Study on the Effect of Operational Temperature and Time
on the Characteristics of Slag Aggregate
Paper #56
Jeng-Shiow Hsiung a, Yi-Chin Huang b, Kung-Cheh Li c
a
Process Engineering, Taiwan Semiconductor Manufacturing Company, Ltd, Hsinchu 300,
Taiwan.
b
Department of Land Management and Development, Chang Jung Christian University, Tainan
711, Taiwan
c
Graduate Institute of Environmental Engineering, National Taiwan University, Taipei 106,
Taiwan
ABSTRACT
The objective of this study is to investigate the influence of operational temperature and time on
the characteristics of slag, and further evaluate the effect of operational condition changes on
molten sludge recovering as fine aggregate. The co-melting mixing ratio of calcium fluoride
sludge and water works sludge are four to six, at which ratio a relatively low eutectic
temperature could be found. The experimental melting temperatures were set at 1200, 1250,
1300, and 1350 °C, and the experimental melting time were 10, 20, and 30 mins. Each batch of
the molten sludge was cooled in water and ground to match the aggregate gradation code. After
that, gravity, absorption, void, unit weight, and hardness of the slag samples were measured. The
experimental results indicated that gravity and void are essentially the same in all conditions. The
values of absorption and hardness decreased accompany with operational temperature or time
increased, but unit weight increased. The results of Toxicity Characteristic Leaching Procedure
(TCLP) revealed that all slag samples are harmless to use as fine aggregate. Furthermore, among
all conditions, the effect of temperature on properties of slag aggregate is higher than that of the
operational time, suggested that it is necessary to operate in higher temperature to obtain better
quality of slag aggregate.
INTRODUCTION
The high percentage of people served by the water supply system and the development of
semiconductor industry produce huge amounts of water works sludge and calcium fluoride
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sludge in Taiwan. Nowadays, at least 6,000 cubic meters of water works sludge cake and 66 tons
of the calcium fluoride sludge are produced every month. Since the main method of treating
these two kinds of sludge is currently disposal in landfills, the public resistance and the
unavailability of landfill sites makes the disposal of sludge a major issue in Taiwan. The best
strategy is to reduce sludge quantity during the treatment process, or recovery sludge should be a
top priority in the subsequent sludge management system.
Among the alternatives for sludge quantity reduction, sludge melting process has received
considerable attention since 1990s to recover sewage sludge 1, 2. The sludge melting process is to
heat sludge in 1200-1600 °C, to compose organic matter and melt the inorganic matter. At this
high temperature, more than 60% of cadmium, lead, zinc and copper present in the slag and 60%
of nickel, chromium and arsenic are associated with fly ash of melting furnace that air pollution
control equipments have to set after the melting process After the sludge is melted, the melted
material was cooled in either air or water to form slag. The structure of slag is considered as the
network of SiO4 tetrahedra, which could fix heavy metals in the structure and reduce their
leaching rate. The obtained slag could used as paving, fine aggregate, block or cement additives.
Therefore, melting process is considered as a solution for sludge final disposal due to its
characters of stabilization, materialization, and environmental friendly.
It is suggested that sewage sludge co-melting with industrial waste can reduce the potential
problem of industrial waste-related pollution. Sakai et al. indicate that asbestos co-melting with
sewage sludge make asbestos dehazardous 3, and Wang et al. apply melting process on municipal
incinerator fly ash treatment 4. Furthermore, the feasibility of apply the melting process to
recover calcium fluoride sludge and water works sludge as constructional material has been
studied. The water-cooled and air-cooled slag obtained from this co-melting system can replace
20% and 40% of fine aggregate in the cement mortar, respectively 5, 6 . Thus, use calcium
fluoride as a metallurgical flux not only effectively reduces the hazard potential of calcium
fluoride sludge on environment and human health, but also beneficially result from the
operational temperature reduction.
Several factors influence the operational temperature of the melting process, such as basicity of
sludge 7, additive into sludge 8, and ambience gas in furnace 9. It is suggested that there is a low
eutectic temperature of the sewage sludge when the basicity of sludge adjust to one. The eutectic
temperature is further reduced when the ambience gas in furnace is reduction, or when additives,
such as borax (sodium borate), soda (sodium carbonate) and lime (calcium carbonate), are added
in the melting system 8.
Although the low eutectic temperature sludge is obtained, the actual operational temperature of
the sludge is at least 100 °C higher than the eutectic temperature to ensure the sludge was melted
complete, and this might affect the characters of slag. However, the study concerns about the
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operational temperature and time are very few. Thus, this study aimed to discuss the influence of
different operational temperature and time on slag characters to obtain a better operational
condition.
EXPERIMENTAL METHOD
The calcium fluoride sludge was gathered from a semiconductor plant sedimentation tank. The
coagulants were calcium chloride and calcium hydroxide. The gathered calcium fluoride sludge
was dried at 105 °C in an oven before the experiments were performed. After acid digestion,
according to NIEA R109.01C, the main components of the sludge were determined using
ICP-AES (Inductivity Coupled Plasma- Atomic Emission Spectrometer) with sensitivity in the
mg/L range. The Water works sludge was collected from a water treatment plant in Taipei, and
dried at 105 °C before mixing with calcium fluoride sludge. The main components of the sludge
were measured using ICP-AES after acid digestion. Due to the basicity of calcium fluoride
sludge and water works sludge are 2.5 and 0.001, respectively, the mixing proportion was set at
60% (w/w) calcium fluoride sludge to 40% (w/w) water works sludge to adjust basicity to 1. At
the mixing ratio, a relatively low melting temperature was obtained at 1197 °C. Furthermore,
both calcium fluoride and water works sludge were ground to 0.075 mm before mixing.
In order to investigate the affect of operational temperature and time on slag, the operational
temperatures were set at 1200, 1250, 1300, 1350 °C, and the operational times were 10, 20, 30
mins. After each batch of melting process, the molten sludge was cooled in water, and then the
obtained slag was crushed to match the aggregate gradation code. After that, gravity, absorption,
void, unit weight, and hardness of the slag samples were measured to compare the difference
between each experimental condition.
RESULTS AND DISCUSSION
In all experimental conditions, the 1200 °C-10 min-batch of melted material was too hard to flow
out due to its high viscosity. Thus, this batch of slag could not collected after the sludge
co-melting was done, and the following measurement of slag characters could not proceeded.
This result also indicated that the operational temperature is necessary raised to increase the
operational convenience.
1. Effects on Gravity
Gravity means the weight ratio of aggregate to water in same volume. Figure 1 displays the
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gravities of each batch of slag obtained from different operational conditions. The results
indicated that the gravities had unapparent relationship with operational temperatures, but
slightly increased when the operational time was increased.
The gravity of nature fine aggregate are 2.4 to 2.9, and it is better when the value is higher than
2.55. From the experimental results, the gravities of each batch of slag fell within 2.65 to 2.75,
indicated that every batches of slag had fine quality in gravity.
Figure1. Effects on Slag Gravity in Different Operational Temperature and Time
2.80
Operation Time
Gravity
2.75
10 mins
20 mins
30 mins
2.70
2.65
2.60
2.55
1150
1200
1250
1300
1350
1400
Operation Temperature (oC)
2. Effects on Absorption
Absorption is one of the important characters of fine aggregate, it has positive tendency with the
porosity in aggregate. Different absorption will affect the water-cement ratio, and further affect
the strength of concrete. As the fine aggregate used in concrete, the absorption should be slower
than 3%.
Figure 2 displays the absorption changes of each batch of slag in different operational conditions.
It shows that all of the absorption obtained from this study fell within 0.6 to 2.4%, indicating that
each batch of slag could used as fine aggregate. As the operational temperature increased, the
absorption of slag significantly reduced. The relationship between operation time and absorption
has a similar tendency, but the decrement is lower than that of operational temperature.
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Figure 2. Effects on Slag Absorption in Different Operational Temperature and Time
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Operation Time
10 mins
20 mins
30 mins
Absorption (%)
3
2
1
0
1150
1200
1250
1300
1350
1400
o
Operation Temperature ( C)
3. Effects on Unit Weight
Unit weight means the weight of aggregate in a unit volume (including aggregate, voids between
aggregates, and pores in aggregate). Different destination of engineer used different unit weight
of fine aggregate. As the fine aggregate used in normal concrete structure, the unit weight should
within 1100 to 1750 kg/m3.
Figure 3 displays the tendency of unit weight in each experimental condition. It shows that the
values of unit weight increased as the operational temperature or the operational time increased,
and the increment is higher when raising the operational temperature. It is suggested that the
viscosity is lower in a higher temperature, and the gas produced by organic matters
decomposition could release more easily. Therefore, the obtained slag had a lower porosity and a
higher unit weight.
The experimental results reveal that the unit weight of slag fell within 1734 to 1930 kg/m 3,
meaning that each batch of slag obtained in this study has potential to use as fine aggregate in
normal concrete structure.
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Figure 3. Effects on Slag Unit Weight in Different Operational Temperature and Time
2000
Operation Time
Unit Weight (kg/m3)
1900
10 mins
20 mins
30 mins
1800
1700
1600
1150
1200
1250
1300
1350
1400
Operation Temperature (oC)
4. Effects on Void
Void is calculated from gravity and unit weight. For a nature aggregate, the range of void is 0.25
to 0.4. In this study, the voids fell within 0.29 to 0.34, and there is no significantly tendency
between void versus operational temperature or operational time (Figure 4).
5. Effects on Hardness
Hardness is an index to establish the ability of aggregate to anti-weathering. Either saturated
sodium sulfate or magnesium sulfate is used to determine the hardness of aggregate. In saturated
sodium sulfate or magnesium sulfate, the loss rate should lower than 10% or 15%, respectively.
The saturated sodium sulfate is used in this study, and the results displayed in Figure 5.
Figure 5 shows that the hardness is in the range of 1 to 8%, and there is a significantly difference
between 1250 °C-10 min-batch of slag to others. Except the 1250 °C-10 min-batch of slag, the
values of hardness are slightly reduced when operational temperature or operational time
increased. Although the hardness of 1250 °C-10 min-batch of slag is 8%, lower than 10%, the
result reveals that the slag obtained from this situation is easily weathering. Moreover, when this
slag used as fine aggregate, the surface of concrete is easily cracked. It is suggested that the
hardness has positive relationship with vitrification, thus, it is worthy to raise operational
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temperature or time to acquire a low hardness.
Figure 4. Effects on Slag Void in Different Operational Temperature and Time
0.6
Operation Time
0.5
Void
0.4
10 mins
20 mins
30 mins
0.3
0.2
0.1
0.0
1150
1200
1250
1300
1350
1400
Operation Temperature (oC)
Figure 5. Effects on Slag Hardness in Different Operational Temperature and Time
10
Operation Time
Hardness (%)
8
10 mins
20 mins
30 mins
6
4
2
0
1150
1200
1250
1300
1350
o
Operation Temperature ( C)
7
1400
6. Effects on TCLP
The Toxicity Characteristic Leaching Procedure (TCLP) is used to adjudge the hazardous of slag.
Table 1 shows that most of the leached concentrations are too low to determine, and all slag are
harmless to use as fine aggregate.
Since the total heavy metals in calcium fluoride sludge and water works sludge are low, the
leached results are reasonable. Moreover, the results showed that there is no tendency between
TCLP and operational temperature or time.
Table 1. The leached concentrations of each batch of slag in different operational conditions
Operational Temperature
1200 °C
1250 °C
1300 °C
1350 °C
mins
20
30
10
20
30
10
20
30
10
20
30
Item
Pb
ND 0.05 0.05 ND ND ND ND ND ND ND ND
Zn
0.14 0.10 0.04 0.01 0.01 0.01 0.01 0.05 0.05 0.04 0.04
Ni
0.01 0.01 0.03 0.03 0.03 0.03 0.03 0.04 0.09 0.05 0.04
Mn
ND 0.01 0.04 0.02 0.02 0.04 0.04 0.06 0.01 0.01 0.06
Cu
ND ND ND ND ND ND ND ND ND ND ND
Cr
ND ND ND ND ND ND ND ND ND ND ND
Cd
ND ND ND ND ND ND ND ND ND ND ND
As
ND ND ND ND ND ND ND ND ND ND ND
ND ND ND ND ND ND ND ND ND ND ND
Hg
ND: Not Detected
-: No regulatory limitation in Taiwan.
ROC-EPA
Regulatory
Limitation
5
15
5
1
5
0.2
CONCLUSION
The experimental results of this study indicated that gravity and void are essentially the same in
all conditions. The values of absorption and hardness decreased accompany with operational
temperature or time increased, but unit weight increased. The results of Toxicity Characteristic
Leaching Procedure (TCLP) revealed that all slag samples are harmless to use as fine aggregate.
Furthermore, among all conditions, the effect of temperature on properties of slag aggregate is
higher than that of the operational time, suggested that it is necessary to operate in higher
temperature to obtain better quality of slag aggregate.
ACKNOWLEDGMENTS
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The authors would like to thank the National Science Council of the Republic of China, Taiwan
for financially supporting this research under contract No. NSC 91-2211-E-002-063.
REFERENCE
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3. Sakai S., Hiraoka M., Takeda N. and Tsunemi T. Water Sci. Technol. 1990, 22(12), 329-338.
4. Wang, K.S., Chiang, K.Y. and Shao, B.S. Toxicol. Environ. Chem. 1997, 61, 69-81.
5. Lo, Y. H., Huang, Y. C. and Li, K.C. A&WMA’s 96th Annual Conference and Exhibition, San
Diego, CA, June 22-26, 2003. Paper AC121520.
6. Huang, Y.C., Li, K.C. and Chiang, H.H. J. Environ. Sci. Health, Part A. 2005, 40(1), 193-202.
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23(10/12), 2019-2028.
8. Hsiau, P.C. Master thesis. National Central University, Jul. 1992. (in Chinese)
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KEY WORDS
Sludge melting process, slag, aggregate
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