Ligand-Assisted Extraction of Metals from Fly Ash with Supercritical

Chapter 7

Ligand-Assisted Extraction of Metals from Fly Ash with Supercritical CO : A Comparison with Extraction in Aqueous and Organic Solutions Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 20, 2016 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch007

2

1

1

2

Christof Kersch , Daniela Trambitas , Geert F. Woerlee , and Geert J. Witkamp 1

1

Laboratory for Process Equipment, Leeghwaterstrasse 44, 2628 C A Delft, The Netherlands FeyeCon D&I B.V., Rijnkade 17a, 1382 GS Weesp, The Netherlands 2

The extraction efficiencies (EE) for the removal of Cu, Cd, Pb, Mn, and Zn from fly ash are investigated for supercritical fluid extraction (SFE) with CO /Cyanex 302, for leaching with different acidic solutions and for solvent extraction with Cyanex 302 in Kerosene. Although initially delayed, after some time the E E obtained by S F E is promoted by water. Leaching at pH=3 proceeds faster and more complete than SFE, although both have a similar expected p H at the solid surface. A n empirical fitting model is presented for the aqueous leaching o f fly ash. The dissolution-desorption step from the solid phase is probably the limiting step of the overal extraction. 2

80

© 2003 American Chemical Society

In Supercritical Carbon Dioxide; Gopalan, Aravamudan S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 20, 2016 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch007

81 Large volumes o f fly ashes are produced by municipal waste incinerators. The amount o f produced fly ash is increasing due to an increasing number o f incineration plants. In the Netherlands and Germany, together approximately 35 million tons o f municipal waste are burned annually, generating about 1.5 million tons o fflyash (1,2). Fly ashes contain a complex system of crystalline phases and a range of leachable metals (3,4). This fly ash is contaminated with toxic heavy metals. These (toxic) metals leach out after contact with water, and pollute the groundwater. Therefore, isolated and expensive disposal of the ash is required. It is becoming increasingly important to find ways of utilising this fly ash. For reuse o f ash as filler for cement or pavements only a minimal teachability of metals is allowed by national legislation. SFE offers a method to reduce the metal content to such an extent, that teachability is reduced and the demands of legislation are observed. Supercritical (SC) C 0 is advantageous for extraction of metals from solid particles such as e.g. fly ash (5), because C 0 is a benign and cheap solvent for which suitable extractants are available. S F E does not require any expensive drying o f the final product. Evaporation o f solvent C 0 by release of pressure results in both a solvent free matrix and a separate metal-extractant complex. The extraction efficiency is enhanced by addition of small amounts of water to the solid phase before or during SFE. This positive effect has been shown by the group o f Wai (6, 7) and by Kersch et al (8). Water leads to dissolution of metal-anions such as oxides MeO, eq. 1, and dissociation to metal cations Me , The metal cation then further reacts, according to eq. 2, with a monovalent extractant (ligand) anion X: 2

2

2

2+

(S)

HL}

MeO +2H

(G)

2HX

2+a)

+Me

2+(l)

iL)

^

Me +H O

^

MeX

(1)

2

iG) 2

(L)

+2H+

(2)

The aim of this work was to study the influence of water on metal extraction from the fly ash, for S F E at a scale o f 2 kg solids. The final aim is to provide design parameters for a larger scale S F E unit We concentrate on the main metals o f municipal solid waste incinerator (MSWI) fly ash (Zn, Pb, Cu, Cd, and M i , Figure 1). Since the ZnO content of the studied ash was high, Zn was chosen as model compound. Three types of experiments were performed: 1. 2.

S F E from M S W I fly ash (2 kg) with various ash humidities (2wt%. and 38wt.%, see Figure 2) Aqueous teaching of M S W I fly ash under various p H values: The presence o f C 0 influences the p H o f an aqueous phase, due to formation and dissociation o f carbonic acid. The resulting p H is equal to values o f about 2.9 at 200 bar and 40°C (9). The dissolution of metals in water determines the extent o f leaching and extraction. T o study the p H 2



82

Figure L XRF-analysis of the original MSWI fly ash (wt.%) calculated as oxides


83


3.

effect on that dissolution, aqueous leaching experiments at constant pH=3, pH=4, and pH=5 have been carried out. Solvent extraction of M S W I fly ash with various ash humidities: In a third series of experiments, extractant Cyanex 302 (Bis(2,4,4 trimethylpentyl)monothiophosphinic acid) was dissolved in Kerosene. These experiments were carried out to (i) study the influence of extractant on the metal ions in solid phase with and without a layer o f water, and to (ii) compare the extent of extraction with SFE.

Theory A stepwise extraction mechanism is shown in Figure 3. The fly ash contains a number of metals, as presented in Figure 1. Exemplarily the extraction of zinc(II) ion (Zn ) with the acidic chelating agent HX=Cymex 302 is described. 24

Dissolution and desorption ofZnO at the solid-liquid phase 2+

This step includes both dissolution of ZnO and then desorption of Zn . The p H dependent solubilisation of ZnO with the release of oxide and the consumption of a hydrogen ion is expressed with (S)

+w

ZnO +2H with

2Hsr

^

Zn

(L)

+H0 .

(3)

2

® in the solid phase, ^ in the liquid phase, and ^ * at the solid surface. 2+

The desorption not only depends on the dissolved concentration of Zn { s

r

at

3

the surface o f the solid particleC „ [mol m" ] but also on the gradient of that z

concentration perpendicular to the surface. The desorption is described by j * = v ( c r - c r ) s

2

1

2+

(4)

where J [mol m* s* ] is the amount of Zn transferred into the liquid phase, k 1 *s [m s ] is the overall rate constant, and ^zn

[mol m ] the concentration in the liquid near the solid surface.



84



Figure 2. SEM photos of MSWI fly ash (a) 2wt.% humidity (x2 000), and (b) 38wt % humidity (x750)


go


86

Figure 3: Process steps ofZn dissolution, transfer and extraction from wet fly ash


87 Diffusion of Zw through the aqueous phase 2+

A n assumed fickian diffusion of Zw through the aqueous phase to the H 0C0 interface is expressed as, 2

2


(5)

D

2

1

where J [mol m" s" ] is the Z n to the C 0 phase,

2 +

flux transferred through the aqueous phase

2

D^l

1

[m s" ] is the binary diffusion coefficient,

ε [1] the bed porosity, τ [1] the pore tortuosity, and 3

2 +

C 2 [mol m" ] the concentration of Z n in the liquid phase. The average porosity ε is the ratio of porous volume V to total volume apparent volume V . Zn

+

P

A

Chemical reaction at liquid-SC C 0 interface 2

At the liquid-SC C 0 interface two reactions takes place, 2

iG)

C0

^

2

iL)

(L)

H O +C0 2

(6)

2

L)

^

2

(L)

C0

H COl 2

{L)

^

+{L)

HCO; +H

(7)

Due to the reactions in eq. 6 and 7, the p H of the liquid phase is estimated to be about 3 (9). The zinc cation reacts with the extractant as: Κ iG)

2+{L)

2HX +Zn

==^

{G)

+iL)

ZnX +2H 2


(8)

88 From this reaction, only the reaction to the right is considered, resulting in Zncomplex. Whereas the extractant Cyanex 302 is virtually insoluble in the aqueous phase, the Zn ion is insoluble in pure SC C 0 . Due to its insolubility in C 0 , the released hydrogen is transferred into the aqueous phase. Since that reaction is fast, its rate does not limit the overall extraction. The reaction of Zw into the SC C 0 phase depends only on a reaction rate constant k and the concentration of Zn , 2+

2

2

2+

2


2+

apparent 2

=

^

+

* ^zJ

^

1

where apparent [mol m" s" ] is the rate of Zn reaction, 3

2+

Cf}* [mol m* ] the concentration of Zn 1

in the liquid phase, and

1

k

[m s" ] rate constant.

Experimental

Reagents Fly ash was supplied by a municipal waste incinerator plant ( A V R ) in Rotterdam. For S F E the acid extractant Cyanex 302* (Bis(2,4,4 trimethylpentyl)monothiophosphinic acid), kindly donated by Cytec Industries, Canada, was employed as extractant. The p H o f the aqueous solution was set with analytical grade nitric acid from J.T. Baker. Solvent extraction was carried out with Kerosene from Chemproha.

Supercritical Fluid Extraction (SFE) A flow diagram of the process is shown in Figure 4. C 0 was supplied from a liquid storage vessel (V4), maintained at a pressure of approximately 5 MPa. 2

* Cyanex 302 has been shown to be an effective extractant for a variety of metal ions. In combination with organic solvents successful extraction of Cd* (10) and Pb (11) was carried out from aqueous solutions and at p H 3 the extraction efficiency for Cu \ Zn \ and Mn was 100%, 100%, and 20% respectively (12, 13). With SFE at 200 bar, the extraction efficiency of Cê\ Cu \ Pb \ and Zn from sand was about 100%, 90%, 80%, and 75% respectively (14). +

2+

2

2

2+

2

2


2+

89


E2

V1

Rotating extraction vessel

V2

Separator vessel

V3

Carbon dioxide storage vessel

Ρ

Membrane pump

Ε1

Double pipe heat exchanger (cooler)

E2

Double pipe heat exchanger (heater)

Vent

Ligand supply

Effluent

£ 0 2 > Carbon dioxide supply) Hot water supply

Temperature indicator

Hot water return

Θ

Mass flow indicator

ool water supply

[ PIC )

Pressure control

Cool water return

Θ

Pressure indicator

Figure 4. Flow diagram of the supercritical extraction system


90 That vessel was filled with liquid C 0 from the recycle stream and fresh C 0 from storage. C 0 was pressurised and passed through a heat exchanger (E2) before it entered the preheated 12 litre extraction vessel ( V I ) . After the extraction vessel, the pressure of the C 0 stream was reduced to 5 M P a via a pressure control valve. This extract was fed into a 17 liter separator (V3) containing 3 litre of 3 M nitric acid to collect the metal-ligand complex. The expanded neat C 0 was evaporated from the separator, condensed in a heat exchanger ( E l ) and then returned to the C 0 storage vessel (V4). In each o f the experiments 2000 g fly ash was placed in the pre-heated (about 40°C) extraction vessel. After pressurisation, warm neat C 0 was passed through the vessel for about 60 min to heat the fly ash, to attain a constant flow. Metal extraction was commenced by continuously adding the complexing agent into the warm C 0 flow. The working conditions were 40°C and 20 M P a . The tilted extraction vessel contained a mixing device that maintained both continuous mixing and extraction when the vessel was rotated at (50 rpm). After extraction, a washing step with pure C 0 removed die remaining complexing agent and metal complexes. The fly ash that was used for all experiments came from a single homogenous 200 kg batch. This batch showed variation in metal concentration of up to 15% (sample size 0.5 g). To compensate for this variation a homogenous sample (0.5 g) o f untreated ash was taken prior to each extraction and analysed. The variation of those samples was then about 5%. A n average of the analyses served as a reference for every set of extraction experiments, allowing a more reliable expression of the extraction efficiency. For the studies of wet ashes, homogeneous humidification was desired. The ash was mixed with neutral water (pH=7) prior to S F E , an excess o f water was removed after sedimentation, and the ash was dried to a humidity of 38wt.%. 2

2

2

2

2


2

2

2

2

Aqueous leaching A l l tools or parts used during the experiments were either cleaned or new to insure analytical metal free conditions. A 500 ml glass beaker was filled with 400 g water and the p H was adjusted to the desired values (pH=3, pH=4, pH=5) at 21(±0.5)°C. The p H electrode (model PHM95, MeterLab) was connected with a dosimeter (model 614+665, Metrohm), and the setpoint for the desired p H was chosen. The leaching experiment was commenced by addition of 4.00 g (±0.01) fly ash into the stirred mixture. A magnetic stirrer was used to obtain homogeneity and complete suspension o f fly ash. The dosimeter ensured a constant p H by addition of a 6wt.% metal free nitric acid solution. The addition of acid was registered during die experiment to correct for concentration changes. Leaching samples were withdrawn periodically with a PP+PE syringe,


91 about 1cm below the liquid surface. To restrain ash particles, the samples were injected through a filter into P E sample pots and diluted with nitric acid and analysed by ICP-MS. The volumes withdrawn were sufficiently small (1.5 ml) and influences on the extraction were negligible. A l l experiments were carried out at 20.5-22°C and the temperature change during each test was °>k \-

(10)

c™L, " metal,i \ppm] J=0

Liquid phases were analysed by ICP-MS for the determination of E E after both aqueous leaching and solvent extraction. Based on these analyses, the amount of extracted metals per mass of dry initial fly ash was related to the initial metal concentration of the applied fly ash as, m E E

=

tZl / * W Q [mg/kg]


(

n

)

92

Results and discussion

SFE Figure 5 shows the extraction efficiency (EE) for six different experiments at T=40°C p=20 M P a , with a C 0 flow o f about 9.5 kg/h and concentration of 0.11 mol% Cyanex 302 in C 0 . The water content was 2wt.% and 38wt.%, and time between 3600 s and 22000 s. For both, Zn and Pb addition o f water enhanced the E E . Where after 3600 s almost no extraction o f dry ash was obtained, the wet ash resulted in E E o f Zn and Pb o f 14±2% and 2 1 ± 3 % respectively. The curves for Zn and Pb in Figure 5 suggest a slight increase of E E with longer time. The extraction level o f Cd, Mn, and Cu was fair, in die order of about 50%. However, the decrease o f E E in time, for Cd and M i , was unexpected. Further research is required, looking for co-precipitation for these elements as (bi-) carbonates. Prolonged extraction with SC C 0 at 40°C lead to decrease in fly ash humidity from initially 2wt.% and 38wt.% to 0.6wt% and 18wt% (after 6h). The continuous addition o f water to the SFE extraction system might be recommendable.


9

2

2

9

9

2

Aqueous leaching A first leaching test with neutral water (pH=7) and a liquid solid ratio of L/S=10 resulted in a final p H value o f about pH=11.7. This effect is due to the basic character of the fly ash, due to its high content o f calcium (20wt.%), aluminum (9wt.%), potassium (5wt.%), and sodium (4wt.%) mainly as oxides (Figure 1). After 4 1 0 s, the obtained leaching level was constant at a very low level for both Zn (0.11±0.01%) and Pb (0.98±0.01%). In a second series of leaching experiments, the three p H values in the relevant range for water-C0 were studied. Throughout the experiments at 21(±0.5) C, the p H was kept constant at pH=5±0.01, pH=4±0.0i, pH=3±0.01 by controlled addition of acid. Figure 6 shows that extraction efficiencies of Zn, Pb, Cd, Cu, and Mn increased dramatically with decreasing pH. After 5 1 0 s the E E of Zn varied from 15% at pH=5, to 78% at pH=4 and 83% at pH=3, E E of Cd was 20%, 36%, and 51%, and E E o f CM was 10%, 60%, and 78% respectively. The extraction of Zn and Cu at pH=3 seemed to approach almost 100%, but also the curves at pH=4 and pH=5 promise a further increase for longer extraction time. Whereas extraction of P i was poor at pH=5 and pH=4 (2-7%), a great rise in extraction was obtained at pH=3 to values o f 45%. However, after an extraction maximum at 3 1 0 s, triple reproducible extraction tests at pH=3 3

2

e

4

3


93

100% 80% i

Ο 2wt.% humidity A 38wt.% humidity

fi 60% UJ


UJ

40% 20%

a)

1E+03

1E+05

0% 1E+03

b)

100%

1E+04

Time (s)

1E+05

100% .

02wt.% humidity 80% + A 38wt.% humidity

Ο 2wt.% humidity 80%+ A 38wt% humidity £ 60% m UJ

c)

0% 1E+03

1E+04

1E+05

Time(s)

d)

40% 20% 0% 1E+03

1E+04 Time (s)

1E+05

100% .

Ο 2wt.% humidity 80% 4 4 38wt.% humidity

e)

1E+03

1E+04 Time (s)

1E+05

Figure 5: Extraction efficiency (%) of SFE, (total L/S ratio): 1800 s (14), 3600 s (18), 11000 s (36), 22000 s (53), a) Zn, b) Pb, c) Cd, d) Cu, e) Mn


94

100% 80%

fi 60%

pH3

111


ω 40% 20% 1E+03

0% 1E+01

1E+05

1E+03

1E+05

Time (8)

Time (s)

a)

b)

1E+03

1E+01

1E+05

c)

1E+03

1E+05

Time (s)

Time (8)

d)

1E+03

1E+05

T i n » (s)

e) Figure 6: Extraction efficiency (%) of aqueous leaching from MSWI fly ash a) Zn ±1% b) Pb ±2%, c) Cd±4%, d) Cu ±5%, ande) Mn±2%), L/S=100


95 resulted unexpectedly in a decrease of E E with time. This might be related to precipitation or reverse reactions, due to an enrichment of other compounds during the batch-process.Whereas the extraction rate of Zn, Pb, Cd, and Cu declined with time, the extraction rate of Mn increased over the whole time span (Figure 6e) for pH=4 and pH=3. The sequence of E E at e.g. 5*10 s is for pH=3 and pH=4 Zn>Cu>Cd>Mn>Pb, is for pH=5 Cd>Zn>Cu>Mn>Pb. The addition rate o f the acid is an indication for the reactivity and leaching behaviour o f the metals. The semi-log plot in Figure 7 indicates, that lower p H values lead to fast leaching, in agreement with the metal leach plots in Figure 6. Addition of 10 ml H N 0 dissolved about 0.3 g fly ash corresponding with almost 10% of the applied sample.


4

3

Solvent extraction Figure 8 shows an increasing extraction behaviour with time for all studied metals. The extraction of Zn from dry ash initially 0=30 s) was about 20%, whereas with increasing humidity to 4.3wt.% and 16wt.% the initial E E minimised to 16% and 0% respectively. After 6 1 0 s , however, the extracted values were similar for all three humidities. The extraction of Pb from dry ash commenced with 25% and increased to 70% for dry ash and 80% for wet ashes. In both cases, dry ash resulted in initially similar E E and same or lower E E after some process time. Dry ash gave a faster start. On the longer run, however, dry ash gave no better performances than the humid ash. The extraction from humid ash increased, due to constant removal of metal from the water phase into the solvent phase. Only for Cd die humid fly ash lead to higher E E values than the dry ash. During S E with Kerosene, the p H of aqueous phase surrounding the particle can not be adjusted. A n equilibrium p H of about 12 in that phase is expected, as in an experiment with neutral water (see previous section 'Aqueous leaching'). Such high p H retards dissolution. Built-up compounds (such as Na, K, Ca) in the more alkali system may retard the dissolution of all metals. 4

Comparison of aqueous leaching, SE and SFE From the above it can be concluded that for extraction with SC C 0 or Kerosene, water is not a necessary attribute. With SFE, however, an expected extraction as fast as for leaching at pH=3 was not observed. This might be explained either by formation and/or precipitation of (bi-)carbonates or by decrease o f humidity. In contrast with aqueous leaching of Pb at p, where unexpectedly the E E decreased with time, the extraction of Pb during the continuous SFE kept rising in time. With SE, the initial extraction of Zn from dry 2



96

1E+00 1E+01 1E+02 1E+03 1E+04 1E+05 1E+06 Time (s) Figure 7: Required addition of 6wt. % HN0 (ml) to maintain a constant pH during aqueous leaching from MSWI fly ash at pH=3, pH=4, and pH=5 3


97

100% 80%


β UJ

β

0

Δ water free ash χ 4.3% humidity. • 16% humidity

%

Δ water free ashl χ 4.3% humidity! • 16% humidity

tu 40% 20% 0% 1E+01

a)

0% 1E+03

b)

Time(s)

100%

1E+03

1E+05

Time (s)

100% , Ι Δ waterfreeash 80% -I huro'd'ty-

Δ waterfreeash χ 4.3% humidity _ • 16% humidity

80%

1E+01

1E+05

x

43%

g 60% 111

ui 40% 20% 0% C)

1E+01

1E+03

1E+01

1E+05

1E+03

1E+05

Time (s)

Time (s)

20% 15% -

Δ water free ash 4-3% humidity _ • 16% humidity

x

1E+01

1E+03

Timers)

1E+05

e) Figure 8: Extraction efficiency (±1%) ofsolvent extraction from MSWIfly ash with Kerosene and 0.9 mol% Cyanex 302, L/S=84, a) Zn, b) Pb, c) Cd, d) Cu, e) Mn


98 4

ash was faster compared to S F E , but it had the same values after 2 1 0 s. For extraction o f Zn from wet ash, however, S F E was better. A lower p H o f the water phase, caused by dissociation o f carbonic acid, seems to have improved that Zw-extraction. S E with either dry or wet ash resulted in more extraction of Pb than SFE. For Cd, Mn, and Cu similar plots were obtained. The fast and complete aqueous leaching at pH=3, however, shows a tempting possibility to enhance the extraction by either extraction with excess acid or a pre-wetting step and a larger amount of (dispersed) water in SC C 0 . Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 20, 2016 | http://pubs.acs.org Publication Date: August 31, 2003 | doi: 10.1021/bk-2003-0860.ch007

2

Empirical model of aqueous leaching The experimental data o f aqueous leaching under constant p H were fitted with Avrami-Erofeev rate law (15) for reactions in the solid state. The reactions of metal dissolution, desorption and transport were combined into one reaction, which is expressed as

-In

EE

(12)

EE,, where m [1] is an index of reaction, ο [s] is a rate constant, and EE andEE

l=a>

[1] are extraction efficiencies

The best fits of each set of m, ko, and EE,.„ (in Table I ) were optained by eq. 12 and combination with the least square of

where

σ

2

[ 1 ] is the least square deviation o f fitted and experimental data,

η [ 1 ] is the number of experimental values, sT [ 1 ] a relative error of 0.02, s/** [ 1 ] an absolute error of0.005, x?*? [ 1 ] the experimental value of EE, and a * [1] the fit value of EE. l


99 For Zn, Pb, and Cd the reaction index m results in values of -2.5, whereas for Cu m=-33 and for Mn a value of m=-5 is best. With decreasing p H value the ko increases, expressing the lower leaching rate. The highest ho values for Mn, express the increasing leaching rate within the first 10 s. The plots of measured and calculated EE in Figure 9 illustrates the sigmoidal curves of Zn, Pb, and Mn and the fits of the Avrami-Erofeev rate law. The experimental data revealed a decrease in EE with increasing pH. This phenomenon is included in the present model by estimation of a maximum EE at infinite time (EE^). The fit of the acid addition in order to maintain a constant p H constant (Table II, Figure 7) results in decreasing reaction indices m such as -3.5 (pH=3), -3.7 (pH=4), and -4.8 (pH=5). These reaction indices are in the order of the indices, obtained for each metal (between - 2 and-5, Table I) and combine, thus, the reactions of all metals. The resulting exponential behaviour of the leaching rate with time is yet unknown but possibly due to the complex structure of the studied municipal fly ash. Further research, however, is necessary to assess the validity of these fits.


5

Rate limiting step for aqueous leaching The overall rate o f ZnO aqueous leaching might be controlled by mass transfer in the boundary layer solid-liquid (16). At this layer, a constant low p H in the aqueous phase suggests a saturation of dissolved Zn . Kakovskii et al (16) have reported, that at high acid concentrations the rate of ZnO dissolution is controlled by diffusion of Zrc away from the surface. This local saturation appears to influence eq. 4 and suggests a p H dependent dissolution-desorption step. This step was estimated with the Avrami-Erofeev rate law applying the results of the aqueous leaching data. A n estimation of the diffusion time for the solute in the aqueous phase, shows that limitation of diffusion is negligible. The measured values of a total fly ash volume i^=1.710" m /g, and a pore volume F > = l . H 0 " m / g of initial fly ash, results in a porosity of £=0.64. The binary diffusion coefficient for Zn in the liquid phase is about 5.4-10' m /s, estimated with the Stokes-Einstein equation. With an estimated length o f a single pore o f 2^=20· 10" m, the estimated diffusion time for the solute in the pore is less than 2 s with eq. 14: 2+

2+

6

3

6

3

2+

10

2

6

(14)


100


100%

Ί

1Ε+01

1Ε+02

1Ε+03

1Ε+04

1Ε+05

1Ε+06

Time (s) Figure 9: Plots of measured (symbols) and with Avrami-Erofeev calculated (line) extraction efficiency (%) of aq. leaching from MSWIfly ash at pH=3

Table I. Avrami-Erofeev constants for aqueous leaching of metals from MSWI fly ash under constant pH values. m

ko

EE^ao

σ\

Zn

pH=3 pH=4 pH=5

-2.5 -2.5 -2.5

55 391 600

0.91 0.89 0.16

0.16 0.03 0.02

Pb

pH=3 pH=4 pH=5

-2 -2 -2

44 108 108

0.54 0.10 0.01

0.02 0.003 0.003

Cd

pH=3

-2.5

35

0.56

0.07

pH=4 pH=5

-2.5 -2.5

293 5323

0.40 0.31

0.04 0.08

Cu

pH=3 pH=4 pH=5

-3.3 -3.3 -3.3

64 466 21

0.90 0.71 0.11

0.15 0.07 0.04

Mn

pH=3 pH=4 pH=5

-5 -5 -5

35430 120000 120000

1.00 0.66 0.11

0.03 0.01 0.0003


Λ

101 Table IL Avrami-Erofeev constants of overall reaction during aqueous leaching at constant pH (addition of 6wt.% HN0 ) 3


pH=3 pH=4 pH=5

m

ko

-3.5 -3.7 -4.8

448 2140 5739

27.5 22.9 12.5

0.73 0.97 0.24

Rate limiting step for SFE A t the liquid-SC C 0 interface, a constant, and sufficient high concentration of extractant Cyanex 302 is assumed. Concentration effects of generated metalcomplexes are assumed to be negligible, due to diffusion coefficient of solutes in supercritical fluids of about 10" m /s (17), which is approximately 2 orders of magnitude faster than in the aqueous phase. A continuous flow of solvent during extraction even reduces surface effects, due to both continuous supply of extractant and continuous removal of metal-extractant complex. Thus, mass transfer is a limiting factor at the liquid-SC C 0 interface, as studied previously by Tai et al. (18). Further research is required to study a possible impact on the overall extraction of SFE from humid M S W I fly ash. 2

8

2

2

Conclusion and Recommendation The SFE can be performed with almost the same results with both dry and humid fly ash. The limiting step of the reaction is most probably the dissolutiondesorption step from the solid phase. For a good extraction of the studied metals from municipal waste fly ash, SFE might be enhanced by a pre-leaching step. Leaching efficiency by aqueous acidic solutions obeys a logarithmic power rate law model.

Acknowledgement Acknowledgment is made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial travel support to Orlando for G . Witkamp.


102 References 1.


2. 3.

4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Puch, K.H.; Hartan, J. In Proceedings of Int. Workshop on Novel Products from Combustion Residues; Nugteren, H . , Ed; Delft Univ. of Technology, The Netherlands, 2001, pp 43-46. Verwoerd, J., internal communication, AVR-Rotterdam, 2002 Fermo, P.; Cariati, F.; Pozzi, Α.; Demartin, F.; Tettamanti, M.; Collina, E . ; Lasagni, M.; Pitea, D . ; Puglisi, O.; Russo, U. Fresenius J. Anal. Chem. 1999, 365, 666-673 Towler, M. R.; Stanton, K. T.; Mooney, P.; H i l l , R. G.; Moreno, N.; Querol, X . J. Chem. Technol. Biotechnol. 2002, 77, 240-245 Glennon, J.D.; O'Connell, M.; Leahy, S.; Mehay, H.; Smith, C . M . M . In Proceedings of the "REWAS'99: Global Symposium on Recycling, Waste Treatment and Clean Technology" San Sebastian, Spain, Sept. 5-9 (1999) Volume III, pp 2329-2336 Wai, C.M.; Lin, Y . ; Brauer, R. Talanta 1993, 40 (9), 1325-1330 Wang, S.; Elshani, S.; Wai C M . Anal. Chem. 1995, 67 (5), 919-923 Kersch, C . ; van Roosmalen, M. J. E.; Woerlee G . F.; Witkamp, G. J. Ind. Eng. Chem. Res 2000, 39, 4670-4672 Toews, K . L . ; Shroll, R. M.; Wai, C . M.; Smart, N. G Anal. Chem. 1995, 67(22), 4040-4043 Almela, Α.; Elizalde, M.P. Hydrometallurgy 1995, 37, 47-57 Menoyo, B . ; Elizalde, M.P.; Almela, A . Solvent Extr. Ion Exch. 2001, 19, 677-698 Sole, K . C . ; Hiskey, J.B. Hydrometallurgy 1995, 37, 129-147 Cyanamid Co. Technical Brochures of Cyanex 301 and Cyanex 302 (1993) Smart, N . G . ; Carleson, T.E.; Elshani, S.; Wang, S.; Wai, C . M . Ind. Eng.Chem. Res. 1997, 36, 1819-1826 House, J.E. in Principles of Chemical Kinetics, WC. Brown Publ.: Times Mirror Higher Education Group, U S A , 1997 Kakovskii, I.A.; Khalezov, B.D. Izv. vyssh. ucheb. Zaved., Tsvetn. metall., 1977, 2, 26-31 (Russian text) Chem. Abstr., 1977, 87, 12172s Paulaitis, M. E.; Krukonis, V . J.; Krunik, R. C.; Reid, R. C . Rev. Chem. Res. 1983, 32, 179 Tai, C . Y . ; You, G.S.; Chen S.L. J. of Supercritical Fluids, 2000, 18, 201212


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