Leaching behaviour of Cu and Pb form Municipal Solid Waste
Incineration Bottom Ash
Gi-Chun Hana, Ji-Whan Ahna, Hee-Chan Chob
a
Korea Institute of Geoscience and Mineral Resources
Gajeong-dong 30, Yuseong-gu, Daejeon, 305-350 South Korea
b
Seoul National University
San 56-1, Sillim-dong, Gwanak-gu, Seoul, 151-742 South Korea
Abstract
With the purpose to be used as raw materials for ordinary Portland cement and construction materials
like aggregates, etc., bottom ashes, which are produced through the incineration of municipal solid
wastes, were washed with water and its heavy metals were removed. Bottom ash contained 5.2% and
6.2% water-soluble salts while containing 1.3-1.5% of chlorine component. When bottom ashes were
washed, with 10 Liquid/Solid ratio (L/S), and 300rpm of agitation speed, almost all soluble salts were
dissolved within 30 minutes, lowering the chlorine content to 0.2-0.3%. The residual chlorine
component existed as Friedel's salt (3CaO․ Al2O3․ CaCl2․ 10H2O). For other heavy metals, a small
amount was found or not found completely. As a result of washing bottom ashes with water, it was
dissolved to below than the permitted value, so that it was verified that washing with water would
stabilize bottom ashes of incinerated municipal solid wastes.
Keywords: Municipal solid waste incineration ashes, bottom ash, copper, leaching
1. Introduction
The disposal of municipal solid wastes in Korea went to landfill at 72.3%, while only 4.0% were
incinerated and 23.7% were recycled in 1995. However, the landfill ratio has continuously decreased
while incineration and recycling ratio has increased improving waste disposal in 2003 showing 40.8%
went to landfill, 14.5% incinerated and 45.2% recycled. Considering more than 50% of municipal
solid wastes are inflammable, however, it is still desirable to increase the ratio of waste disposal
through incineration.
As the incineration ratio gradually increases, it is estimated that its residual by-products would also
increase. For incinerated ashes produced from municipal solid wastes, about 20wt.% of municipal
solid wastes, came to incineration facilities; it is estimated that about 90wt.% of incinerated ashes
produced is bottom ashes, while 10wt.% is fly ashes.
Based on this calculation, in 2003, when the incineration ratio of municipal solid wastes was about
13.6wt.%, 400,000tons of incinerated ashes were produced; where about 350,000tons are estimated to
be bottom ashes while about 50,000 tons were fly ashes. Since the Government are going to increase

E-mail : hsue@dreamwiz.com, Tel : +82-42-868-3597, Fax : +82-42-861-3990
the incineration ratio of municipal solid wastes to 30wt.% till 2011, it is estimated that incinerated
ashes would increase to about 10% every year with an increase of about 1 million tons in 2008 in case
that there is no big change in municipal solid wastes produced from 2003 with an increase of 2% in
incineration ratio.
Fly ashes exceeded the permitted standard of heavy metals, and they are treated by landfill through
intermediate treatment process such as solidification and stabilization, etc. Since bottom ashes are
consisted by steel, glass, ceramic, etc., recycling is available through a proper pre-treatment. But, they
are usually reclaimed in Korea. To secure landfill becomes difficult due to discharge of secondary
pollutants, NIMBY (Not In My Back Yard) and so on.
From a viewpoint of recycling of resources, it is desperately needed to develop a technology to
recycle bottom ashes and change relevant policies. On the other hand, in European countries like
Germany, Denmark, Netherlands, etc., bottom ashes are used as light aggregates for road construction,
or 60-90% of bottom ashes are recycled as asphalt or concrete aggregates.
In order to solve chlorine or heavy metal production, which becomes a recycling problem for
incinerated bottom ashes from municipal solid wastes, in this study, water-washing method was
investigated. It aims to reduce the chlorine content of incinerated ashes and the environmental
harmfulness of heavy metals.
2. Samples and experimental methods
2.1. Samples
For samples of bottom ashes, bottom ashes produced in municipal solid wastes incinerator located in
Gyeonggi-do l Region (here-in-after will be referred to as bottom ash I) and bottom ashes produced in
incinerator in Busan D Region(here-in-after will be referred to as bottom ash II) were used. In each
incineration facilities, wastes are incinerated with stocker at 200 – 300tons a day. Samples collected
from the ash bunkers of each incinerator are moved to laboratory and sealed, dried at 100±5℃, and
kept in a desiccator to be used as reagent in each laboratory.
2.2 experimental methods
For washing test, water bath, where temperature control is available, was connected to cylinder type
reactor and the test was carried out. After the reactor was filled with distilled water, stirred with
shaker, the temperature was adjusted. Then, bottom ashes were mixed with fixed Liquid/Solid ratio
(L/S); washed out; and the solid part was separated using glass fiber filter of 1.0 ㎛; the density of
heavy metals and chlorine ion from the solution was measured. Dehydrated cake was dried and the
chemical component and mineral phases, etc. were analysed.
3. Results and discussion
Table 1 shows the content of soluble salts of the bottom ashes according to time when I and D bottom
ash was washed at 20oC, the agitation speed of 300rpm. As seen in Table 1, the weight of bottom
ashes diminished to 5.2%, 6.2% within 30 minutes, respectively. As investigated in the USA,
therefore, for bottom ashes of soluble materials harmfully influencing the environment and also
influence the quality of recycling products, it is desirable to remove these before water washing.
Table 1. Content of soluble salts for bottom ash
Content of soluble salts(%)
Agitation time
30 min.
60 min.
90 min.
120 min.
Ⅰ
Bottom ash
5.2
5.3
5.3
5.4
Ⅱ
Bottom ash
6.2
6.3
6.3
6.3
3.2 Dissolution characteristic of chlorine
When incinerated ashes are used for civil engineering materials, cement or other purposes, the
following tests were carried out to investigate the dissolution characteristic of the chlorine component
influencing environment and cement characteristics. Fig. 1 shows the result of the process carried out
for bottom ashes. It shows the dissolution amount of chlorine according to the time interval of
agitation at 300rpm with 1/10 of Liquid/Solid ratio(L/S) each at 20℃. According to the test result, In
case of bottom ash I and II, 80%(residual amount inside bottom ash: 0.6%) and 85%(residual amount
inside bottom ash: 0.4%) of the chlorine component was removed respectively within 30 minutes. The
reason why chlorine removal ratio of bottom ash in incinerator I and II is high looks like mainly due
to the dissolution of soluble salts like KCl, NaCl, CaClOH, etc., and the remaining residual chlorine
component, which is not soluble, looks like chlorine component combined with Friedel's salt.
Table 2 shows the test result to find out the chlorine dissolution rate according to each fixed
temperature. Test was carried out with the following conditions; 30 minutes of shaking time,
solid/liquid ratio of 1/10; individual testing at 5℃, 25℃, 50℃ at 300rpm of agitation time. As
temperature increases, the dissolution ratio of chlorine also increases, and there’s a tendency for the
residual chlorine of incinerated ashes to increase. It is predicted that the dissolution ratio of Freidel's
salt to increases as the temperature also increases. Accordingly, it is decided that to be effective, the
temperature of washing water be increased to above 25℃ so as to enhance the removal effect of the
chlorine.
100
3
2
60
bottom ash I
bottom ash D
40
1
20
Residual Cl content(wt.%)
Cl removal rate(wt.%)
80
0
0
0
30
60
90
120
Time(min)
Fig. 1. Chlorine removal rate and Residual Chlorine content in bottom ashes depending on washing time(L/S =
10, , washing temperature : 20℃).
Table 2. Residual chlorine content in bottom ash after washing according to temperature.
Sample
o
5C
0.91
0.58
Bottom ash I
Bottom ash II
Residual chlorine content in bottom ash(wt.%)
20oC
50oC
0.20
0.15
0.30
0.14
Fig. 2 is XRD patterns for each bottom ash of below 100mesh 2 days and 1 month before and after
sudden cooling into water. In the reagent before sudden cooling with water, it is noted that there are
Ca(OH)2, CaCO3, SiO2, CaSO4, CaO, etc. After 3 days, it is seen that CaO almost becomes Ca(OH)2,
and Ca, Al and Cl is combined with water to formulate Friedel's salt (C3A․ CaCl2․ 10H2O). After 1
month, the reagent shows a big peak of Friedel’s salt, and only CaCO3 appears as the main
component.
Since the reaction continues as times goes after the bottom ash is rapidly cooled with water, and the
composition of Friedel's salt with low dissolution ratio takes place, it is found from these results that
chlorine removal from bottom ash by water washing would be advantageous than none rapid cooling
with water.
m : CaCO3
n : CaSO4
t : Ca(OH)2
m
l : CaO
q : SiO2
s : friedel's salt
water quenched(30days)
q
s
m
m
m
m
m m
m
mm
m
n
q
t
s
water quenched(2days)
m
q
15
20
25
m
m
m
n
10
t
30
m
m
m
m
air quenched
l
ml
m
35
2
mm
40
l
m
m
45
50
55
60
Fig. 2. XRD patterns before and after water quenching the bottom ash of below 100mesh.
3.3 Leaching of heavy metals
Table 2 and Table 3 show the concentration of leached heavy metals contained in residual liquid as
water washing is repeated for solid by-products under the same washing conditions. For this, bottom
ash of incinerator I and II of less than 19mm each was washed with water at 20℃ for 30 minutes at
the solid/liquid ratio of 1/10 at the speed of 300rpm.
According to the test results, for Pb, Cd, As, Cr, from both bottom ashes of incinerators I and II were
dissolved at less than permitted value regardless of washing time. For Cu, however, 5.1ppm was
dissolved in bottom ash I, exceeding the permitted value, while 2.0ppm was produced in bottom ash
II. After analyzing the residual liquid that was washed 2 times, however, Cu also was dissolved at less
than the permitted value. Since the amount of heavy metal in bottom ash is dissolved less than the
permitted value with only 2 times of washing, bottom ashes can be said to have stabilized according to
the test regulations in Korea.
Table 2. The amount of heavy metals according to washing time of bottom ash I
Heavy metal amount (ppm)
No. of washing
Cu
Cr
Pb
Cd
As
1
5.21
0.24
0.66
ND*
ND
2
0.12
ND
0.6
ND
ND
3
*ND : Not detected
0.02
ND
0.6
ND
ND
Table 3. The amount of heavy metals according to washing time of bottom ash II
Heavy metal amount (ppm)
No. of washing
Cu
Cr
Pb
Cd
As
1
2.10
ND
0.65
ND
ND
2
0.12
ND
0.6
ND
ND
ND
ND
0.6
ND
ND
3
*ND : Not detected
4. Conclusions
1. The incinerated bottom ash of municipal solid wastes contains about 5.2% or 6.2% soluble salt,
and 1.3-1.5% of chlorine component is also contained in it. When bottom ash was washed with
water with 10 of Liquid/Solid ratio(L/S) at 300rpm of shaking speed at 20℃, most soluble salts
were dissolved within 30 minutes and the chlorine content lowered to 0.2-0.3%. The residual
chlorine content remained as Friedel's salt.
2. As a result of heavy metal production test of incinerated bottom ash for municipal solid wastes, Cu
exceeded the permitted value of heavy metal, while the other heavy metals were slightly found or
not found. After washing bottom ash with water, it was dissolved at below the permitted value.
Therefore, washing with water was proven to help to stabilize the incinerated bottom ash of
municipal solid wastes.
Reference
The Ministry of Environment of Korea, 2004, Waste generation and treatment situation of Korea 2003.
Clive Brereton, 1994, Resources, Conservation and Recycling, 16.
C. Wiles and P. Shepherd, 1999, Beneficial Use and Recycling of Municipal Waste Combustion Residues,
National Renewable Energy Laboratory.
Ji-Whan Ahn, et al., 2001, Journal of the Korean Institute of Resources Recycling, Vol.10, No. 4.
Ji-Whan Ahn, et al., 2001, Journal of the Korean Institute of Resources Recycling, Vol.10, No. 5.
U.A. Birnin-Yauri, F.P. Glasser, 1994, Cement and Concrete Research, Vol. 28, No.12.