Volatilization Mechanism of Pb from Fly Ash in Municipal Waste Incinerator
Mototsugu Matsuno
1, Katsuhiro Tomoda
1and Takashi Nakamura
2 1Sumitomo Metal Mining Co., Ltd, Tokyo 105-8716, Japan2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8517, Japan
Fly ash produced by waste combustion is designated as specially controlled waste in Japan due to harmful heavy metals contained in it and is legislated to be processed properly before disposal to landfill. As for technologies for making the fly ash harmless, several methods are recommended for the treatment, but each of them has some problems to be solved. The fly ash from a municipal solid waste incinerator contains large amounts of chlorinated compounds that make the harmful treatment difficult. A complete recycling of waste residue left behind after waste incineration, however, is required to solve the shortage of available final disposal sites. With this background, a new technology needs to be developed, that the heavy metals in the fly ash are removed sufficiently, no harmful heavy metals are leached and the residue ingredients are used effectively. In addition, Japanese government revised the Soil Contamination Countermeasures Law in February 2003 to strengthen the soil environmental evaluation standards. This could also promote the development of above technology. This paper focuses on lead that is particularly emphasized in the law, and proposes an advanced technology to remove lead from the fly ash, discussing the removal mechanism and removing conditions of lead. The newly developed technology is associated with the sintering process using a rotary kiln, and effectively uses only the chlorinated compounds contained in the fly ash to volatilize and remove the heavy metals. The technology achieves satisfactory low level of dioxins as well as prevention of lead leach. The fly ash treated by the technology also has passed a leaching test at pH 4 simulating typical acid rain circumstances.
(Received June 27, 2003; Accepted October 8, 2003)
Keywords: lead volatilization, HCl, fly ash, waste incinerator, sintering
1. Introduction
Fly ash produced from a municipal solid waste incinerator is designated as specially controlled waste in Japan since it contains large amounts of heavy metals such as lead (Pb) and cadmium (Cd). There is a legal obligation to make it harmless before the landfill disposal. The Ministry of Health, Labor, and Welfare of Japan recommends several basic technologies for making the fly ash harmless and at the same time points out some problems inherent to technology. Therefore a new reliable technology need to be developed.
On the other hand, availability of new waste disposal site becomes very difficult in Japan so that a recycling-oriented technology is urgently required to avoid landfill disposal of wastes.
Technologies for making specially controlled wastes harmless are required, not only to meet the requirements of their disposal to landfill waste site, but also to secure environmental safety satisfying safety standards applied in the areas where the treated wastes can be used effectively, for instance the soil environmental evaluation standard. In any event the conventional technologies are difficult to meet these requirements. This paper focuses on Pb that is emphasized in the soil environmental standards revised in February 2003, and proposes an advanced technology to remove lead from fly ash, discussing the removal mechanism and removing conditions of lead. The developed technology consists of removing heavy metals from the fly ash, through the chloride volatilization method, to allowably low level in the resultant pellets. In this sense, the technology can be differentiated from the existing harmless technologies which simply immobilize heavy metals by adding reagents to the fly ash.
And the technology presented here is also completely different from the volatilization reaction of chloride in oxidizing atmosphere that is adopted by the conventional
technologies,1) and is able to sufficiently volatilize and remove heavy metals in weak HCl atmosphere under which only Cl contained in the fly ash is effectively utilized. Thus the novel technology can be differentiated from the conven-tional chloride volatilization method which requires the additional chlorine source such as CaCl2 to achieve high PHCl2, aiming at recovering valuable heavy metals in
oxidizing atmosphere.
The technology is also able to volatilize and remove heavy metals other than Pb simply by maintaining reducing atmosphere in the bed layer.2–9)
2. Outline of Treatment Method and Testing Method
Figure 1 shows schematically the treatment flow of the newly developed technology. The dimension of the applied kiln in this study is 900 mm in diameter and 12000 mm in length. The kiln was operated under the following conditions: rotation 16rpm, throughput 100 kg/h (typical) and in-creased up to 250 kg/h (maximum).
Additive agents such as coke and hematite were added to the fly ash. They were mixed by a mixer and crushed by a vibrating rod mill. Then water was added to the mixture to knead them together. The mixture is extruded to prepare columnar pellets of 10 mm in diameter and 5 to 15 mm in length. After dried, the pellets were charged into a rotary kiln, where the pellets are sintered at temperatures higher than 1000C. The flue gas is cooled down rapidly to less than
200C by water spraying to prevent dioxin formation.
Secondary fly ash is collected in a bag filter. Since the flue gas contains HCl, the gas is passed through a scrubber to neutralize it, and then entered into an activated carbon absorber to further decrease the concentration of dioxins before venting to atmosphere. Table 1 shows the typical chemical compositions of the fly ash used in the tests.Num-bers of test run were conducted in order to indicate the
applicability of the technology to the wide range of the fly ash compositions.
3. Test Results
3.1 Variations in concentration of Pb,etcin kiln
Figure 2 shows the test results under typical test con-ditions, giving profiles of various elements as a function of the distance from inlet (left end of the figure) to outlet (right end) of the testing kiln (total length of 12 m).
As can be seen from the figure, Cl gradually decreases as the distance from the intlet (representing a relatively low temperature) increases and shows a rapid decrease at about 950C. As for K and Na, however, such a rapid decrease can
not be seen. This different behavior suggests that some phenomena regarding Cl different from NaCl and KCl volatilization occur in the kiln. On the other hand, Pb shows similar behavior with Cl, giving a rapid decrease at 950C.
This observation suggests some relation between Pb and Cl.
3.2 Amount of Pb remaining in sintered pellets
Figure 3 shows amounts of Pb remaining in sintered pellets for every test run. It can be seen that the volatilization and removal of Pb are fully generated regardless of the testing conditions employed. Among varied conditions, a few test runs show quite higher values than the average because of the insufficient temperature.
3.3 Pb leaching test
Also for the leach of Pb, as a whole stable and low level of Pb is attained as shown in Fig. 4. Since there appears a phenomenon of irregularly exceeding the standard level, the test data including cases of the increased charging amount are rearranged in Table 2 for individual conditions. The data exceeding the standard level were found to be in the condition of accelerating the oxidization, or 4 rpm or higher rotational speed of the kiln.
3.4 Evaluation by Toxicity Characteristic Leaching Procedure (TCLP) and for the test simulating acid rain
Tables 3 and 4 show the evaluation results of leached amount for TCLP and for pH 4 condition. Both cases produced satisfactory results, and it is clear that quite small amount of Pb remaining in sintered pellets is fully stabilized after the treatment.
4. Discussion
The results mentioned above have shown that Pb contained in the fly ash is volatilized and removed to a sufficiently low level. From the fact that Cl is detected in the collected volatilized matter, it is obvious that Pb is volatilized like chloride. As any additive containing Cl is not used in the verification tests, Cl ingredient used in the chloride reaction
Mixing
Pellet Forming
Drying
Rotary Kiln
Sintered Pellets Waste Gas
Cooling
Bag Filter
Scrubbing
Adsorption Tower
Waste Gas Pellets
2nd. Fly Ash
Hydrometallurgy
Zn, Pb, Cd Fly Ash Flux
<Markets> <Smelter>
[image:2.595.46.551.660.767.2]Fig. 1 Flow sheet of the fly ash treatment process.
Table 1 Typical chemical compositions of fly ash used in the tests
(Unit: %)
Type of
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O ZnO PbO SO3 Cl C
fly ash
A
10.2 5.8 1.3 15.8 2.9 12.7 12.0 1.3 0.2 10.8 23.8 0.8
(EP ash)
B
1.3 3.5 0.8 39.6 4.3 3.9 4.0 0.3 0.1 3.7 13.3 1.9
(Ca ash)
C
20.2 13.0 1.5 17.0 3.2 9.1 8.5 2.4 0.8 4.8 13.0 2.4
(EP ash)
is only Cl contained in the fly ash. Therefore Cl which has been regarded as a problem-inducing ingredient in the fly ash is expected to be utilized effectively in the process of volatilization and removal of harmful heavy metals.
4.1 Reaction mechanism by equilibrium calculations
Since chloride is detected in volatilized matter, the volatile compounds containing Pb are determined to be PbCl2.
Pb in green pellets before sintered is confirmed to be PbO so that PbCl2formation reaction is written as follows:
PbOþ2HCl¼PbCl2þH2O
GðJÞ ¼ 189;570þ94:44T ð1Þ
G873ðJÞ ¼ 107;124
K¼PbCl2PH2O=PbOP
2
HCl¼2:56106 ð2Þ
0 0.2 0.4 0.6 0.8 1
0 1 2 3 4 5 6 7 8 9 10 11 12
Distance from Kiln End , d/m
Residual Amount (g/100g of
Green pellet)
400 500 600 700 800 900 1000 1100
T
e
mperature , T/
°
C
Pb
Temp.
Inlet Outlet
0 2 4 6 8 10
0 1 2 3 4 5 6 7 8 9 10 11 12
Distance from Kiln End , d/m
Residual Amount
(g/100g
of
Green pellet)
400 500 600 700 800 900 1000 1100
T
e
mperature , T/
°
C
Cl K Na Temp.
Inlet Outlet
Fig. 2 Amounts of Cl, K, Na and Pb remaining in pellets within rotary kiln.
0.000 0.005 0.010 0.015 0.020 0.025 0.030
1 6 11 16 21 26 31 36 41 46 51 56 61 66
Runs
Residual Percentage (%)
Pb Standard (<150ppm)
Fig. 3 Amount of Pb remaining sintered pellet.
0.001 0.01 0.1 1
1 6 11 16 21 26 31 36
Runs
Elution,E/mg.L
-1
Pb Cd Cr Cr standard:<0.05
Pb,Cd standard:<0.01
Oxidized Oxidized
Low sitering Temp. Low sitering Temp.
[image:3.595.69.529.71.256.2]Fig. 4 Results of Pb, Cd and Cr elution. Table 2 Result of lead elution tests for different feeding rate.
(Rotation of kiln is less than 4 rm1)
Feed rate No Pb elution Feed rate No Pb elution Feed rate No Pb elution
(kg/h) (mg/L) (kg/h) (mg/L) (kg/h) (mg/L)
120 1 <0:01 150 1 <0:01 200250 1 <0:01 2 <0:01 2 <0:01 2 <0:01
3 <0:01 3 0.031(*) 3 0.010
4 <0:01 4 0.036(*) 4 0.013(*)
5 <0:01 5 <0:01(*)
6 <0:01 6 0.015(*)
7 0.020(*) 8 <0:01
(*): Rotation of kiln is 4 rm1or more.
Extraction conditions: A sample of 100 g is dissolved in 1000 mL of water
Table 3 Result of elution tests of sintered pellets (I).
Elution of Pb (mg/L)
Residusl amount
Sample No. TCLP of USA Notification No. 46 of Pb (<5:0mg/L) of JAPAN (%)
(<0:01mg/L)
F-1 0.10 0.004 0.006
F-2 0.53 0.032 0.009
L-1 0.15 <0:001 0.006
L-2 0.065 0.017 0.004
[image:3.595.57.281.299.411.2]Extraction conditions: A sample of 100 g is dissolved in 2000 mL of acetic-acid solution
Table 4 Result of the elution tests of sintered pellets (II).
Elution result by
Elution result in pH4 Japanese soil
Notification No. 46 environmental
of JAPAN standard
[image:3.595.315.543.302.442.2] [image:3.595.47.291.469.599.2] [image:3.595.305.550.498.552.2] [image:3.595.48.290.664.765.2]Since the oxygen combustion rate in LPG combustion is estimated to be about 80% based on the analysis of oxygen concentration in kiln flue gas, the partial pressure of H2O in
the flue gas is calculated to be about 12%, (pH2O =
0:12105(Pa)). It is calculated with the combustion of LPG gas fuel by air(C3H8+5O2=3CO2+4H2O). Assuming that
PbO and PbCl2 exist as simple substances and hence their
activities are defined to be 1, the partial pressure of equilibrium HCl is calculated to be 2:17101 (Pa). The
value is very low so that the chlorination reaction of Pb possibly occurs.
As shown in Fig. 5, the vapor pressure of PbCl2 is
sufficiently high, thus PbCl2 is fully volatilized after its
formation, which suggests that PbCl2 is relatively hard to
remain in pellets.
As for the source of HCl necessary in the chloride reaction, two reactions related to aluminosilicate and sulfate might occur taking into account the ingredients of the fly ash.2–9) These two possible reactions are following.
NaClþ1=2SO2þ1=2H2O
þ1=4O2¼HClþ1=2Na2SO4 ð3Þ
G1173¼ 21:0ðkJÞ
NaClþ3SiO2þ1=2Al2O3
þ1=2H2O¼HClþNaAlSi3O8 ð4Þ
G1173¼ 7:6ðkJÞ
A comparison of standard free energy of formation for each of the reactions at 900C shows that the reaction with
SO2 contribution more easily occurs. These reactions,
however, are highly temperature-dependent as shown in Fig. 6, and the reaction (4) is likely to occur in the low temperature zone while the reaction (3) does not occur. On the other hand, at temperatures of 900 to 950C, it can be seen
that the values of standard free energy of formation for the reactions are inverted, and at the high temperature zone, the reaction (3) becomes dominant.
On the assumption ofPHCl¼101(Pa), the partial pressure
of SO2necessary in the reaction (4) is calculated to be about PSO2¼101(Pa).
HCl produced presumably proceeds through the reaction (3) in the low temperature zone and through the reaction (4)
in the medium temperature zone. If HCl is produced in the high temperature zone, similar to SO2, there may occur the
migration of HCl to the lower temperature and the migration into the bed layer caused by mechanical entrapment resulted from the rotation of the kiln.
4.2 Basic test relating to the source of SO2and to the Pb
volatilization
The source of SO2is CaSO4, which is contained in the raw
material fly ash, and the thermal decomposition of CaSO4
produces SO2.
4.2.1 Testing method
As for thermal decomposition of CaSO4, a basic test was
conducted in helium gas atmosphere so as to avoid the influence of oxidization. This condition simulates the actual bed layer. A test atPO2¼3103 (Pa) was also conducted
simulating the oxygen partial pressure in actual gas layer. The apparatus used consists of a differential thermal balance and a mass spectrometer (TG-MS), schematically shown in Fig. 7. Table 5 shows the main specifications of TG-MS used in the experiments.
Pellets containing CaSO4 was used as CaSO4 reagent for
thermal decomposition. The sample of fly ash was a powder mixture of fly ash and auxiliary raw materials with the same mixing ratio as that used in the verification test. Aiming at understanding both reactions in internal zone of bed and in gas layer, two types of the atmosphere were chosen: the reducing atmosphere (simulating the bed) of 100% helium as stream; and oxidizing atmosphere (simulating the gas layer) of 3% oxygen-contained helium gas stream.
It should be noted that, different from actual rotary kiln, the test does not form any bed and does not apply counter-flow of gas to the sample. Also note that the gas analysis data is measured in the gas layer.
The variations in the composition of the residue of pellets sintered and the compounds were measured using chemical analysis and X-ray diffractometry (XRD).
4.2.2 Test for producing SO2gas
Figure 8 shows the test result of CaSO4decomposition and
of SO2production. The results include both oxidizing and
un-oxidizing conditions. The figure clarifies that SO2 produced
increases with the decrease in CaSO4 in un-oxidizing
condition. Taking into account the fact that SO2 was
0.0001
0.001
0.01
0.1
1
400
600
800
1000 1200 1400
Temperature ,
T
/
°
C
Pressure /10
5
Pa
ZnCl
2PbCl
2
KCl
NaCl
vapor pressure for volatilization
volatile start temperature
Fig. 5 Vapor pressure of chlorides versus temperature.
-100 -75 -50 -25 0 25 50 75
200 400 600 800 1000 1200
Temperature,T / °C
∆
G
°
/kJ
.mol
-1
0.5H2O [g] + 3 SiO2 [s] + 0.5Al2O3 [s] + NaCl [sl] = HCl [g] + NaAl3SiO4 [s] (4)
0.25O2 [g] + 0.5H2O [g] + 0.5SO2 [g] + NaCl [sl] = HCl [g] + 0.5Na2SO4 [sl] (3)
[image:4.595.51.287.70.235.2] [image:4.595.309.547.73.229.2]produced at more than 600C, it is supposed that the SO2
contributing reaction is unlikely to occur at least below 600C. In oxidizing atmosphere, SO2 is produced at higher
temperatures than that of un-oxidizing atmosphere.
4.2.3 NaCl decomposition
A basic test was conducted on NaCl decomposition, as a source of HCl produced. Fly ash containing NaCl and moisture was prepared and used as experimental reagent. As shown in Fig. 9 (in un-oxidizing atmosphere), HCl is produced along with the decrease in NaCl, thus the production of HCl caused by the NaCl decomposition was confirmed. The amount of HCl produced, however, was relatively small.
4.2.4 Volatilization and removal of Pb from pellets
The amount of HCl produced and the decrease in Pb in the pellets were also measured to find that HCl production and the decrease in Pb occur almost simultaneously in un-oxidizing atmosphere (see Fig. 10).
4.2.5 Phenomena in oxidizing atmosphere
Figures 8, 9 and 10 also show the data obtained in oxidizing atmosphere. As shown in Fig. 8 (see the case of oxidizing atmosphere), SO2caused by CaSO4decomposition
occurs in a delayed time compared with the case of un-oxidizing atmosphere, indicating a rapid increase in SO2 at
around 950C. This is due to the delay of CaSO
4
decom-position, since the decreasing rate in CaSO4 is smaller than
[image:5.595.305.544.72.226.2]that of un-oxidizing atmosphere. It is, therefore, obvious that the difference in bed atmosphere also gives significant influence on SO2 production. Further It implies that the bed Table 5 Specifications of TG-MS.
Apparatus TG-DTA2020S (MAC make)
TG-DTA Heating furnace Resistance furnace Temperature RT1100C
Atmosphere He, N2, Air,etc
Capillary temperature 250C
Apparatus THERMOLAB (VG make)
Q-MS Ionizing EI method (voltage=70eV)
Filament Iridium
Mass range 1300amu
0 50 100 150 200 250 300
0 200 400 600 800 1000 Temperature , T/°C
Ion Intensity (M-12)
0 20 40 60 80 100
Relative Intensity of
XRD(%)
SO2-Oxidized SO2-Un-oxidized
CaSO4-Oxidized CaSO4-Un-oxidized
Oxidized Oxidized
Un-oxidized
Un-oxidized
Fig. 8 SO2gas produced and residual amount of CaSO4as a function of
sintering temperature.
0 50 100 150 200
0 200 400 600 800 1000
Temperature , T/°C
Ion Intensity (M-12)
0 20 40 60 80 100
Relative Intensity of
XR
D(%)
HCl-Oxidation HCl-Un-oxidation
NaCl-Oxidation NaCl-Un-oxidation
Oxidized
Un-oxidized Oxidized
Un-oxidized
Fig. 9 HCl gas produced and residual amount of NaCl as a function of sintering temperature.
0 50 100 150 200
0 200 400 600 800 1000
Temperature , T/°C
Ion Intensity (M-12)
0 0.2 0.4 0.6 0.8 1 1.2
Residual
Amount of Pb (%)
HCl-Oxidized HCl-Un-oxidized Pb-Oxidized Pb-Un-oxidized
Oxidized Oxidized
[image:5.595.73.266.73.335.2]Un-oxidized Un-oxidized
Fig. 10 HCl gas produced and residual amount of Pb as a function of sintering temperature.
Capillary inlet
Ceramic probe heater
Sample
Tube Seal
Glass Standard sample
Protecting tube Heating furnace
Sample holder
Sleeve Ion
source
Molecular leak
[image:5.595.308.545.282.439.2] [image:5.595.47.292.380.695.2]atmosphere gives influence also on the formation reaction of lead chloride.
On the contrary, as shown in Fig. 9 for HCl produced by NaCl decomposition, the decomposition of chloride is enhanced rather in oxidizing atmosphere. Similarly, as seen in Fig. 10 for the volatilization of Pb in pellets by HCl, the decreased amount of Pb is large owing to the high partial pressure of HCl.
These basic tests indicate that HCl formation reaction from SO2and the lead chloride formation occur very easily. Thus,
even in the low temperature zone, the SO2 contribution
reaction occurs only if the partial pressure of SO2 is at a
satisfactory level.
4.2.6 Evaluation of basic test results
The test results show that the mechanism to produce HCl differs with the kind of atmosphere. It implies that the SO2
-contributing reaction (the reaction (4)) may firstly occur in the bed layer, and that the SiO2-contributing reaction (the
reaction (3)) may firstly occur at the contact zone with the gas layer under the presence of a certain level of oxygen partial pressure.
4.3 PHClandPSO2in the kiln verification test
To confirm the basic test results, gas analysis was conducted at various positions in the kiln. The result is shown in Table 6.
4.3.1 Sampling method
Gas sampling was made through the sampling holes opened on the kiln at specified positions. For sampling, the kiln was temporarily stopped during the continuous operation when the thickest portion of the bed came to the opposite side of the sampling hole concerned. The sampling device was quickly inserted through the sampling hole into the depth center of the bed to collect sample by sucking the sampling device. For the gas layer, the gas sample was collected at about 50 mm above the bed surface. The sampling was completed in a relatively short period, then the kiln was restarted. By repeating the steps, total sampling work was done.
4.3.2 Analysis of the results
From the data based on the procedures mentioned above in kiln verification tests, the following results were obtained as for HCl:
a) Small amount of HCl produced was observed at relatively low temperature zone of the charging side.
b) Significant amount of HCl produced was observed at around 900C.
c) At the high temperature zone, however, expected amount of HCl was not observed.
As for SO2 the results were obtained as follows:
d) The amount of SO2 produced was very small at the low
temperature zone of the charging side, not reaching an equilibrium pressure necessary for producing HCl. (PSO2¼10
1Pa).
e) Very large amount of SO2 was produced at the high
temperature zone, significantly exceeding the amount con-tained in the raw material. S circulation, therefore, that may occur within the kiln needs to be taken into consideration.
The above observations clarify that the partial pressure of SO2 is low in the low temperature zone, and that SO2
contributing reaction in the zone is not so active. In order to enhance HCl formation the supply of SO2from the gas layer
having high SO2 partial pressure is required obviously. At
around 900C, the amount of HCl formed is satisfactory, and
it might be produced through both SO2-contributing and
SiO2-contributing reactions. Due to the fact that the
meas-ured values of SO2showed very high level, there might occur
both SO2 absorption by the pellets at the charging side and
the internal circulation of SO2 within the kiln in the high
temperature zone caused by SO2 decomposition.
4.4 Analysis of reaction in kiln and material balance relating to the gasification
4.4.1 Reaction at the low temperature zone
Based on the results described above, the results of verification test are analyzed as follows:
As shown in Fig. 2, the slow decrease in both Pb and Cl at the relatively low temperature zone suggests that HCl producing reaction is not significant, or that the decompo-sition of NaCl does not occur and the partial pressure of SO2
is low. Thus, the decomposition of CaSO4 observed at
temperatures below 700C under the un-oxidizing condition
shown in Fig. 8, presumably begins at a zone of slightly higher temperature than 700C. As clearly shown in Table 6,
since the partial pressure of SO2 detected in the low
temperature zone is lower than the equilibrium partial pressure, SO2 is not necessarily a controlling variable to
[image:6.595.46.549.84.207.2]produce HCl within the zone. The speculation agrees well with the fact that Cl in charged materials does not decrease significantly in the low temperature zone in the kiln (Fig. 2).
Table 6 The SO2and HCl concentrations measured in the bed layer and the gas zone of the kiln.
Sampling point (Temp.) No. 6 (650C) No. 3 (900C) No. 2 (1000C)
Gas component SO2 HCl O2 SO2 HCl O2 SO2 HCl O2
Gas zone (ppm) a) 2200 76 0.4 50000 150 1.8 4100 350 <1
b) 440 1300 590
Bed layer (ppm) a) 35 12 <0:1 12000 150 <0:1 610 50 <0:1
SO2Partial pressure 100
(ppm)(*1)
HCl Partial pressure 64
(ppm)(*2)
(*1): Calculated equilibrium SO2partial pressure at the time of HCl producing
4.4.2 Material balance in the gasification reaction
Based on the kiln test, the material balance relating to the gasified ingredients is expressed in Table 7. Na and K are fed as respective chlorides and oxides. They are partly volatilized as chlorides and then decomposed to produce HCl.
Most of supplied Ca is in a form of oxide, partly with sulfate. Within the kiln, the sulfate is decomposed to produce SO2, while a part of the sulfate reacts with HCl to produce
CaCl2.
S is fed as CaSO4, and it is decomposed in the kiln to
produce SO2.
In that case, the amount of HCl produced is calculated to be 0.105 kmol at 100 kg/h of charging. If the gas amount is assumed to be 1000 Nm3/h of flue gas at the exit of kiln, HCl concentration in the kiln is calculated to be about 2350 ppm. The analyzed values of individual ingredients in the kiln, however, are somewhat different from the calculated values, as shown in Table 6, giving higher level in the gas layer than in the bed. The observation suggests that HCl is produced in the bed, and then HCl vapor migrates into the gas layer. Sulfur dioxide (SO2) shows a surprising result (see Table 6),
giving a peak of several tens of thousands of ppm significantly exceeding the predicted level at around 950C,
while giving several thousands of ppm at around 700C at the
charging side. These values are much larger than the values simply converted from the amount of S charged continu-ously. This observation suggests that SO2 produced in the
high temperature zone is consumed in the low temperature zone through some possible reactions. At the center portion of the kiln, SO2concentration was higher in the gas layer than
in the bed layer, which could induce SO2migration from the
gas layer into the bed layer at the kiln center.
S circulation is established by, for instance, alkali sulfate, and alkali earth sulfate, and CaSO4was found in the charged
material. Although CaSO4 was detected in the charged
material by X-ray analysis, no CaSO4 was detected in the
sintered material. Since CaSO4 can be produced within the
kiln, the CaSO4 may be produced and increased within the
kiln, and may be decomposed before the sintered material is discharged. Possible components assumed as the S medium include Na, K,etc. as well as Ca, but these are not confirmed in the present experiment. If all of Ca, Na, and K in the charged materials are converted to respective sulfates, the sum of the partial pressure of SO2 produced by their
decomposition becomes about ten thousand ppm, which is far below the observed values. Consequently, the presence of another medium for S circulation is expected, and the possibility of Al, Fe, S, etc. should be investigated. The
production of Smay not be impossible because the oxygen
partial pressure in the bed layer in the kiln is as low as
<104Pa and because the kinetic data suggests.
5. Conclusions
Regarding the advantages of the developed technology giving excellent performance of making harmless and high environmental safety, we made an analysis on the mechanism of volatilization of lead chloride, and obtained the following conclusions:
(1) The basic tests proved that HCl can be produced from NaCl contained in fly ash from waste incinerator. (2) HCl is produced through decomposition of NaCl by
alminosilicate and by the action of SO2.
(3) HCl producing reaction is highly susceptible to the temperature and the partial pressures of SO2 and O2,
and the dominating reaction differs with the reaction conditions.
(4) The amount of SO2 produced was measured to be
surprisingly large at a temperature of around 900C, which suggests the presence of S circulation within the kiln in some compound modes. Although the produc-tion and decomposiproduc-tion of Na2SO4and CaSO4might be
associated with this circulation, other causes should be investigated further.
[image:7.595.45.548.95.318.2]The above-described findings show that stable operation is attained by many reactions, each of which plays the dominant
Table 7 The material balance rerating to gasified components.
(Feeding rate of green pellets: 100 kg/h)
Elements Input (k mol/h) Output (k mol/h)
Na as NaCl: 0.129 as NaCl (vaporized): 0.040
as Na2O: 0.071 as Na2O (in pellet): 0.160 (0.089 from NaCl)
Total 0.200 0.200
K as KCl: 0.091 as KCl (vaporized): 0.070
as K2O: 0.049 as K2O (in pellet): 0.070 (0.021 from KCl)
Total 0.140 0.140
Ca as CaSO4: 0.062 as CaSO4: 0 (decomposed completely)
as CaO: 0.257 as CaO: 0.304 (0.319-0.015)
as CaCl2: 0 as CaCl2: 0.015 (estimated from remained chloride)
Total 0.319 0.319
Cl as NaCl: 0.129 as NaCl (vaporized): 0.040
as KCl: 0.091 as KCl(vaporized): 0.070
as CaCl2: 0.030
as PbCl2: 0.002 (calculated from analytical data)
as HCl gas: 0.078 (calculated by total method)
role responding to the intra-kiln conditions depending on various variables specific to individual incinerator fly ashes, independent of the differences in treatment conditions. These advantageous characteristics make the technology presented versatile as industrial technology.
Acknowledgements
This research constitutes part of the joint research with the New Energy and Industrial Technology Development Or-ganization (NEDO) and The Center For Eco-Mining under the title of ‘‘Development of Technology for Recovering Valuable Metals from Incinerator Fly Ash and Making It Harmless’’.
REFERENCES
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