Method and apparatus for processing contaminated soils

A method of and apparatus for decontaminating contaminated soils and similar materials, the method includes (a) drying and comminuting contaminated soil and similar material in a mill through which a flow of hot gases is passed to provide a mixture of solids and gases; (b) introducing the mixture into a cyclone separator where the mixture is separated into gaseous and solid components; (c) thermally treating the solid components in a decontamination zone at a temperature effective to decontaminate at least a portion of the solid components; (d) passing the solid components after thermally treating same in step (c) into a dwell zone configured as a flow-through region; (e) thermally treating the solid components within the dwell zone at a temperature effective to decontaminate at least a portion of the solid components; (f) cooling the solid components after step (e) by transferring same into a cooling region of a cooling line; (g) thermally afterburning gaseous components from at least one of steps (b) and (c) in a burner; (h) cooling and filtering at least a part of the gaseous components from step (g); and (i) discharging the gaseous components from step (h) to the atmosphere.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method of processing contaminated soils and 
similar materials, particularly from coke making locations, by removing, 
decontaminating and possibly replenishing the removal location with 
purified soil in such a manner that the removed soil is dried and 
comminuted in a mill through which hot gases flow, the solids/gas mixture 
is introduced into a cyclone separator and is there separated into gaseous 
and solid components; the solids are thermally treated at a high 
temperature, are then cooled and possibly employed to replenish the 
removal location; and, after being subjected to a cooling and filtering 
process, the exhaust gases are discharged, at least in part, to the 
atmosphere. 
2. Background of the Related Art 
It is known to excavate rivers in order to remove deposits therein and to 
introduce the material, after preprocessing it by comminuting it in a 
crusher and screening it, into a rotary kiln system. The rotary kiln 
system is composed of two series connected revolving drums, the first one 
of which operates as a drying drum to pre-dry the material. The second 
rotary kiln following in the stream of material is fired by an open flame 
which acts directly on the solids to be purified at temperatures between 
400.degree. C. and 800.degree. C. The purified soil is removed from the 
rotary kiln and installed again at the location where it was obtained. 
After the dust components contained therein have been removed, the exhaust 
gases generated in the rotary kiln system are heated in a combustion 
chamber equipped with a separate burner to about 1200.degree. C. and are 
thus decontaminated. After subsequent cooling and filtering, the purified 
exhaust gases are discharged to the atmosphere. Part of the exhaust gases 
are fed to the rotary drying drum to intensify the drying process and to 
improve the energy balance. The drawback noted in the prior art method is 
that the preprocessing by crushing and screening does not produce 
sufficiently small grain sizes in the contaminated material so that the 
subsequent drying process step is ineffective. 
In the subsequent process step of directly charring the solids by a flame 
in the rotary kiln, there exists the additional drawback that uniform 
heating does not take place, rather streaks of cold gas develop which 
result in untreated streaks of solids in the discharged material, the 
latter thus being purified only insufficiently. Finally, the temperatures 
realized in a rotary kiln system are not sufficient to decompose all 
noxious substances so that the thus purified soil no longer meets present 
requirements for environmental protection. Also, the return of purified 
hot exhaust air to the drying drum through a branch provided downstream of 
the second burner is unfavorable from an energy engineering point of view. 
German Laid-Open Patent Application 3,623,939 which corresponds to U.S. 
Pat. No. 4,750,436, discloses a method and an apparatus operating 
according to the method for processing contaminated soils and similar 
materials by removing, decontaminating and replenishing the removal 
location with purified soil, wherein the removed material, possibly after 
pre-processing, is dried and then thermally treated at a high temperature. 
In a mill through which hot gases flow, the drying is combined with 
comminution of the material to a grain size range from 0 to 10 mm in such 
a manner that a significant portion of the contaminants are transferred 
into the gas phase. After drying, the material together with the gas phase 
is treated in the form of a solids/gas dispersion in a burner-equipped 
heating and decontamination line at a temperature in a range between 
800.degree. C. and 1200.degree. C. and is subsequently separated in a 
cyclone separator stage from the gas phase containing all of the noxious 
substances, with the gas phase being included in the circuit with the 
burner to completely decompose the noxious substances. A partial stream of 
the combustion gases is fed to the combined drying and comminution stage, 
while the purified solids are discharged after cooling. The cooling air is 
fed to the burner as preheated primary air. 
The drawbacks of the prior art discussed are essentially that the circuitry 
of the system components only permits a combustion temperature, with 
respect to the solids, of a maximum of 800.degree. C. Periods of dwell of 
only two seconds are realized for the solids in the decontamination line. 
This relatively short passage time is only sufficient, however, to 
decontaminate a grain size range of &lt;3 mm at a temperature of 800.degree. 
C. so that the easily volatile components can be separated from the solid. 
For grain sizes above 3 mm, it may even happen that sufficiently thorough 
heating cannot be realized in this short period of passage, even to remove 
the easily volatilized components from the solid. Aromatic hydrocarbons, 
aliphatic hydrocarbons, cyanides, easily volatilized heavy metals, PCB's 
and chlorinated hydrocarbon cannot be removed during this short passage 
time and at these temperatures. 
Another drawback is that the hot gas circulation between decontamination 
and burner is a stress for the fan employed. Moreover, the large quantity 
of dust in the separated gases also constitutes a strain on the burner, 
with the resulting percentage of filter dust to be disposed of making the 
process more expensive. 
Based on German Laid-Open Patent Application 3,623,939, it is the object of 
the invention to provide an improved method, as well as apparatus, for 
processing contaminated soils and similar materials particularly from coke 
making locations, by removing, decontaminating and possibly replenishing 
the removal location with purified soil in such a way that the removed sol 
is dried and comminuted in a mill through which hot gases flow, the 
solids/gas mixture is introduced into a cyclone separator and is there 
separated into gaseous and solid components; the solids are thermally 
treated at a high temperature, are subsequently cooled and possibly 
employed to replenish the removal location; and at least part of the 
exhaust gases are discharged to the atmosphere after having undergone a 
cooling and filtering process, in that the flow path is decoupled without 
the gas being circulated between the decontamination line and the 
combustion chamber. In order to increase the flexibility, that is, the 
degrees of freedom of the combustion system so that substances, such as 
easily volatized heavy metals, PCB's, CHC and the like can be destroyed 
thermally, it is desirable to modify the combustion system. Also the 
atmosphere in the entire system should be as dust-free as possible to thus 
prevent the formation of a molten phase and clogging in the combustion 
chamber. 
SUMMARY OF THE INVENTION 
This is accomplished with respect to the process in that the solid 
components are introduced, subsequent to the decontamination, into a dwell 
zone configured as a passage region and are there treated further at 
temperatures essentially corresponding to the decontamination temperature 
to then be introduced into the region of the cooling line; and the exhaust 
gases from the drying mill and/or the decontamination are again subjected 
to thermal afterburning. 
Modifications of the process according to the invention includes, in 
dependence on the degree of contamination, passing the solid components 
through the well zone in a period of time between 5 seconds and 10 
minutes. 
The method may include charging the mill with hot gas of at most 
700.degree. C. and heating the fraction, in dependence on its initial 
moisture content, to about 100.degree. C. to 300.degree. C.; heating the 
solid components in the region of the decontamination line to about 
800.degree. C. to 1100.degree. C. in dependence on the degree of 
contamination to decontaminate same; and maintaining the temperature of 
the decontamination line in the region of the dwell zone essentially 
without further influx of external heat. 
The method may include clearing the exhaust gases from the drying mill 
and/or the decontamination line of dust at least in part and subjecting 
same to thermal afterburing; and conducting the flue gases from the 
thermal afterburning process through a gas cooler and introducing same 
into the filter device where the filter dust, on the one hand, and the 
dust from the preliminary dust removal, on the other hand, are returned at 
least in part into the region of the decontamination line. 
The method may include mixing the part of the exhaust gases from the drying 
mill from which the dust was not removed with part of the flue gases from 
the thermal afterburning process and making same available to the mill as 
heating gas. 
The method may include conducting the exhaust gases from the drying mill, 
as well as the flue gases from the thermal afterburning process, through a 
heat exchanger; conducting the heated exhaust gases from the drying mill 
into the region of the decontamination line and bringing same there to the 
decontamination temperature; and introducing the flue gases, after passing 
same through the heat exchanger, into a flue gas dust removal system and, 
in dependence on the degree of contamination of the materials to be 
purified, feeding same to a flue gas sulfur removing system and/or a flue 
gas nitrogen removing system. 
The method may include introducing the flue gases from the thermal 
afterburning process, subsequently to passing same through the heat 
exchanger, into the filter device, with the filter dust being returned to 
the region of the decontamination line. 
The method may include removing the dust content of the gases separated by 
the cyclone separator subsequent to their decontamination and subjecting 
same to thermal afterburning, with the dust being charged downstream of 
the dwell zone. 
The method may include effecting the energy supply of the drying mill by 
means of the heat discharged from the cooling device, with at least one 
further hot gas generator being added in dependence on the moisture 
content of the materials to be decontaminated. 
The method may include utilizing energetically the thermal energy from the 
thermal afterburning process, particularly to generate steam, to generate 
current or for remote heating purposes. 
Further, the method may include making available the energy discharged by 
the heat exchanger as primary or secondary energy, respectively, for the 
burner or burners and/or to the mill. 
The method features permit the problem-free thermal treatment of solid 
which are contaminated with organic contaminants, particularly aliphatic 
and aromatic hydrocarbons, polycyclic aromatic hydrocarbons, phenol 
compounds and derivatives cyanide compounds, and halogen-organic compounds 
as well as easily volatilized heavy metal compounds and similar wastes, 
since the incorporation of a dwell zone permits the period of dwell to be 
extended into the range of minutes. The setting of the period of dwell is 
here regulated as a function of the degree of contamination of the soils 
and waste materials. In contrast to the prior art, circulating gas between 
decontamination and burner is eliminated due to the change in the gas 
flow. The gases supplied to the burner are first subjected to a dust 
removal process. The resulting filter dust is returned at least in part 
into the decontamination region. 
Compared to the rotary kiln processes, the method according to the 
invention achieves a reduction in the consumption of energy, lower 
investment costs as well as a more compact structure. Compared to German 
Laid-Open Patent application 3,623,939, a better purification effect is 
realized, on the one hand, for the solids, as well as the exhaust gases, 
and, on the other hand, there is less susceptibility to malfunction. 
The apparatus according to the invention for processing contaminated soils 
and similar materials by removing, decontaminating and possibly 
replenishing the removal location with purified soil is characterized by a 
dwell zone configured as a flowthrough device for the decontaminated 
materials provided downstream of the decontamination line, as well as a 
device for thermally afterburning the exhaust gases from the drying mill 
and/or decontamination line. 
The invention thus provides an apparatus for processing contaminated solid 
and similar materials by means of removal, decontamination and possibly 
replenishment of the removal location with purified soil, the apparatus 
including a drying mill, a device for thermally treating the material, at 
least one cyclone separator for separating the solids from the gas phase 
and a heating device, a cooling and dust removal device for the exhaust 
gases as well as a cooling device for the purified materials, 
characterized by a dwell zone configured as a flow-through device for the 
decontaminated materials, said device being provided subsequent to the 
decontamination line as well as a device for thermally afterburning the 
exhaust gases from the drying mill and/or the decontamination. 
The apparatus may have the dwell zone formed by a multi-stage cyclone 
arrangement. The dwell zone may be formed by a turbulence layer/fluidized 
bed. 
The apparatus may include a device for pneumatically sucking off the fine 
particles and the exhaust gases in the entrance region of the turbulence 
layer/fluidized bed. The device for thermally afterburning may be composed 
of a burner followed, if required, by a gas cooler, with the burner being 
preceded by a dust removal device for the gases from the drying mill and 
the decontamination, respectively. 
The apparatus may have the cyclone separator followed by a heat exchanger 
through which can be conducted, on the one hand, the exhaust gases from 
the drying mill and, o the other hand, the flue gases from the thermal 
afterburning process. A second fuel/air intake device may be provided in 
the region of the decontamination line. The system blower may be connected 
to the cold side downstream of the heat exchanger. Finally, a jet pump may 
be provided upstream of the device for thermally afterburning. 
By configuring the dwell zone as a multi-stage cyclone apparatus, periods 
of dwell of about 1 minute can be realized for the solids. A turbulence 
layer is able to realize solids dwell periods of several minutes. The 
system is preferably laid out in such a way that the finer particles are 
discharged pneumatically and the coarser particles--the only ones that 
require additional dwell time--remain in the fluidized bed. 
The mill is charged with a hot gas temperature of no more than 700.degree. 
C., with the material to be purified having a temperature, depending on 
its initial moisture content at the mill outlet, of about 100.degree. C. 
to 300.degree. C. The solids/gas dispersion is fed to a cyclone separator 
which separates the solids from the gaseous components. The solids mixture 
charged into the decontamination line is heated to 800.degree. C. to 
1100.degree. C. and is thereafter again separated from the gaseous 
components in a further cyclone separator. The partially decontaminated 
solids are introduced into the dwell zone according to the invention where 
they are moved as a function of the degree of contamination for a period 
between 5 seconds and 10 minutes so that they will not bake on. As already 
mentioned, the dwell zone is composed of a multi-stage cyclone apparatus 
or a turbulence layer/fluidized bed. Both types have in common a heat 
insulated shield against the environment so as to avoid a drop in 
temperature. After passing through the dwell zone, the solids are 
introduced in the usual manner into the cooling device and are there 
cooled to a temperature between 100.degree. C. and 150.degree. C. The 
purified soil may be stored intermediately or incorporated directly in, 
the area from which it was removed. The gas may be conducted according to 
the variations defined in the patent claims which should be selected in 
dependence on the materials to be decontaminated and from a cost aspect. 
If heat exchangers are employed, the energy obtained here may be utilized, 
for example, to generate steam, to generate current or for remote heating 
purposes. There exists the additional possibility of making this energy 
available to the burner or burners and/or to the mill to thus improve the 
heat balance of the entire system. 
In dependence on the degree of contamination of the material to be 
purified, it may be appropriate to take measures on the exhaust gas side 
to remove the nitrogen from the flue gases and/or to desulfurize the flue 
gases. These measures may be such, for example, that after the dust is 
removed from the flue gases, an activated carbon filter is connected 
downstream of the flue gas dust removal station to separate heavy metal 
compounds. The filter dusts obtained during the removal of dust from the 
flue gases can be introduced completely into the decontamination system. 
Removal of dust at this location thus becomes superfluous.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The system according to FIG. 1 is essentially constructed of the following 
apparatus components: 
The soils to be decontaminated are pre-broken and put into piles 1 (rough 
comminution). By way of loading units (excavators, conveyor belts or the 
like) which are not shown in detail the coarse fractions are introduced 
into a mill 2 through which flow hot gases and which is connected by way 
of a pipeline 3 with a cyclone separator 4. From there, the solids travel 
through a pipeline 5 into a heating and decontamination line 6 which may 
be configured as a combustion chute or the like. If required, further fuel 
and air may be supplied at 7 in the region of decontamination line 6. 
During the flameless combustion taking place there, the released energy is 
spontaneously converted to heating energy, that is, overheating of the gas 
phase is avoided. The heating and decontamination line 6 is followed by a 
further cyclone separator 8 in which the purified solids are separated 
from the gas phase. The solids, that is, the partially decontaminated 
soil, is then transferred to a dwell zone 9 which in the flow chart of 
FIG. 1 is shown merely as a black box. Various types of dwell zones can be 
found in FIGS. 4 and 5. For the present purpose, dwell zone 9 is 
configured as a flow-through device so that the material is prevented from 
baking on. Due to the extended period of dwell of the solids, which may be 
increased into the range of minutes in dependence on the degree of 
contamination, it can be ensured, with the appropriate temperature, that 
even contaminants posing particular problems, such as easily volatilized 
heavy metal compounds, PCB's and CHC, can be separated from the materials. 
After passage through dwell zone 9, the solids flow through a further 
pipeline 10 into a three-stage cooling apparatus. The three cooling stages 
are formed by subsequently connected cyclone separator stages 11. Through 
discharge devices that are not shown in detail, the stream of the solids 
leaves the cooling system at a temperature which lies between 100.degree. 
C. and 150.degree. C. 
A first contaminated gas phase is present in cyclone separator 4. By means 
of a fan 12, at least some of the gas phase from cyclone separator 4 is 
made available through a pipeline 13 as heating gas for mill 2. The 
remaining gas phase of cyclone separators 4 and 8 is fed by means of a fan 
14 and a pipeline 15 to a subsequently connected burner 16 and is there 
thermally afterburned, with complete decomposition of all contaminants 
being realized here. Through a pipeline 17, the flue gases from the 
thermal afterburning phase are made available as heating gases to mill 2, 
with these gases being mixed with the colder exhaust gases of the gas 
phase that had been extracted by fan 12 from cyclone separator 4. Mixing 
of the gas streams from pipelines 13 and 17 results in an approximate 
entrance temperature at mill 2 of at most 700.degree. C. Exhaust gases 
from cooling stage 11 are made available to burner 16 as secondary air by 
way of a conduit 18 and a blower 19. The gas phase not made available to 
burner 16 is conducted through a conduit 20 to a heat exchanger 21. Here, 
the exhaust gases are cooled and are then discharged to the atmosphere by 
way of a filter 22 and a fan 23. By way of heat exchanger 21 energy can be 
recovered in a manner not shown in detail here in that the thermal energy 
from afterburner line 17 is used as energy through a further pipeline 24 
(e.g., heat reduction kettle, steam generation, generation of electrical 
current and/or remotely supplied heat). The heated air from heat exchanger 
21 may also be utilized as combustion air for burner 16 and as energy to 
be introduced into mill 2. Another portion of the gas phase from cooling 
stage 11, which is not made available to burner 16, is conducted as 
pre-heated air through a conduit 25 and a fan 26 into the region of the 
additional fuel supply 7. At 27, the cooling air required for multistage 
cooling device 11 is sucked in from the atmosphere and, as already 
mentioned, is returned at least in part into the region 7 of 
decontamination line 6 for the purpose of recovering the energy after 
passage through the cooling device. 
The described system operates as follows: 
In mill 2, the contaminated soils are simultaneously dried by a stream of 
hot gas of no more than 700.degree. C. and are comminuted to a grain size 
range between 0 and 3 mm. Due to the comminution of the solids to grain 
sizes of &lt;3 mm while simultaneously charging them with heat, it is ensured 
that the solids are heated through to the fullest extent. In dependence on 
the initial moisture content, the starting temperature of the solids at 
the outlet of mill 2 is about 100.degree. C. to 300.degree. C. This 
heating already converts a considerable amount of the contaminants to the 
gas phase. The comminuted material is discharged from mill 2 as a 
solids/gas dispersion and is fed by action of fan 12 to cyclone separator 
4. Part of the gas separated there returns as heating gas to mill 2 and 
the other part is fed by means of jet pump 14 to burner 16 to provide for 
thermal afterburning. The solids that are completely dried there are 
transferred to the heating and decontamination line 6 and are there 
thermally treated at about 800.degree. C. to 1100.degree. C., a 
temperature that is sufficient to produce a first vaporization during a 
passage time of about 2 to 3 seconds, which enables the easily volatilized 
components such as, for example, solvents, BTX aromatics or the like, to 
be separated from the solids. The separation of the prepurified solids is 
effected in cyclone separator stage 8, where the gas phase is introduced 
into burner 16 by way of jet pump 14. The pre-purified solids now reach 
dwell zone 9 where they are moved, in dependence on the degree of 
contamination, in the minute range. Dwell zone 9 is provided with a 
heat-tight shield against the environment to avoid a drop in temperature. 
Due to the increase in the period of dwell at approximately the same 
temperature as in decontamination line 6 (800.degree. C. to 1100.degree. 
C.), contaminants that pose problems, such as, inter alia, easily 
volatilized heavy metals, PCB's and polycyclic aromatic hydrocarbons can 
also be evaporated out of the solids. From dwell zone 9, the now 
completely purified soil is moved by way of a cooling stage 11 and a 
discharging device not shown in detail, for example, to a stockpile 28. 
FIG. 2 shows another alternative of the apparatus according to the 
invention. This system is essentially composed of the following 
components: 
The already pre-broken material is fed, as already described in FIG. 1, 
into a mill 29 through which flow hot gases, where it is dried, on the one 
hand, and comminuted, on the other hand, to fractions of &lt;3 mm. By way of 
a pipeline 30, the solids-gas dispersion is introduced into a cyclone 
separator 31 from where the solids are brought into the region of 
decontamination line 32. There, a burner 33 is provided which generates 
the appropriate temperature level (800.degree. C. to 1100.degree. C.) in 
decontamination line 32. Decontamination line 32 is preferably configured 
as a combustion chute. As already mentioned in FIG. 1, during the passage 
of the material, a first decontamination takes place here. The prepurified 
solids then enter a cyclone separator 34 where the solids are again 
separated from the gas phase. The purified solids are now transferred into 
a dwell zone 35 again shown as a black box which is described in greater 
detail in FIGS. 4 and 5. Here again the temperatures generated in 
decontamination line 32 are essentially maintained so that the dwell 
period can be varied from seconds to minutes as a function of the degree 
of contamination of the material. The thus purified solids now enter a 
multi-stage cooling device 36, with the cooling air being introduced into 
the system from the atmosphere at location 37. The purified solids are 
transferred by way of discharging devices which are not shown in detail to 
a stockpile 38. The exhaust gases and residues are conducted as follows: 
The solids/gas mixture coming from the drying mill 29 is sucked in by way 
of a fan 39 and is at least in part made available again as heating gas to 
mill 29 through a pipeline 40. The gas portion not made available to mill 
29 travels through a dust removal device 41 to a burner 42 where a thermal 
afterburning process takes place. The same applies for the gas phase 
coming from cyclone separator 34 which is also introduced through a 
conduit 43 into dust removal device 41 and is afterburned in burner 42. 
The dust generated there, which may under certain circumstances be highly 
contaminated, is returned at least in part into decontamination line 32 
through a pipeline 44. The flue gases resulting from the thermal 
afterburning, which have an exit temperature of approximately 1200.degree. 
C. are separated. By way of a pipeline 45, a part of the flue gases is 
mixed with the exhaust gases from cyclone separator 31 and made available 
as heating gas to mill 29. The hot gas temperature present at the mill 
inlet amounts to a maximum of 700.degree. C. The part of the flue gas not 
made available to mill 29 travels through a gas cooler 46, a further fan 
47, as well as conduit 48 into a filter 49 from where the purified exhaust 
gases are introduced by way of a fan 50 and a filter for heavy metals 51 
to chimney 52. The dusts from filter 49 are charged through a conduit 53 
directly into decontamination line 32. Cooling stage 36 also has an 
associated dust removal device 54, with the still dust-shaped components 
present here being turned over directly to stockpile 38 since they no 
longer constitute contamination. The gaseous components leaving cooling 
stage 36 are returned as secondary air to burner 42 by way of a conduit 55 
and a fan 56. 
FIG. 3 shows a further alternative. As already described in the preceding 
figures, the prepared material is again introduced into a mill 57 through 
which flows hot gas and where a fraction of &lt;3 mm is produced. The thus 
pre dried mixture is fed into a cyclone separator 58 that separates the 
solid phase from the gas phase. The solids travel through a conduit 59 
into the region of decontamination line 60 which is heated by means of a 
burner 61 (800.degree. C. to 1100.degree. C.). In this decontamination 
line, the easily volatilized contaminants are evaporated out due to the 
relatively short period of dwell (about 2 to 3 seconds). The mixture then 
enters a cyclone separator 62 where a further separation of the solid 
phase from the gas phase takes place. As already discussed in FIGS. 1 and 
2, the solid phase travels into a dwell zone 63 shown here again only as a 
black box, with it being possible again to remove at essentially the same 
temperature as in decontamination line 60 the problem causing contaminants 
in dependence on the degree of contamination. Subsequent to dwell zone 63, 
the now purified solids are introduced into a multi-stage cooling device 
64 where they are cooled from an entrance temperature of about 800.degree. 
C. to 1100.degree. C. to about 100.degree. C. to 150.degree. C. The 
cooling air required for this purpose is taken from the atmosphere at 
location 65 and, after passing through cooling stage 64 is made available 
by way of a fan 66 as heating gas for mill 57. If the energy discharged 
from cooling stage 64 should not be sufficient to ensure complete drying 
in mill 57, a further hot gas generator 67 may be included in hot gas 
conduit 68. The gas phase separated in cyclone 58 is fed by way of a 
filter 69 and a fan 70 to a heat exchanger 71. Filter dusts developing in 
filter 69 are charged through a conduit 72 into conduit 59 and are thus 
introduced directly into decontamination line 60. The gas phase separated 
in cyclone 62 is also fed through a further conduit 73 into a dust removal 
device 74 and is then thermally afterburned in a burner 75. Dusts 
appearing here, which no longer contain any contaminants, are charged 
through a conduit 76 directly into the region of cooler 64. The exhaust 
gases coming from the drying mill, which had been introduced into heat 
exchanger 71 through filter 69, are heated by the flue gases from the 
thermal afterburning process, which have an approximate temperature level 
of 1200.degree. C., in a conduit 77 and, after passing through heat 
exchanger 71, are made available as primary air by way of a further 
conduit 78 to burner 61. The exhaust gases from the thermal afterburning 
process are fed by way of an exhaust gas conduit 79 to a filter 80, with 
the exhaust gases 96 being cooled by means of air and/or water before 
entering into the filter. The filter dust generated in filter 80 is 
returned through a conduit 81 into the region of decontamination line 60. 
The thus purified exhaust gas is conducted by a fan 82 and possibly also 
through a filter for heavy metals 83 and then into the chimney 84. 
FIGS. 4 and 5 show dwell zones (as they are discussed in FIGS. 1 to 3 as 
black boxes 9, 35, 63). 
FIG. 4 shows a dwell zone configured as a multistage cyclone device 85, 86. 
The solids discharged from cyclones 8, 34, 62 travel through pipelines 87, 
88 into the first cyclone 85, where a first separation occurs between the 
solid and the gas phase. The gas phase is conducted through a conduit 89 
directly into the entrance region 90 of the subsequently connected second 
cyclone 86. The solids are conducted through a further conduit 91 also 
into region 90 of the second cyclone stage 86, with a further separation 
of the phases taking place there. In region 92, the thus purified solids 
are returned to the region of the respective cooling stages 11, 36, 64. 
FIG. 5 shows an alternative to FIG. 4. The dwell zone shown here is 
configured as a turbulence layer/fluidized bed 93. The solids mixture 
discharged from cyclones 8, 34, 62 reaches the entrance region of the 
fluidized bed where fine particles sucked off, i.e., are pneumatically 
separated, at orifice 94. Since only the coarse particles require an 
additional period of dwell in fluidized bed 93, they are guided around 
appropriate baffles 95. A turbulence layer is able to realize periods of 
dwell of several minutes for the solids. After passing through fluidized 
bed 93, the solid streams freed of all contaminants enter into the region 
of the respective cooling stage 11, 36, 64.