design of a leaching test framework for coal fly ash: …suwic.group.shef.ac.uk/posters/mz2.pdf ·...
TRANSCRIPT
Chemical & Process Engineering
Intr
oduc
tion
R
esul
ts
Con
clus
ion
s
Fut
ure
Design of a Leaching Test Framework for Coal Fly Ash: Accounting for Environmental Conditions
Dr Mohammad Zandi • Dr Nigel V. Russell [email protected] [email protected]
‘Engineering from Molecules’ www.sheffield.ac.uk/cpe
PFA A PFA B PFA C PFA D PFA E PFA F Major oxides (%) Al2O3 SiO2 Fe2O3 CaO Trace elements (mg/kg) As Cd Cr Mo Ni Se Zn
31.80 47.30 4.12 7.37
19.8 < 0.2
71 11.4 51.6
3 50.1
30.60 49.10 5.90 5.97
44.7 < 0.2 40.5 11.5 68 4.2
42.1
29.00 47.30 8.17 4.68
0.3 0.18 51.1 11.5 82.8 0.3
94.2
26.50 49.50 11.70 2.49
264.3 0.17 73.5 24.1 98.2 38
212.8
28.10 48.4 8.69 6.3
185.5 < 0.2 134.2 42.9
107.4 1
140
28.60 49.9 7.90 5.3
59.2 < 0.2 85.9 29.3 94.1
1 112
Six fly ash samples, collected from UK power stations, were selected for this study and characterised to BS 3892: Part 1. PFA E and PFA F are re-fired fly ash. No attempt was made to identify the coal or to determine the combustion and post-combustion conditions that control the mineralogy of the fly ash samples.
It has been established that fly ash from coal combustion contains relatively high concentrations of trace elements distributed heterogeneously. When fly ash comes into contact with an aqueous environment some constituents will dissolve to a greater or lesser extent and become mobile. The product of this contact is known as the leachate. The degree of mobilisation and dissolution of the constituents in the leachate (extract composition) is of interest for different purposes. Leaching tests are widely used as indicators to estimate the release potential of these elements from fly ash in order to assess environmental impacts of utilisation or deposition of the fly ash under laboratory conditions. Standard leaching tests determine mobility of trace elements from solid matrix under specific leaching conditions and the results are not necessarily representative of leaching behaviour of examined fly ash under its application conditions. Hence different information will be obtained from different leaching tests and this might lead to different management and/or disposal of the same material. Therefore, it is vital to study the effects of different factors controlling trace element leachability in order to better estimate fly ash leaching behaviour under different application conditions.
Fly
ash
sam
ples
Exp
erim
ents
o Availability test
o Standard batch leaching tests
o Titration test
o LS test
o Batch leaching tests o Effects of pH o Effects of LS ratio
The Dutch availability test NEN 7341 was used to determine leaching potential.
Two standard leaching tests (TCLP 1311 and BS EN 12457-2) were selected for this study to determine the stability of trace elements in fly ash samples under two different conditions.
BS EN 12457-2 TCLP 1311 Leachant pH LS Leaching time (h) Temperature (°C) Number of extraction
Deionised water Not a factor
10 24
18 – 25 1
Acetic acid buffer 4.93 or 2.88
20 18
18 – 25 1
This test has been designed to study effects of pH on mobility of trace elements from fly ash samples. Fly ash samples have been titrated with 1 N nitric acid or sodium hydroxide to reach specific pH. After calculating volume of needed acid or base, fly ash has been leached for 24 hours and LS = 100.
Liquid-to-Solid (LS) test has been developed to determine impacts of liquid to solid ratios (LS) on release of trace elements from fly ash. Fly ash samples have been leached for 24 hours with DI water in different LS ratios (LS = 5 to 100).
Availability test The main aims of this test are (1) to indicate what quantity of a particular component may leach out from a material under extreme circumstances and (2) to quantify the elements not bound in silicate minerals or other poorly soluble mineral phases.
TCLP 1311 and BS EN 12457-2 Most of the elements showed higher leaching rates under TCLP 1311 test than BS EN 12457-2. This is due to the acidic conditions of the TCLP leaching test (pH < 4 .30) . However, there are some exceptions.
PFA B
0.001
0.01
0.1
1
10
100
As Be Ca Co Cr K Mg Mo Ni Se Ti V ZnElement
Leac
hed,
(%)
Availability Test TCLP 1311 (pH=3.64) BS En 12457 (pH=11.86)
PFA A
0.001
0.01
0.1
1
10
100
As Be Ca Co Cr K Mg Mo Ni Se Ti V ZnElements
Leac
hed,
(%)
Availability Test TCLP 1311 (pH=4.21) BS EN 12457 (pH=12.11)
PFA C
0.001
0.01
0.1
1
10
100
Be Ca Co Cr K Mg Mo Ni Ti V ZnElement
Leac
hed,
(%)
Availability Test TCLP 1311 (pH=1.87) BS EN 12457 (pH=8.90)
PFA D
0.001
0.01
0.1
1
10
100
As Be Ca Co Cr K Mg Mo Ni Se Ti V ZnElement
Leac
hed,
(%)
Availability Test TCLP 1311 (pH=3.83) BS EN 12457 (pH=5.33)
PFA E
0.001
0.01
0.1
1
10
100
As Be Ca Co Cr K Mg Mo Ni Ti V ZnElement
Leac
hed,
(%)
Availability Test TCLP 1311 (pH=2.54) BS EN 12457 (pH=11.46)
PFA F
0.001
0.01
0.1
1
10
100
As Be Ca Co Cr K Mg Mo Ni Ti V ZnElement
leac
hed,
(%)
Availability Test TCLP 1311 (pH=4.27) BS EN 12457 (pH=11.11)
10
100
1000
10000
100000
1000000
2 3 4 5 6 7 8 9 10 11 12pH
Leac
hed
quan
tity,
(ppb
)
Ca K Sr
0.001
0.01
0.1
1
10
100
1000
10000
100000
2 3 4 5 6 7 8 9 10 11 12
pH
Leac
hed
quan
tity,
(ppb
)
Be Mn
1
10
100
1000
2 3 4 5 6 7 8 9 10 11 12pH
Zinc
leac
hed,
(ppb
)
0.1
1.0
10.0
Zn Cd
Cad
miu
m le
ache
d, (p
pb)
1
10
100
1000
2 3 4 5 6 7 8 9 10 11 12
pH
Leac
hed
quan
tity,
(ppb
)
Cr V
1
10
100
1000
10000
100000
1000000
10000000
100000000
0 10 20 30 40 50 60 70 80 90 100
LS ratio
Leac
hed
quan
tity,
(µg/
kg)
Ca Cr K Mo Sr
1
10
100
1000
10000
100000
1000000
0 10 20 30 40 50 60 70 80 90 100
LS ratio
Leac
hed
quan
tity,
(µg/
kg)
As B Ba Mg Na Ni Se V Zn
ð Leachability of highly soluble elements. ð Leachability of cationic elements.
ð Leachability of amphoteric elements. ð Leachability of oxyanionic elements.
ð Elements, on the basis of their leaching behaviour in different LS ratios, can be classified into two groups: Type 1 comprises elements which are most soluble elements (Ca, Cr, K, Mo and Sr) and their release is independent of LS ratio or is a very week function of LS. Type 2 includes elements where their release is a strong function of LS ratio for LS ratios less than 50. Leachability of these elements becomes independent for LS>50.
Elements showed four different leaching behaviours against pH.
Total concentration in fly ash
Available or potentially leachable element from fly ash
Fly ash
Amphoteric
Oxyanionic
Cationic
Highly soluble
Che
mic
al s
peci
atio
n of
el
emen
t in
leac
hant
Associated with fly ash matrix
Total concentration in fly ash
Available or potentially leachable element from fly ash
Fly ash
Amphoteric
Oxyanionic
Cationic
Highly soluble
Che
mic
al s
peci
atio
n of
el
emen
t in
leac
hant
Associated with fly ash matrix
0 50 LS
Type I
Type II
Leac
hed
quan
tity
100
Material
Characterisation
Availability Test
Equilibrium Tests
Mass Transfer Tests
Application Scenario
Regulations
Leaching pre-tests
Titration pre-test
Titration Tests
LS Tests Contact Time Tests
Effects of controlling factors on leaching Equilibrium Modelling
Mass Transfer Modelling
Com
plia
nce
test
ing
Design leaching procedure
Start
1
2
3
Long term leaching prediction
Classify the materialend
The authors would like to thank Prof. J. Williamson and Mr. F. Wigley (Imperial College, London), Mr. M. Wilde, Mr. R. Stacey Dr. R. Bachmann and Dr. D. Poole (The University of Sheffield) and financial support from DTI Cleaner Coal Technology R&D Programme (Project 325).
Acknowledgements
o Results from this study led to the construction of a leaching test framework which can be used to evaluate the potential trace element release from a fly ash sample in the context of known disposal or utilisation conditions. A leaching test should reflect the range of conditions (pH, redox potential, LS ratio, contact time, etc.) that will be present in the fly ash and its interface with its environment during leaching in the field.
The main objective of this framework is to provide an evaluation of trace element release from particular fly ash samples under realistic conditions. This framework helps to determine the stability of trace elements in fly ash using standard methods in a reasonable time. Also the framework makes currently available leaching tests more flexible by rearranging and redesigning the standard leaching test.
Existing standard methods have been shown to be inadequate by not representing possible disposal or utilisation scenarios. A number of factors which affect leaching have, therefore, been examined to determine a framework to better understand the leaching behaviour of trace elements from fly ash.
o pH was found to be the overriding parameter affecting the mobility of trace elements from fly ash.
o LS ratio was found to be the second most important parameter affecting the leachability of fly ash constituents
o The fly ash samples were examined under TCLP 1311 and BS EN 12457-2. Trace elements were found to be more mobile under TCLP conditions. TCLP results indicated the worst case scenario;
Ground water Ground water
ð Two application scenarios of fly ash: Left) utilising fly ash in construction materials such as asphalt, Right) depositing fly ash in lagoons.
q Investigating effects of surface area and porosity on leachability of trace elements; q Developing mathematical models to predict long-term leaching;
q Applying Leaching Test Framework on other type of wastes; and
q Stabilising heavy metals in an industrial waste in order to reduce risks of reusing the waste (converting a waste to a product).