U.S. patent application number 10/396516 was filed with the patent office on 2003-10-23 for method of treating heavy metal and/or organic compound.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho(Kobe Steel, Ltd.). Invention is credited to Harada, Takao, Michishita, Haruyasu, Sugitatsu, Hiroshi, Tamazawa, Hiroshi, Tanaka, Hidetoshi.
Application Number | 20030196517 10/396516 |
Document ID | / |
Family ID | 28677652 |
Filed Date | 2003-10-23 |
United States Patent
Application |
20030196517 |
Kind Code |
A1 |
Harada, Takao ; et
al. |
October 23, 2003 |
Method of treating heavy metal and/or organic compound
Abstract
The present invention provides a method of purifying polluted
soil and/or burned ash containing heavy metals and/or organic
compounds with a higher throughput than a conventional method. In
this method, polluted soil and/or burned ash is dried by, for
example, a rotary dryer so that the moisture content is 5% by mass
or less, preferably 3% by mass or less, and then large lumps having
a particle diameter of 10 mm or more are removed by a vibrating
screen. Only an undersize portion is formed into a briquette having
a volume of about 6 cm.sup.3 by a press molding machine. The
briquette is charged in a rotary hearth furnace together with the
large lumps, and heated in the furnace to remove or detoxify the
heavy metals and organic compounds by evaporation with high
efficiency.
Inventors: |
Harada, Takao; (Kobe-shi,
JP) ; Michishita, Haruyasu; (Kobe-shi, JP) ;
Tanaka, Hidetoshi; (Kobe-shi, JP) ; Tamazawa,
Hiroshi; (Kobe-shi, JP) ; Sugitatsu, Hiroshi;
(Kobe-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko
Sho(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
28677652 |
Appl. No.: |
10/396516 |
Filed: |
March 26, 2003 |
Current U.S.
Class: |
75/401 ;
75/392 |
Current CPC
Class: |
B09B 3/40 20220101; F23G
2900/50209 20130101; F23G 2201/50 20130101; F23G 2201/701 20130101;
F23G 2209/30 20130101; F23G 2201/602 20130101; B09B 3/00 20130101;
B09C 1/06 20130101; F23G 7/14 20130101 |
Class at
Publication: |
75/401 ;
75/392 |
International
Class: |
C22B 011/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 2002 |
JP |
2002-114884 |
Aug 19, 2002 |
JP |
2002-238371 |
Claims
What is claimed is:
1. A method of treating a heavy metal and/or organic compound
comprising a heating step of heating a material containing a heavy
metal and/or organic compound, in a radiant rapid heating furnace
to purify the material.
2. A method of treating a heavy metal and/or organic compound
comprising an agglomeration step of agglomerating at least a
portion of a material containing a heavy metal and/or organic
compound, to form agglomerates, and a heating step of heating the
agglomerates and the unagglomerated residue of the material in a
radiant rapid heating furnace to purify the material.
3. A method of treating a heavy metal and/or organic compound
according to claim 2, further comprising a screening step of
screening the material to separate the material into an oversize
portion and an undersize portion before the agglomeration step, the
undersize portion being supplied to the agglomeration step, and the
oversize portion being supplied to the heating step without the
agglomeration step.
4. A method of treating a heavy metal and/or organic compound
comprising a mixing step of mixing a carbonaceous reducing agent
and a metal oxide-containing material with a material containing a
heavy metal and/or organic compound, and a heating step of heating
the resultant mixture in a heating furnace to purify the
material.
5. A method of treating a heavy metal and/or organic compound
according to claim 2, wherein the material is heated at an ambient
temperature of 700.degree. C. or more at least in the second half
of the heating step.
6. A method of treating a heavy metal and/or organic compound
according to claim 5, wherein the material is heated at an ambient
temperature of 700.degree. C. or less in the first half of the
heating step.
7. A method of treating a heavy metal and/or organic compound
according to claim 2, wherein the radiant rapid heating furnace is
a rotary hearth furnace.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of purifying soil
and/or burned ash containing heavy metals and/or organic compounds.
Particularly, the present invention relates to a method of removing
or detoxifying heavy metals and/or organic compounds contained in
soil and/or burned ash.
[0003] 2. Description of the Related Art
[0004] In recent years, various heat treatment methods have been
proposed as methods of removing or detoxifying pollutants contained
in soil and/or burned ash. Herein, the pollutants include heavy
metals such as cadmium (Cd), lead (Pb), hexavalent chromium
(Cr.sup.6+), arsenic (As), mercury (Hg), and the like,
organochlorine compounds such as dioxines, and the like, other
organic pollutants (volatile organic compounds VOC and persistent
organic pollutants POPs), and the like.
[0005] As a heat treatment method for purifying soil polluted with
an arsenic compound, a method is used, in which a mixture of the
soil and a-reducing agent is heated, or the soil is heated in the
presence of a reducing gas to reduce and evaporate the arsenic
compound from the soil at a low temperature. In this method, carbon
or ammonium sulfate is used as the reducing agent, and carbon
dioxide, carbon monoxide, ammonia, or methane gas is used as the
reducing gas.
[0006] However, when only the reducing agent such as carbon is
added, the amount of the CO gas generated is limited, and thus
reduction of the arsenic compound is not sufficiently promoted,
thereby quite possibly failing to achieve a high rate of
removal.
[0007] A rotary kiln is used as a waste reclamation furnace. For
example, residual soil is molded by an extrusion granulator, burned
by an outside heating type or inside heating type rotary kiln, and
used as burned modified soil.
[0008] However, in the use of the outside heating type rotary kiln,
heat is mainly transferred by conductive heat transfer through the
furnace wall, causing difficulties in rapid heating. On the other
hand, in the use of the inside heating type rotary kiln, heat is
mainly transferred by radiant heat transfer, but a heat transfer
area is small as compared with the ratio of the occupied area of a
material in the furnace, thereby causing difficulties in rapid
heating. Therefore, both the inside and outside heating types have
a fault that heat treatment requires a long time, and the apparatus
used is greater than enough as compared with its throughput. Also,
in a rotary kiln, a gas discharge port is limited to one position,
and thus the harmful materials removed by evaporation cannot be
separately recovered according to the types of the materials.
[0009] Another method is disclosed, in which a reducing agent
containing a formic acid and/or oxalic acid salt is added to a
material to be treated, and the resultant mixture is heated to
reduce heavy metals or organochlorine compounds, to remove or
detoxify the heavy metals or organochlorine compounds. Besides the
rotary kiln, a fluidized bed type heating furnace can be used.
[0010] However, this method requires a relatively expensive formic
acid and/or oxalic acid salt to increase the treatment cost,
causing a problem of practicability. In the use of the fluidized
bed type heating furnace, heat is mainly transferred by convective
heat transfer between a heated gas and a material to be treated,
and a heat transfer area is extremely larger than the rotary kiln
to permit the realization of rapid uniform heating. However, when
the material to be treated contains a lump material, the material
must be previously ground because the material to be treated, which
can be used in the fluidized bed, is limited to a powdery material.
Also, a large amount of dust contained in the exhaust gas must be
treated, a countermeasure against abrasion of the furnace wall is
required, and the temperature cannot be set to high because
adhesion easily occurs. Therefore, the fluidized bed type has low
handleability and thus has the problem of increasing the treatment
cost.
SUMMARY OF THE INVENTION
[0011] The present invention has been achieved in consideration of
the above-described situation, and an object of the present
invention is to provide a method of purifying soil and/or burned
ash with a higher throughput than a conventional method, the method
comprising heat-treating soil or burned ash containing heavy metals
and/or organic compounds.
[0012] In a first aspect of the present invention, a method of
treating heavy metals and/or organic compounds comprises a heating
step of heating a material containing heavy metals and/or organic
compounds, in a radiant rapid heating furnace to purify the
material.
[0013] In a second aspect of the present invention, a method of
treating heavy metals and/or organic compounds comprises an
agglomeration step of agglomerating at least a portion of a
material to be treated, which contains heavy metals and/or organic
compounds, to form agglomerates, and a heating step of heating the
agglomerates and the unagglomerated residue of the material in a
radiant rapid heating furnace to purify the material.
[0014] In a third aspect of the present invention, a method of
treating heavy metals and/or organic compounds further comprises a
screening step of screening a material to be treated to separate
the material into an oversize portion and an undersize portion
before an agglomeration step, the undersize portion being supplied
to the agglomeration step, and the oversize portion being supplied
to a heating step without the agglomeration step.
[0015] In a fourth aspect of the present invention, a method of
treating heavy metals and/or organic compounds comprises a mixing
step of mixing a carbonaceous reducing agent and an metal
oxide-containing material with a material to be treated, which
contains heavy metals and/or organic compounds, and a heating step
of heating the resultant mixture in a heating furnace to purify the
material.
[0016] In a fifth aspect of the present invention, a method of
treating heavy metals and/or organic compounds comprises heating a
material to be treated at an ambient temperature of 700.degree. C.
or more at least in the second half of a heating step.
[0017] In a sixth aspect of the present invention, a method of
treating heavy metals and/or organic compounds comprises heating a
material to be treated at an ambient temperature of 700.degree. C.
or less in the first half of a heating step.
[0018] In a seventh aspect of the present invention, in a method of
treating the heavy metals and/or organic compounds, a radiant rapid
heating furnace is a rotary hearth furnace.
[0019] The radiant rapid heating furnace comprises a hearth (or
grate) continuously or intermittently horizontally moved, and a
heating furnace body which covers an upper portion of the hearth
(or grate). In the radiant rapid heating furnace, soil and/or
burned ash as the material is placed on the hearth (or grate), and
mainly radiantly heated from above by passing through the heating
furnace body together with the hearth. Another furnace may be used,
in which the soil and/or burned ash placed on the hearth is moved
by a scraper without movement of the hearth.
[0020] Examples of the radiant rapid heating furnace include a
moving-bed furnace, a tunnel kiln, and the like, and examples of
the moving-bed furnace include a circular rotary hearth furnace,
and a linear or grate-type furnace. A multi-hearth furnace is also
included in the radiant rapid heating furnace.
[0021] A heat transfer mechanism of the radiant rapid heating
furnace includes radiant heat transfer from a space (combustion gas
and the furnace wall) above the material to the surface of the
material, and conductive heat transfer in the material from the
upper surface of the material to the lower surface thereof. The
radiant heat transfer rate is proportional to a difference between
the fourth powers of the absolute temperatures of the heated
material and the material to be heated, while the conductive heat
transfer rate is proportional to a temperature gradient from the
upper surface and the lower surface of the material. Therefore,
conductive heat transfer is generally a rate-determining step.
[0022] By using the radiant rapid heating furnace as the heating
furnace, the material can be thinly placed on the hearth (or
grate). The thickness of the material placed on the hearth (or
grate) depends upon the type of the charger used, but the thickness
can be generally minimized to about 10 mm. Therefore, the area (the
upper surface area of the material) of radiant heat transfer from
the heated material (combustion gas and the furnace wall) in the
upper space to the material can be increased to significantly
increase the amount of radiant heat transfer.
[0023] Furthermore, the layer of the material to be heated is thin,
and thus heat transferred to the upper surface of the material by
radiant heat transfer is transferred to the lower surface of the
material layer to be heated by conductive heat transfer in the
rate-determining step within a short time, thereby achieving rapid
uniform heating of the material. Therefore, the whole material is
uniformly rapidly heated to remove or detoxify the heavy metals or
organic compounds adhering to the surface of the material (soil
or/or burned ash) within a short time.
[0024] The inside of the radiant rapid heating furnace is divided
into zones by the furnace walls so that the temperature and
atmosphere of each of the zones can easily be-controlled.
Consequently, the heavy metals can easily be reduced and evaporated
by suppressing an oxidizing atmosphere in the furnace.
[0025] On the other hand, in a heating system using a conventional
inside heating type rotary kiln, radiant heat transfer from the
heated material (combustion gas and the furnace wall) in the upper
space to the surface of the material is mainly performed. However,
in heating by the rotary kiln, the ratio (occupancy ratio) of a
sectional area of the material to the total sectional area of the
kiln is 5 to 17% in an ordinary operation. For example, in the kiln
having an inner diameter of 1 m, the maximum thickness of the
material in the kiln reaches 95 to 230 mm (average thickness of
about 50 to 150 mm). Therefore, the heat transfer area (the upper
surface area of the material) relative to the amount of the
material staying in the kiln, is extremely smaller than that in the
radiant rapid heating furnace. Although the rate-determining step
of conductive heating is absent from the radiant rapid heating
furnace because the material is agitated by rolling the kiln, the
heating rate is significantly lower than that in the radiant rapid
heating furnace, and thus a long time is required for heating.
[0026] Preferably, at least a portion of the soil and/or burned ash
containing the heavy metals and/or organic compounds is
agglomerated to form agglomerates, and the agglomerates and the
residue of the soil and/or burned ash are placed on the hearth (or
grate) of the radiant rapid heating furnace. Agglomeration of at
least a portion of the soil and/or burned ash increases the packing
density of particles of the soil and/or burned ash in the
agglomerates, and thus the packing density of the whole layer of
the material is also increased as compared with a case in which the
whole of the soil and/or burned ash is placed on the hearth (or
grate) without agglomeration, thereby further increasing the
conductive heat transfer rate. Furthermore, handling after heating,
such as transport or the like, is facilitated, and the agglomerates
having high strength are preferably used as a roadbed and an
aggregate.
[0027] Furthermore, the soil and/or burned ash containing the heavy
metals and/or organic compounds frequently contains a large amount
of moisture, and is thus difficult to handle without any treatment.
Therefore, at least a portion of the soil and/or burned ash is
preferably dried before the agglomeration.
[0028] Also, the soil and/or burned ash containing the heavy metals
and/or organic compounds frequently contains large lumps, and thus
the large lumps are preferably previously removed by a screen
having a predetermined screen mesh (for example, 10 mm) for
protecting an agglomerator used for agglomeration. After the large
lumps are removed, only un undersize portion is agglomerated, and
the resultant agglomerates and the oversize portion containing the
large lumps are preferably charged in the radiant rapid heating
furnace. In the large lumps, most of pollutants such as the heavy
metals and/or organic compounds adhere only to the surfaces of the
lumps, and thus the pollutants can easily be removed or detoxified
by heating without grinding, to cause no problem.
[0029] The radiant rapid heating furnace preferably has at least
two exhaust gas discharge ports so that the exhaust gas is
separately discharged from the discharge ports. Therefore, the
exhaust gas containing heavy metals having different evaporation
temperatures can be separately discharged, thereby facilitating a
subsequent treatment of the exhaust gas and reclamation of
recovered materials.
[0030] Furthermore, a carbonaceous reducing agent and a metal
oxide-containing material are preferably mixed with the soil and/or
burned ash containing the heavy metals and/or organic compounds,
and then the resultant mixture is charged in the heating furnace.
Therefore, in heating in the heating furnace, a carbon content (C)
in the carbonaceous reducing agent reacts with the metal oxide
(M.sub.xO.sub.y) to activate the generation of CO gas, as shown by
the following reaction formula (1):
M.sub.xO.sub.y+zC.fwdarw.M.sub.xO.sub.(y-z)+zCO (1)
[0031] As a result, reduction of the heavy metal oxides is promoted
to bring the heavy metals into a volatile state, thereby
accelerating evaporation removal.
[0032] In this method, reduction can be promoted without an
increase in the heating rate, and thus the heating furnace is not
limited to the radiant rapid heating furnace. Thus, the method can
also be applied to a conventional rotary kiln.
[0033] At least in the second half of heating, the ambient
temperature in the heating furnace (heating step) is preferably
700.degree. C. or more. Therefore, As and Hg which evaporate at
relatively low temperatures, and Pb and Cd which evaporate at
higher temperatures, can be evaporated and removed from the
material.
[0034] Furthermore, in the first half of heating, the ambient
temperature in the heating furnace (heating step) is preferably
700.degree. C. or less. Therefore, low-temperature volatile metals
such as As and Hg, and high-temperature volatile metals such as Pb
and Cd can be separately evaporated and removed in the first half
and the second half, respectively, and thus the low-temperature and
high-temperature heavy metals can be separately recovered, thereby
facilitating final treatment and reclamation after recovery.
[0035] Furthermore, a rotary hearth furnace is recommended as the
heating furnace. In this case, the agglomerates can be treated
without pulverization, and the inside of the furnace can easily be
divided, thereby facilitating separated recovery of the volatile
materials removed by evaporation from the treated material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a drawing schematically showing a construction of
an apparatus used in example 1;
[0037] FIG. 2 is a vertical sectional view of a rotary hearth
furnace used in example 1; and
[0038] FIG. 3 is a graph showing the relation between the elapsed
time from sample charging and the CO concentration of an atmosphere
gas for heating each sample containing polluted soil in example
2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0039] An embodiment of the present invention will be described in
detail below with reference to examples.
EXAMPLE 1
[0040] FIG. 1 is a drawing schematically showing a construction of
an apparatus used for a method of treating heavy metals and/or
organic compounds according to an embodiment of the present
invention. In FIG. 1, reference numeral 1 denotes a dryer for
drying polluted soil A.sub.1 as a material to be treated; reference
numeral 2, a screen for screening soil A.sub.2 dried by the dryer
1; reference numeral 3, an agglomerator for agglomerating an
undersize portion A.sub.3 put through the screen 2; and reference
numeral 4, a radiant rapid heating furnace for heating agglomerates
A.sub.5 produced by agglomeration by the agglomerator 3 and an
oversize portion A.sub.4 put through the screen 2. Also, reference
numerals 5a and 5b denote exhaust gas discharge ports provided at
two positions of the radiant rapid heating furnace 4, and reference
numerals 6a and 6b denote dust collectors connected to the exhaust
gas discharge ports 5a and 5b, respectively, through exhaust gas
ducts (not shown).
[0041] As the polluted soil A.sub.1, soil containing pollutants
such as heavy metals such as cadmium (Cd), lead (Pb), hexavalent
chromium (Cr.sup.6+), arsenic (As), mercury (Hg), and the like,
organochlorine compounds such as dioxines, and the like, and other
organic pollutants is treated.
[0042] When the polluted soil A.sub.1 has a high moisture content,
the moisture content is preferably previously decreased to 5% by
mass or less, more preferably 3% by mass or less, by drying with
the dryer 1. This is because handling is facilitated by drying.
Also, screening is also facilitated by drying. Furthermore, drying
before agglomeration has not only the effect of preventing the
occurrence of excessive bursting even when the agglomerates A.sub.5
are charged in the heating furnace 4, but also the effect of
decreasing the time required for drying the agglomerates A.sub.5 in
the heating furnace 4 to improve a throughput, and saving the fuel
required for drying in the heating furnace 4. Of course, the dryer
1 is unnecessary for treatment of soil A.sub.1 having a low
moisture content. The drying step may be omitted according to a
treatment scale, and thus the soil may be dried in the heating
furnace. A treatment without agglomeration has the low probability
of bursting, but drying is preferably performed to an extent which
causes no trouble such as clogging in charging in the heating
furnace. In a treatment without agglomeration, more preferably the
density of the soil A.sub.1 is increased by compression with a
press after charging in the furnace to improve a heat transfer
efficiency, or a groove is, formed in the surface of the soil
A.sub.1 by a scraper or the like to increase a heat receiving area.
The type of the dryer 1 is not limited, and any one of a rotary
dryer type, a fluidized bed dryer type, and the like may be used.
However, in this example, a simple rotary dryer with rich
performance is used (drying step).
[0043] Instead of drying before agglomeration, drying of the
agglomerates A.sub.5 after agglomeration has the effect of
suppressing bursting in the heating furnace 4, improving
productivity and decreasing fuel consumption. However, in this
case, the soil containing a large amount of moisture is difficult
to agglomerate. In addition, while drying can be carried out with a
simple dryer such as a rotary dryer or the like before
agglomeration, a relatively expensive apparatus such as a
moving-bed dryer or the like is required for drying the
agglomerates A.sub.5 in a stationary state after agglomeration so
as to prevent bursting and powdering.
[0044] When the soil A.sub.1 (A.sub.2) contains large lumps, the
large lumps are preferably removed by the screen 2 after drying in
order to protect the agglomerator 3. For example, a vibrating
screen can be used as the screen 2, and the screen mesh is 10 mm in
consideration of the protection of the agglomerator and the
screening efficiency. In a conventional technique, the large lumps
A.sub.4 on the screen 2 are ground, and then mixed with the
undersize portion A.sub.3 or used for other applications. However,
in this example, the large lumps A.sub.4 are charged in the heating
furnace 4 together with the agglomerates A.sub.5 after
agglomeration described below (screening step).
[0045] The undersize portion A.sub.3 from which the large lumps
A.sub.4 are moved by the screen 2 is preferably agglomerated by the
agglomerator 3 to produce the agglomerates A.sub.5 having a
predetermined size. The type of the agglomerator 3 is not limited,
and any one of a press molding machine, an extrusion molding
machine, tumbling granulators such as a dish-shaped granulator and
drum granulator, an agitating granulator, a fluidized bed
granulator, and the like may be used. Particularly, the press
molding machine, the extrusion molding machine, or the tumbling
granulator, which can produce agglomerates with a high density, is
preferably used. Particularly, a press agglomerator capable of
achieving a high density is recommended. Furthermore, a binder and
moisture may be added to the undersize portion A.sub.3 before
agglomeration according to demand. Although the shape and size of
the agglomerates A.sub.5 are not limited, a pillow-shaped briquette
of 39 mm.times.20 mm.times.12 mm (volume of about 6 cm.sup.3) is
formed by compression molding using a press agglomerator, and used
in this example (agglomeration step).
[0046] The agglomerates A.sub.5 agglomerated as described above are
charged in the radiant rapid heating furnace 4 together with the
large lumps A.sub.4 remaining on the screen 2. As the radiant rapid
heating furnace 4, a rotary hearth furnace is used in this example.
FIG. 2 is a vertical sectional view showing the rotary hearth
furnace 4 used in this example. The rotary hearth furnace 4
comprises a doughnut-like hearth 41 which horizontally rotates, a
heating furnace body 42 which covers the upper portion of the
hearth 41, a charger 43 for charging the agglomerates A.sub.5 and
the large lumps A.sub.4 (generically named a "material A.sub.6" to
be heated) onto the hearth 41, and a discharger 44 for discharging
the heated material A.sub.6 (purified soil A.sub.7) after heating
from the hearth 41 to the outside. The material A.sub.6 is charged
onto the hearth 41 by the charger 43 so that the thickness is about
1 to 2 times as large as the thickness (in this example, 12 mm) of
the agglomerates A.sub.5. In this example, the heating furnace body
42 is divided into four zones (I, II, III, and IV zones) along the
movement direction of the hearth 41, and a combustion burner (not
shown) is provided in each of the zones so that the ambient
temperature of each zone can be precisely controlled. In addition,
the ambient temperatures (the temperature near the material
A.sub.6) of the two zones (I and II zones) in the first half are
700.degree. C. or less, and the ambient temperatures (the
temperature near the material A.sub.6) of the two zones (III and IV
zones) in the second half are 700 to 1250.degree. C. Furthermore,
the exhaust gas discharge ports 5a and 5b are provided in the first
zone (I zone) and last zone (IV zone), respectively, so that
exhaust gas from the two zones (I and II zones) in the first half,
and exhaust gas from the two zones (III and IV zones) in the second
half can be separately discharged. During passage through the
heating furnace 4 with rotation of the hearth 41, the material
A.sub.6 is heated to 700.degree. C. or less in the two zones (I and
II zones) in the first half to first remove As and Hg which
evaporate at relatively low temperatures. Then, in the two zones
(III and IV zones) in the second half, the material A.sub.6 is
heated to the maximum temperature of about 1250.degree. C. to
remove Pb and Cd which evaporate at higher temperatures. The
conceivable reason why the heavy metals can easily be evaporated
only by simply heating the material A.sub.6 is that in heating the
material, the organic materials contained in the soil are
decomposed to generate CO gas, and thus the heavy metal oxides are
converted to a volatile state by reduction, as described below with
reference to example 2. In this example, fly ash containing a high
concentration of As can be recovered from the dust collector 6a
connected to the exhaust gas discharge port 5a in the first half,
and fly ash containing a high concentration of Pb can be recovered
from the dust collector 6b connected to the exhaust gas discharge
port 5b in the second half. In a treatment method using a
conventional rotary kiln, the inside of the furnace cannot be
divided into zones from the viewpoint of structure, and thus,
unlike in this example, the temperature cannot be easily precisely
controlled in each zone in the furnace, and exhaust gas cannot be
discharged from a plurality of positions. Furthermore, in the
conventional method, the gas atmosphere in the furnace cannot be
controlled to cause difficulties in evaporating heavy metals by
reduction. After the material A.sub.6 passes through the IV zone in
the heating furnace body 42, the material A.sub.6 is discharged as
the purified soil A.sub.7 to the outside by the discharger 44. In
this example, when the retention time of the material A.sub.6 at an
average ambient temperature of 1200.degree. C. is 12 minutes, the
Pb concentration of the material A.sub.6 of 160 mg/kg before
heating is decreased to the Pb concentration of the purified soil
A.sub.7 of 20 mg/kg after heating. Although not confirmed at
present, it is thought that hexavalent chromium (Cr.sup.6+) in the
material A.sub.6 is detoxified to a valence state (a metal,
bivalent, or trivalent) other than a hexavalent state, which is
hard to elute, and the organochlorine compounds such as dioxine are
also detoxified by heating decomposition (heating step).
[0047] Another soil containing 6800 mg/kg of Pb and 59 mg/kg of As
was heated by the same method as described above, and then an
elusion test was carried out based on Notification No. 13 of
Environment Agency. As a result, the elusion amounts of Pb and As
were as low as <0.01 mg/l (quantitative lower limit) and 0.007
mg/l, respectively.
[0048] As described above, by using the radiant rapid heating
furnace as the heating furnace, the polluted soil A.sub.1 can be
highly purified within a short time, as compared with the
conventional rotary kiln.
[0049] In this example, the ambient temperatures in the first half
and second half of the heating furnace (heating step) are set with
a boundary at 700.degree. C. However, setting of the ambient
temperatures is not limited to this, and the temperatures can be
appropriately changed according to the composition of the material
to be treated, and the evaporation temperature of the material to
be separately recovered, etc. The first half and second half simply
mean the front part and back part of the heating step, not strictly
mean the first half and second half of the heating step. Therefore,
the passage time (retention time) of the material to be treated in
the first half of the heating furnace is not necessarily equal to
that in the second half thereof. For example, when the material to
be treated contains large amounts of Ag and Hg which evaporate at
relatively low temperatures, the first half of the heating furnace
can be set to longer than that of the second half. Conversely, when
the material contains large amounts of Pb and As which evaporate at
higher temperatures, the second half of the heating furnace can be
set to longer than that of the first half.
[0050] Furthermore, in this example, the ambient temperature of the
heating furnace is set in the two steps on the front and back
parts. However, in an operation, the temperature can be set in the
three steps of the front part, an intermediate part, and the back
part. In this case, for example, when the front part, the
intermediate part and the back part are set to 500.degree. C. or
less, 500 to 800.degree. C. and 800.degree. C. or more,
respectively, Hg, As and Cd, and Pb can be separately recovered in
the front part, the intermediate part and the back part,
respectively. The intermediate part means a part between the front
and back parts.
EXAMPLE 2
[0051] In order to confirm the effect of a method comprising mixing
a carbonaceous reducing agent and a metal oxide-containing material
with the polluted soil A.sub.1 as the material to be treated, and
heating the resultant mixture, the experiment below was carried
out.
[0052] Each of single soil (Comparative Example 1), a mixture
(Comparative Example 2) containing 97 parts by mass of soil and 3
parts by mass of coke power serving as the carbonaceous reducing
agent, a mixture (Example 1 of this invention) containing 82 parts
by mass of soil, 3 parts by mass of coke powder serving as the
carbonaceous reducing agent, and 15 parts by mass of an iron ore
composed of an metal oxide was formed in a pillow-like briquette
sample of 39 mm.times.20 mm.times.12 mm (volume of about 6
cm.sup.3) by compression molding using a press agglomerator. Each
of the samples was heated at 1200.degree. C. (constant) in a
N.sub.2 atmosphere in a small tubular furnace for about 12 minutes
to measure changes in the CO concentration in the atmosphere gas
with time.
[0053] FIG. 3 shows the relationship between the elapsed time from
sample charging in the tubular furnace and the CO concentration of
atmosphere gas. An increase in the CO concentration is observed in
the sample of single soil of Comparative Example 1, and this
increase in the CO concentration is possibly due to the fact that
the organic materials contained in the soil are decomposed by
heating to generate CO gas. Therefore, when rapid heating of the
material can be achieved, the heavy metals can be evaporated by
reduction with high efficiency without the addition of a CO gas
generating source to the material to be treated, like in the
present invention. In the sample of Comparative Example 2
containing the soil and the coke powder, the CO concentration is
less increased as compared with Comparative Example 1. Therefore,
oxides of the heavy metals as the pollutants in a portion in direct
contact with the coke powder are reduced, while reduction does not
sufficiently proceeds in other portions out of direct contact with
the coke powder. On the other hand, in the sample of Example 1 of
this invention containing the soil, the coke powder and the iron
ore, the CO concentration of the atmosphere gas is significantly
increased, as compared with Comparative Examples 1 and 2. It is
thus confirmed that a large amount of CO gas is generated.
Therefore, reduction of the heavy metal oxides through the gas is
promoted to permit removal of the heavy metals by reduction
evaporation, and detoxification by insolubilization with high
efficiency.
[0054] The briquette of Example 1 of this invention has a crushing
strength of 530 N after heating, and is thus suitable as a roadbed,
an aggregate, and the like.
[0055] Although, in this example, the material to be treated is
soil, burned ash also can be treated as a material containing heavy
metals and/or organic compounds to produce the same effect as
described below in Example 3.
EXAMPLE 3
[0056] Burned ash is heated by using the same apparatus as that
used Example 1. Each of two samples including single burned ash
(Example 2 of this invention), and a mixture (Example 3 of this
invention) containing 82 parts by mass of burned ash, 3 parts by
mass of coke powder serving as a carbonaceous reducing agent, and
15 parts by mass of iron ore composed of a metal oxide was molded
in a tablet of 20 mm in diameter.times.15 mm in length, and then
heated at an average ambient temperature of 1200.degree. C. in a
rotary hearth furnace for a retention time of 12 minutes. Table 1
shows the Pb concentrations of the tablet before and after heating,
and crushing strength of the tablet after heating. Table 1
indicates that the heavy metals of the burned ash can be removed by
treatment.
1 TABLE 1 Crushing Pb concentration Pb concentration strength after
before heating after heating heating (mg/kg) (mg/kg) (N/tablet)
Example 2 of 530 120 590 this invention Example 3 of 740 50
Unmeasured this invention
[0057] Besides the iron ore, iron-making dust containing an iron
oxide, such as blast furnace dust, converter dust, electric furnace
dust, mill scales, mill sludge, pickling sludge, sintering dust,
and the like, nonferrous oxides such as nickel oxide, manganese
oxide, and the like may be used as the metal oxide. Besides the
coke powder, coal, petroleum coke, waste carbides, RDF carbides,
carbonaceous sludge, scrap tires, blast furnace dust, and the like
may be used as the carbonaceous reducing agent. Particularly, the
blast furnace dust is preferred because it contains an ion oxide
(metal oxide) and a carbon content (carbonaceous reducing
agent).
[0058] As be seen from the above description, according to the
present invention, heavy metals and/or organic compounds contained
in polluted soil and/or burned ash can be sufficiently removed or
detoxified with a high efficiency, as compared with a conventional
method. Also, the apparatus used can be simplified to decrease
apparatus cost and treatment cost.
* * * * *