Note: Descriptions are shown in the official language in which they were submitted.
21 84843
METHOD AND ARRANGEMENT FOR HEAT-TREATING A
MATERiAL
The invention relates to a method for heat-treating a
flowable material, especially for sintering metallic materials
according to the preamble of claim 1. Furthermore, the invention
relates to an arrangement according to the preamble of claim 13.
Method and ci, r~l-ge~ "l of the dru~ l ,liol1ed kind will be
explained first with the aid of Fig. A in which as a treatment
apparatus a conventional sintering device with first auxiliary
devices is S~ ldLiC~ 'y represented.
The arrangement l~ ser,l~d in Fig. A is ~u",,~ ed
si,u~dllli-:ly of a sintering device 1 with an ignition furnace 3
arranged at the upstream side thereof, a sintering bed ll~"~ o, li"~
device 5 which comprises an endless sintering band 7 guided
through the sintering device, a sinter mixture feeding device 9, and
an exhaust gas removal device 11 with which the pressure
yradient extending from the top to the bottom through the sintering
bed is produced for the sinterinr~ process.
The sinter mixture for the sintering process explained with
the aid of Fig. A is comprised of ores, flux materials, fuels,
especially coke fines, quick lime, and return material of the
2~ 848~
sintering process itself. A mixing and rolling drum 13 provides for
an intensive mixin3 of the granular, respectively, flowable mixture
co" ",~lle, Itb as well as for a uniform granule size and shape of the
sinter material. The feeding device 9 is supplied from the mixing
and rolling drum 13 and pours the sinter mixture in a su~ dl~ lly
uniform layer thickness onto the grid of the sintering band 7. In
this known device with the aid of an upstream feeding device 15
a finished sinter layer is applied as a grid coating onto the sinter
band 7 and is arranged between the actual sintering bed, i.e., the
sinter mixture layer and the grid in order to protect in the manner
of a temperature barrier the grid from extreme temperature
exposure and loads. The finished sinter layer remains unaffected
by the subsequent sintering process in the sintering device 1 and
does not interfere with sllhse~lPnt ,ulucess~ steps in a breaking
device 17, a sinter cooling device 19, and in a cold sifting station
21 up to the point of obtaining the ready-to-use finished sinter at
the outlet location 23.
The uul I t!Spu~ 9 conventional sintering process is carried
out as follows. First, a suitable grid coating layer is uniformly
applied with the feeding device 15 onto the sinter band 7. Onto
the grid coating layer the sinter mixture is poured as uniformly as
2~ 848~
possible in a ~ d~ d layer thickness over the entire width
of the bed. From the supply location of the sinter mixture below
the feeding device 9, the sinter band 7 moves to the leff of the
sintering device. The sinter bed is ignited by the ignition furnace
3 at the side facing the furnace upon entering the sintering device
1. Vvith the removal device 11 below the bed a pressure gradient
is produced within the sinter bed over the entire sintering device
1 via which combustion air is introduced into the combustible
sinter mixture and exhaust ~ases are removed at the second
sinter bed side (underside). Affer i~nition of the carbon-~,ol lldil lil l9
fuel of the sinter mixture, a combustion zone is formed within the
sinter bed which with the advance of the sinter bed within the
sintering device from the right to the leff moves from the top of the
sinter bed to the bottom, whereby the sinter material in this zone
is sintered and baked together. The course and the formation of
the combustion or sintering zone over the depth of the sintering
bed is shown as a function of the length of the sintering band in
the interior of the sintering device 1 in Fig. B. The depth of the
bed in this example is 5ûO mm. As can be seen in Fig. B, the
combustion, respectively, sinterin~ zone at the end of the sintering
process, respectively shortly before exiting from the sintering
3 -
~'
21 ~4843
device 1, has reached the bottom of the sintering bed and softens
also in this area the material to be sintered to such an extent that
the individual material granules bake together (ayylu,l,el(A":).
This prfurnace sintering process has, as is well known, the
disadvantage that the exhaust gases withdrawn from the sintering
layer are loaded with a high percentage of pollutants which must
be removed before the exhaust gases can be released into the
surrounding ..~."os~,l,e,~. The purification of sinter exhaust gases
has been carried out in practice with secondary exhaust gas
purification devices. The investment cost of secondary exhaust
gas purification devices which reduce the pollutant emission in a
reliable manner are extremely ~tigh because the exhaust gas is
highly loaded with pollutants over the entire length of the sintering
bed and restrictively large gas volumes must be taken care of
during the scrubbing operation. Therefore, exhaust gas
purification has been limited to the use of dust filters (dust removal
from the band in Fig. A) or has been ptlrullll~d by a somewhat
more effective exhaust gas purification, for example, in scrubbers.
The especially critical cl,loruolydlli,, substances, heavy metals,
etc. in contrast have not been removed because of the high costs
involved .
-- 4 -
2 1 84843
It is therefore an object of the invention to remove the
pollutants produced in a heat-treatment process of the
."t:",t~ io"ed kind substantially completely with comparatively
low investment costs, preferably by reducing the volume of
exhaust gas to be purified.
The solution to this object is based on the idea that the
disclosed conventional sintering process inherently has already
numerous properties and phases which favor exhaust gas
purification but in the past could not be taken advantage of in a
suitable manner. This is achieved inventively, i.e., with the
inventive features of the method of patent claims 1, 2, 3, 4 and 6
and according to the inventive arrangement with the features of
patent claims 13, 14, 15 and 16.
The invention no longer employs secondary exhaust gas
purification measures for great gas volumes which require
extremely high investment costs. Instead, it incorporates in all
alternative solutions the exhaust gas purification suL,~lal llk311y into
the primary sintering process. The invention is based on the idea
that in the known manner of heat treatment a strongly absorbing
fllter layer is positioned downstream of the combustion zone as
long as the combustion zone has not yet penetrated to the
- 5 -
21 8~8~3
. ~
exhaust gas side opposite the ignition side. The purification effectof this "natural" a~ulluliù~l layer is used in the invention. Before
the cleaning effect based on combustion of the carbon-containing
(ad~ol,uliull-effective) fuel ~i~d,upea,~ and the combustion zone
pe~ Les to the other side, in a first inventive solution the
pressure gradient is reversed and a counter combustion is
generated. In this manner adjacent to the second combustion
zone which migrates to the other side another layer is arranged
~Jo...~ lll which initially can act as a filter layer. Before
col"l.;,li"g the two combustion zones, the ulllulùulydlli~
substances which are released in the second combustion zone, for
example, dioxins or furanes, are combusted under the influence
of the high temperature of the first combustion zone, respectively,
are adsorbed by remaining carbon particles and are subsequently
also combusted.
According to another inventive solution the exhaust gases
exiting from the treatment bed are collected and introduced into
the i~qnition furnace. In this context it is possible to return all of
the ,qases exitiny from the treatment device or only the gases
containing a critical amount of pollutants from partial phases,
especially of the end phase of the treatment process. Due to the
21 84843
influence of the high temperatures~within the ignition furnace the
pollutants within the exhaust gas are destroyed. The exhaust
gases are then guided through the treatment bed under the effect
of the pressure gradient present within the treatment bed. Upon
doing so, they are again exposed to high temperatures within the
area of the combustion zone and are further purified in this
manner. The "natural" purification layer positioned downstream of
the combustion zone within the treatment bed adsorbs furthermore
remaining pollutants still contained within the exhaust gas.
In an alternative treatment process the exhaust gases
exiting from the treatment bed at the end phase of the process are
collected and are guided together with the gases required for the
heat treatment into the treatment bed at the initial phase andlor
the central phase of the process. The exhaust gases can be
areally distributed over the treatment bed and can be guided
therethrough. The pollutants therefore must not be co~ ,..~d
within the area of the ignition furnace but can be supplied together
with combustion air in low cullcelllld~iulls~ The pollutants
contained within the exhaust gas, for example, dioxins or furanes,
are destroyed under tlle infiuence of the high temperature upon
passing of the gas mixture through the combustion zone. In this
-- 7 -
2 1 848~3
process alternative the cleaning effect of the natural a,l~or,uliul,layer arranged behind the combustion zone is also employed.
Only minimal technical retrofitting measures are required for
realizing the method are required so that the resulting costs are
CUI I l,Udl dli"ely low.
In a further alternative treatment process the pressure
gradient and thus the combustion of the fuel layers within the bed
from the beginning to the end of the treatment process is
I l Idil lli~lil led. During the migration of the combustion zone from the
top to the bottom the relatively cool, un,treated mixture layer
positioned llle,~uelo.. acts as an adbo"uliull filter. Its filter effect
is reinforced by the lowest (first) layer consisting of ad~ol,u~iull-
suitable material, especially coke. This layer is protected against
combustion by the second layer made of inert material which acts
as a temperature barrier. This layer is an a~ùlluLiùll filter layer
which is carried along within the primary process. Instead of
lumpy or granular coal or coke other lumpy or granular material
with 1ù,ll,ual~le ad~ul,u~iull or filtering properties can be used.
The last named process alternative makes obsolete a
change of the conventional treatment device. The coke which
functions as the adsul,uli~l, medium is pored as a first layer onto
2~ ~4~43
the grid supporting the layer sequence and ac~v""~a~ s the
actual treatment process as a primary filter. This coke layer is not
wasted. Most of the coke, on the one hand, can be separated at
the end of the treatment process from the remaining layers and
can optionally be returned and reused; on the other hand, the
coke used as a filter layer during the heat treatment can be used
in the blast furnace as a l~Jlc,Cts,~ of other coke materials.
In a further alternative method, with a directed displacement
of the pollutant profile in the direction of the profile of the exhaust
gas temperature and especially by overlapping the correspondin~q
profile maxima, the pollutants are col,c~"ll,~lt,.l onto the section
of the device in which the exhaust gas temperature is especially
high. The purification of exhaust gases can thus be limited to a
correspondingly short section of the treatment area in which the
pollutant collcell~l.9i~ll and exhaust gas temperature are at a
maximum. In this manner, purification devices of a small capacity
are sufficient in order to operate the entire treatment device with
very low pollutant emission.
The di:~Jldctelllelll of the pollutant cul,ce,,ll~;vll profile is
achieved by enriching the treatment bed with pollutant-adsorbing
media which retain the pollutants until the last section of the
g
` ~ 218484~
treatment area (for example the sintering apparatus) is reached.
Oniy at the end of the treatment area when the capacity of the
pollutant-adsorbing medium is depleted and the combustion zone
reaches the lower layers in which the pollutants are retained at
high collce~ ;lLiulls the cunce~ iull of pollutants within the
exhaust gas rises greatly. This portion of the exhaust gases
which is greatly pollutant-loaded can then be cleaned separately.
Instead of introducing additional pollutant-adsorbing media
into the treatment bed it is possible to use media with improved
~:,oruli~n properties in ~ ~ ul1S in which the treatment
process requires the use of pollutant-ad~u,L,e"l media within the
treatment bed. A greater specific surface area of the individual
adboruliol~ medium particles can for example lead to the desired
~i_,,lacul"e,l~ of the pollutant uul~c~ Livll profile.
Advantageously the pollutant-ads~ "L media are mixed
well with the material to be treated and are then poured onto the
grid before they are introduced into the treatment area. It is thus
possible SULJ~ldl lli..'l~ without any technical expenditure to
uniformly distribute the pollutant-adsorbing media within the
treatment bed.
In a further development of the invention it is su~gested
- 10 -
21~4843
that the pollutant-adsorbent media are provided within the lower
area of the treatment bed in a higher cullc~rllli~liu" than in the
upper area of the treatment bed. Varying collc~"l,~ n ratios
between the pollutant-adsorbent media and the material to be
treated can be produced during mixing. In order to produce an
especially steep conct~r~ " I profile of the pollutants, it is
favorable to gradually increase the ~oncel ,11 iiliull of the pollutant-
adsorbing medium from the top to the bottom within the treatment
bed. Instead, it is also possible to provide a plurality of two layers
with different c~llce"lliilioll ratios.
A preferred ~IllI.odi,,,~,,l is ull~ ttli~t:d in that the
collected exhaust gases are catalytically purified by employing
their own high temperature. An effective catalytic purification is
possible only at temperatures above 300 C. By overlapping the
pollutant ~unce~ lioll maximum with the temperature maximum
the required high temperature for the catalytic cleaning is achieved
in most heat treatment processes. It is not necessary to provide
additional heat energy; instead, the heat energy released by the
process of heat treatment is collected and is suffficient as a heat
source. It is furthermore advanta~eous, that the heat is not
returned into the surroundings as waste heat. By catalytic
~78484~
puriflcation different pollutants can be removed from the exhaust
gas. For example, ul llul Uolydl ~ic substances such as dioxins and
furanes can be reduced with suitable reduction catalysts. Also,
nitrogen oxides can be reduced without problems in this
temperature range. A catalytic oxidation is, for example, also
possible for pollutants such as SO2; SO2 is oxidized to SO3.
As an advantageous embodiment of the invention it is
suggested that the pollutant-adsorbent media and their
~ul Ice, ,11 d~iUI 1~ within the treatment bed are selected such that the
cul Ice, ,1, dliul I profile of the pollutants resulting within the treatment
process and especially their maximum are overlapped within the
end section of the treatment zone. The more CullC~Illldliol,
profiles of different pollutants are overlapped and the steeper the
.ullCell~l~Liull peaks are, the lower the partial volume of the
exhaust gas to be purified will be and the more effective the
inventive method will operate.
In a further development the exhaust gases exiting from the
treatment bed in the last phase of the process are subjected to a
particle sepdl d~iOll step. For this purpose, a conventional electric
filter can be used. In order to achieve an especially high cleaning
effect it is favorable to arrange downstream of the particle
218~843
separation a catalytic purification device.
Instead of the particle separation the exhaust gas can be
cleaned with a.l~o, ~ media and water after catalytic
purification. This is performed, for example, with the aid of a
spray dryer with which carbon-containing adsor,uliull material in
the form of dust and water are introduced into the exhaust gas
stream. The introduced water reduces the exhaust gas
tempsrature to such an extent that, for example, salts and
chlorides will crystalize. In order to further improve the quality of
the exhaust gas, instead of a spray dryer, or in addition to it, an
active coke device can be used.
Advantageously, the exhaust gas is subjected to a particle
sepdlaliul1 after cleaning with a-iso,,.Liùl~ medium and water.
A portion of the solid materials collected in the particle
s2pdldliun step can be returned and used again as an adaur~uliull
medium for cleaning. Since the ad~ull~ medium-containing
solid materials have not cu~ t~ly exhausted their a~sul~ ull
capacity for pollutants during their first run through the exhaust
gas, it is possible by returning it to l~ad the a.l~o"u~ medium
with pollutants up to its capacity limit and to thus reduce the
operation costs. -13 -
-
2~ ~fi~6F3
Advantageously the pollutant~adsorbent means is a carbon-
cul"~i"i,ly flowable material for example coke fines and/or active
coke. This material is i, ~ ellsive. In heat treatment processes
above the ignition temperature of coke fines it will ignite and will
release its combustion heat as additional heat into the treatment
bed. No interfering foreign substances will remain within the
treatment bed.
Preferably the inventive method is used during sintering of
metallic materials.
Expedient devt~l.,,u" ,el,ts and embodiments of the invention
are defined in the dependent claims.
In the following the invention will be explained in more
detail with the aid of the drawing in which s~ ", ~y
s~llLt~d ~Illbodi,ll~,,LO are shown. It is shown in the drawing:
Fig. 1 a schematic It~ Sellldli~l~ of a sintering
~y~ e~l for p~rullllillg a first sintering
method according to the invention;
Fig. 2 a schematic l~p~se~ldli~ of a sintering
d"d"g~",el,L for performing a second
inventive method alternative;
Fig. 3 a schematic representation of a sintering
- 14 -
. ~ 2~84843
d,l~l,gt""~"l for performing a third inventive
method alternative;
Fig. 4 a schematic representation of a sintering
for performing a fourth inventive
treatment method;
Fig. 5 a schematic representation of a sintering
arran~ement for p~,rul",i"g a fifth inventive
method alternative;
Fig. 6 a schematic l~pl~se,,ld~iull of a sintering
a"~ "el" for performing a sixth inventive
method ~ ; "~t; ~ c;
Fig. 7a a schematic repl~s~,,ld~iùll of a sintering
tll 1~1 ,~e" ,el ,~,
Fig. 7b a diagram of exhaust gas temperature,
plotted as a function of the length of the
sintering band;
Fig. 7c a diagram of the exhaust gas collcel ,I, ;~liull of
SO2 plotted as a function of the length of the
sintering band;
Fig. 7d a diagram of the exhaust gas concentration of
polychlorinated dibenzodioxins and
- 15 -
.
-
2~ ~4~4~
polychlorinated~dibenzofuranes, plotted as a
function of the length of the sintering band;
Fig. 7e a diagram of the exhaust gas ,ullCt~ 1 d~iOl~ of
nitrogen oxides, plotted as a function of the
length of the sintering band.
The ~I ~ ,L~odi" ~"~ of the invention scl1e" Id~iC~lly I ~ 5~ d
in Fig. 1 differs from the conventional sintering d"d"ge",~"~ of
Fig. A on the one hand due to its second ignition furnace 4 which
is arranged in the vicinity of the exit end of the sintering device 1
and which ignites the sintering bed from the underside and on the
other hand by a device for g~ l d~il 15J a pressure gradient in the
counter direction, i.e., from the underside of the bed to the upper
side. This pressure gradient gel1~ld~i"~ device Colll~ 5 an
exhaust removal hood 6 and a suction pump 10 co""~.,t~d to a
return line 8.
The exhaust gas removed via the removal hood 6 can be
freed of dust with a suitable filter 12. In a mixer 14 the exhaust
gas is mixed with the combustion air required in the sintering
device 1 at the forward, respectively, central area and is
introduced together with the combustion air via the hood 16 from
the top to the bottom through the sinter bed. Upon pel1~lldLillg the
- 16 -
2li ~84~
combustion zone, respectively, the high temperature of the sinterbed through which the combustion zone migrates, the
olool!Jdlli~ substances are destroyed sufficiently and reliably.
Behind the combustion zone the exhaust gases newly formed
during sinterin3, respectively, the returned exhaust gases pass
through the sinter mixture in which the fuel cullL~illi,lg a great
amount of carbon, in general the form of coke fines, is finely
distributed. This fuel acts as an adsorption medium in which a
substantial portion of the pollutants, similar to the conventional
secondary purification of the exhaust gases in active coke
reactors, is adsorbed
In the schematic drawing according to Fig. 2 only the
feeding section of the sinter bed transport device 5 is
s~ e",d~ 'ly ,~ se"'~i. Othen~ise, the sintering arld"g~",e,ll
corresponds to the one of Fig. A. As can be seen, the sinter band
7 has arranged thereat in sequence three feeding devices. With
an additional first feeding device 18 a thin a i~ol,ulioll medium
layer 20 is applied to the grid. Do~ d"~ of the device 18 the
feeding device 15, as already described in connection with Fig. A,
for feeding the finished sinter layer 22 onto the a~ liol I medium
layer 20 is arranged, and du~ a~ thereof the feeding device
- 17 -
21 ~4843
9 for feeding the sinter mixture 24 which forms the actual sinteringbed is arranged. During the sintering process the f nished sinter
layer 22, which may be aiso comprised of another material with
sulJ~Idl llidlly inert properties, provides a temperature barrier which
prevents a migration of the combustion zone into the filter layer 20
of adsorptive capacity.
Fig. 3 shows a sintering all dl~yc" ,ent which is principally of
a similar design as the one of Fig. 1. However, in the rearward
area the sintering a, Idl1g~",e"l 1 lacks the second ignition furnace
4. Similar to the dl I dl~gd" ,e~ I~ of Fig. 1, an exhaust gas collecting
device 6' is provided which however is arranged at the underside
of the sintering bed for producing a conventional pressure
gradient. The collected exhaust gas is returned via the return line
8 and the suction pump 10 to the mixing device 14, is mixed with
combustion air, and introduced via the hood 16 in the forward and
central area of the sintering device into the sintering bed. The
purification effect of the sinter mixture on the other side of the
combustion zone within the sintering bed is taken advantage of
with this arrangement according to Fig. 3. The investment costs
are lower especially because the second ignition device is
obsolete and, furthermore, because the additional f Itering device
- 18 -
2 1 84~43
12 can be omitted.
The previously described primary exhaust gas cleaningmeasures can also be combined. For example, the exhaust gas
return according to Fig. 3 can also be used when using the
additional a.lbo"uliulI medium layer 20 of Fig 2. Fu~ ull"ul~, the
exhaust gas can be returned to the ignition furnace or to the inlet
area, respectively, central area of the sintering device external to
the ignition furnace. Also, it can be introduced in cùll,bi, IdliUI1 into
the ignition furnace area and a further area, for example, into the
central area of the sintering device. Upon return of the exhaust
gases only into the ignition furnace, the mixer 14 andlor the hood
16 can be omitted. In respect to the sinter bed conveying device
no limitations are necessar,v in connection with the invention. The
conventional sinter band device can, for example, be replaced by
a so-called pusher type furnace in which the sinter bed is moved
in sequentially arranged baskets and is pushed through the
sintering device 1. The conveyor belt can be a continuously as
well as a discontinuously driven belt.
In the device according to Fig. 4 the feeding device 9
serves for feeding not only the sinter mixture but also an additional
adsorption medium. The sinter mixture is ,o"".rib~d in a manner
- 19-
~ .
~78~843
known per se of ores, flux material, fuels, especially coke fines,quick lime, and return material of the sintering process itself. The
adsorption medium is carbon-~o"ld;"i"g and granular,
respectively, flowable.
By enriching the sinter bed with adsorption media,
pollutants developing within the forward or central area of the
centering device 1 are adsorbed to a greater extent. The
pollutants are retained within the sintering bed so that the exhaust
gases removed within the forward sections already comprises
sufficient purity. The exhaust ~ases removed within the forward
and central areas of the sintering devices with the exhaust gas
removal device 11 (removal line 25), after particle separation with
an eledric filter 25', can be introduced without further purification
steps into the surrounding ~ us~ul~e~. The exhaust gases
removed by the exhaust removal device 11 within the rearward
area of the sintering device 1 are guided via a separate removal
line 26. In this area of the sintering device the temperature of the
exhaust gases is naturally very high. By enriching with pollutant-
adsorbing media, the Cul l~ iol~ maximum of all critical
pollutants, for example, possible ~,I llul uo, ~a"ic substances,
nitrogen oxides, and sulfur dioxide are located within this rearward
- 20 -
.
~ 2~8484}
area. Particles such as fly ash are separated with t~le electric f Ite!27 from the exhaust gas. Subsequently, the exhaust gasses are
catalytically cleaned by adding reduction media. Within the
catalytic cleaning reactor 28 the optionally present dioxins and
furanes as well as nitrogen oxides are reduced by the reduction
catalyst. An additional oxidation catalyst serves to oxidize SO2 to
S03. The catalytic treatment is favored by high exhaust gas
temperatures. The catalytically cleaned exhaust gas can then be
released without further cleaning steps into the surrounding
~ti"ob~ e,~ (removal line 29).
The fifth embodiment represented in Fig. 5 of the inventive
s~lld~lgelllellL differs from the one of Fig. 4 by a purification
d"~ l"er~l dU...lSIl~dlll of the exhaust gas removal line 26. In
this ~ :l l IL,o-li,, ,~I II the catalytic cleaning reactor 28 is directly loaded
with pollutant-loaded exhaust gases. Subsequently, the exhaust
~ases are cleaned in an d.l~l,uIiul1 medium reactor 30, with
addition of adsoll,Iiol, media and water, from sulfur oxides, dust,
and organic substances. A fabric filter 31 arranged do.."~ d,l,
of the ad~o"uIiul1 medium reactor 30 separates the adsorption
medium and further solid material from the flue gas stream. The
filtrate is partially returned via the return line 32 into the reactor in
- 2 1 -
2~84843
order to adsorb more pollutants therein and in order to completelyexhaust the adbor~ capacity of the particles.
Do.~.lsLI~,,, of the fabric filter 31 the exhaust gas has
relatively high purity and can be returned together with the
exhaust gas from the exhaust gas line 25 into the surrounding
~I",ob~ e,~. The exhaust gas in this ~",bodi,l,e"l fulfills the more
stringent pollutant emissions limits and is especially
enYi, ul ~ ,t ~I:y friendly.
The employed d,lbor,uli~," medium reactor 30 can be of a
very small size because by concentrating the pollutant release
within the rearward area of the sintering area only a small portion
of the exhaust gas removed during the heat treatment process
must be cleaned from the pollutants. The constructive
expenditures are thus minimal. By returning the absorption
medium into the reactor the operating cost are kept at a
minimum.
In the ~",I,odi,l,e"l according to Fig. 6 the removal line 26
is flrst introduced into the electric filter 27 in which solid particles
especially fly ash are separated. The actual cleaning is again
carried out in the catalytic cleaning reactor 2~ at temperatures of
dpf.l ~ 'y 300 to 400 C. Subsequently the hot exhaust gas
- 22 -
~ 2~ 8~8~3
is returned via a return line 33 into the ignition furnace. Here theexhaust gas is mixed with combustion air and returned into the
sintering bed. The cleaning effect of the sinter mixture positioned
doL~ l of the combustion zone within the sintering bed is
taken advantage of in order to achieve an even higher purity of
the exhaust gas.
The disclosed exhaust gas purification measures can also
be combined. For example, the exhaust gas return according to
Fig. 6 can also be used in addition to adso"uliull medium reactors.
Furthermore, the adsù",': 1 medium in the lower layer of the
sintering bed can be introduced in higher ~ullC~ iDll than in the
upper layers. The adsorption medium and the sinter mixture can
also be applied onto the band in sequence. A separate adso~liull
medium layer can be provided, separated by an isolation layer
from the sinter mixture. Such an a.l~,,uliul~ medium layer can
also be used in addition to an ~ lll of the material to be
treated with adsorption material.
The ddsol,uliun medium itself can be a flux material and/or
a material mixture used in conventional sintering processes,
whereby its pollutant-adsorbent properties are improved in the
spirit of the present invention. It is important that the
~84843
Co~ r,~, dliOIl profile of the pollutants is displaced according to the
exhaust gas temperature profile. This will be explained in more
detail with the aid of Fig. 7.
The sintering process begins below the ignition furnace 3.
The sinter band 7 moves in the direction of conveyance which is
to the right in the shown drawing. With the conveying movement
of the sinter bed the sinter zone moves simultaneously from the
top to the bottom through the sinter bed.
Fig. 7a shows a diagram of the exhaust gas temperature
plotted as a function of the sinter band length. The solid line
,~p,~:,e"~ the curve for a conventional sintering process and the
dashed line lezl)~se~t~ the curve for the inventive method. This
is true also for diagrams b-e. The curve of the exhaust gas
temperature shows in the rearward area of the sintering device a
strong maximum in the conventional as well as the inventive
method. The course of the temperature is practically not affected
by the invention.
Fig. 7b shows the diagram of the ~ollC~ dli~l~ of SO2 in
the exhaust gas plotted as a function of the sinter band length.
In known sintering processes (solid line) the SO2 c~n~ ldlioll
within the exhaust gas rises shortly behind the center of the
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~ ~ 4 218~843
sintering device. The SO2 peak is very broad. The exhaust gasCo~ ldliùl\ of SO2 in the inventive method is constantly
ri~d"LIy lower in the forward and central areas and rises
sul~:~ld~ lly only at a later point with relatively steep flanks. The
peak is displaced to the rear and su~sld,lli~'ly smaller.
In Fig. 7c the exhaust gas uull~el ~1~ dliOIls of polychlorinated
dibenzodioxins and dibenzofuranes is plotted as a function of the
sintering band length. In the conventional sintering process the
ol)ce~ dliUIl of the ~ luruul ydlli~, substances rises already at the
center of the sintering process. The peak is very broad in analogy
to the peak of the SO2 exhaust gas ~;ullu~l 1ll dliun. In the inventive
method the exhaust gas loading with ulllo~ uu, Udl Ik, substances in
the forward and central areas is suLbldll" "y lower due to the
a-l~or,u~ioll effect of the sinter bed, and the maximum is displaced
by forming a sharp peak to the rear.
In Fig. 7e the exhaust gas col,cel,lldliulls of NOx is plotted
as a function of the sinter band length. In the conventional
sintering process the NOX COll~ll[ldliOIl is sul,~ld"li..lly constant
over the entire length of the sintering band. Only at the end of the
band the NOX drops s~b~ldl lli.~'ly linearly. This nec~ssi'.~'_d in the
past a cleaning of the entire exhaust gas volume for removal of
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` ~ 2~84843
NOx pollutants. The exhaust gas~concentration of NOx in theinventive method is negligibly low in the forward and central areas
and rises to a peak only in the rearward section of the sinter band.
Thus, with the inventive method it is thus possible to
concentrate the pollutants within the rearward area of the sinter
band. The pollutant peaks are displaced to the rear and the
col~ce~ l maxima coincide with the maximum of the exhaust
gas temperature. In this manner, only a small portion of the
resulting exhaust gas volume must be purified. The partial
amount of exhaust gas to be cleaned is collected within the
rearward area of the sintering device, i.e., at a location where the
exhaust ~as temperature is sul,~ '!y within the optimum
temperature range required for catalytic purification.
Various alternatives are possible with the invention.
Especially, the measures represented in Figures 1 to 6 can be
combined in any desired fashion. The method is suitable for a
plurality of heat-treating methods with similar advantages,
especially also for roastin~ methods, for example, for the heat
treatment of metal sulfides, especially lead, zinc and nickel in
oxidizing ~I",o~phel~s.
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