrogen fluoride gas is removed
from the exhaust gas as close to completely as possible, the
exhaust gas is washed twice with water and/or an acid in a
preferred embodiment and, for this purpose, is guided from the
first double-walled graphite scrubber (3) into a second
single-walled graphite scrubber (4). The exhaust gas is washed with
water and/or an acid in both scrubbers (3, 4). In both scrubbing
stages, the initial scrubbing agent is water. The water becomes
acidified by the hydrogen fluoride gas washed out of the exhaust
gas such that, overall, both water and an acid wash the exhaust gas
at the respective stages. If the hydrogen fluoride gas is already
removed completely from the exhaust gas in the first stage, the
second stage entails a cleaning with water only.
[0060] In scrubbing step c), the exhaust gas is neutralised and
acidic components originating from step b) are removed. A base with
a pH of at least 10 or more, in particular of at least 11 or more,
can be used as base in this context. It is preferred to use sodium
hydroxide solution for scrubbing the exhaust gases in step c).
Sodium hydroxide solution does not undergo any undesired reaction
with the exhaust gas components. Moreover, it is inexpensive and
easy to handle.
[0061] The method according to the invention is suitable preferably
for the processing of materials that have a fluorine content of up
to 5% by weight. It is preferred to use fluoro-organic materials,
PTFE films, fuel cells, catalysts and/or pastes as materials to be
subjected to the thermal treatment. As a matter of principle, the
method is suitable for all materials having a fluorine content of
up to 5% by weight that decompose at a temperature of approx.
800.degree. C., in particular of approx. 600.degree. C.
[0062] It is preferred to implement the method according to the
invention in an ashing plant of the type described above. The
ashing plant according to the invention is suitable for the use and
implementation of the method according to the invention.
LIST OF REFERENCE NUMBERS
[0063] 1 Thermal treatment chamber [0064] 2 After-incineration
chamber [0065] 3 Double-walled graphite scrubber [0066] 4
Single-walled graphite scrubber [0067] 5 Alkaline scrubber [0068] 6
Exhaust gas conduit
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