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Integrated Treatment of MSWI-Residues Treatment of Fly Ash in View of Metal Recovery
Environmental Aspects of Consttuctioti with Waste Materials JJJ.M. Goumans, H A . van &r SIoot and Th.G. Aalbers (Edtors) el994 Elsevier Science B. V. AN rights reserved.
525
Integrated treatment of MSWI-residues Treatment of fly ash in view of metal recovery B. Laethema, P. Van Herckb, P. Geuzensa, C. Vandecasteeleb aFlemish Institute for Technological Research, Department of Environment, Boeretang 200, 2400 Mol, Belgium buniversity of Louvain, Department of Chemical Engineering, de Croylaan 46, 3001 Heverlee, Belgium Abstract To minimize the production of residues in municipal waste incineration, concepts are developed, which link fly ash extraction and inertizing at metal recovery To facilitate the possibility for metal recovery an extended study has been performed on the leaching behaviour of the fly ash in function of pH, liquid to solid ratio and time Additional interventions, both on the leaching process and the total concept of fly ash treatment, are examined in order to optimize leaching efficiency and final concentrations 1. INTRODUCTION
Up till now the focus within the environmental research field on municipal solid waste incineration (MSWI) has been laid at first on the maximal reduction of toxic components in the flue gas. Because of a more stringent emission-regulation new flue gas cleaning techniques have been developed. As a consequence higher amounts of residuals are produced. To minimize the amount of residues new concepts of gas cleaning in waste incineration are needed. MSWI-fly ash is one of the most critical products produced during the flue gas cleaning process. Due to the high concentration and mobility of several heavy metals the reuse of MSWI-fly ash as a secondary building material is prohibited in Flanders. At this moment only disposal in a covered landfill is allowed. Within the scope of a project on the treatment of MSWI-fly ashes, by order of the Public Waste Company for Flanders (OVAM), a concept is proposed for inertizing fly ash and metal recovery. The basic concept for the treatment of fly ashes is similar to the German 3Rprocess (3R-proces = Rauchgas-Reinigung mit Ruckstandsbehandlung, which means flue gas purification including residue treatment) [I]. In this process, developed by the Nuclear Research Centre in Karlsruhe, an acid extraction is carried out by which the soluble heavy metals 2re leached from the fly ashes to a certain extent, using the acid flue gas scrubbing solution. The contemporary method for the removal of the heavy metals from the extraction solution is a hydroxide precipitation. In the following text the leaching behaviour of fly ash is discussed in function of the development of a concept which integrates the recovery of heavy metals from the extraction solution.
526 2. LEACHING PROPERTIES OF MSWI-FLY ASH
The basic principle of the 3R-process is the extraction of fly ash with the solution from the acid flue gas scrubber. In addition the recovery potential of the metals is examined. To this goal different leaching procedures can be used. To optimize the leaching procedure an extensive study on the composition and the leachability of the fly ash has been carried out. First priority has been given to the influence of pH, extraction time and the liquid solid-ratio on the leaching behaviour of the fly ash. 2.1. Material and methods Samples of MSWI fly ash were obtained from the Houthalen Waste Incineration Facility, a municipal solid waste facility with a annual capacity of 98 000 ton. The fly ash was collected by a classic electrofilter. The chemical composition of the fly ash was measured by inductively coupled emission spectrometry (ICP-MS) after destruction with aqua regia. The extracts obtained from the leaching tests were analyzed by the same method. Simple batch extractions on lab scale were used to characterize the leaching properties. To simulate the acid flue gas scrubber solution, and since the main input of acidity into the scrubber unit is represented by absorbed HCI, synthetic solutions of HCI were used as extraction medium. 2.2. Results of elemental analysis Table 1 shows results of the elemental analysis on the incinerator fly ash samples. Results
are compared with literature data of the elemental analysis of the fly ash used during experiments with the German 3R-process [I]. We note a generally good agreement among the element composition of the fly ash used in both experments with the exception of Ca, Fe, Cr and Sn. Especially the difference in the amount of Ca in the fly ash is important since Ca is the most important element with respect t o neutralization capacity and leaching behaviour. Table 1 Composition of the fly ashes (mg/kg d.m.) Parameter This work Ref [I] Parameter Al 85 000 a2 000 Cr Ca 187 900 89 000 cu Fe 11 250 30 000 Ni Pb 13 000 16 700 Mg As 115 100 Sn Cd 305 280 Zn
This work 120 810
215 3 250 500 1 1 970
Ref [I] 810 1100 140
5 300
1800 16 000
2.3. Results of the leaching tests
2.3.1. Leachability in function of pH Influence of pH on the leaching of several elements has been measured during a set of extraction procedures with different additions of acid. Each extraction is carried out at a liquid solid ratio of 10 (L/S=lO Vkg) during 3 hours.
527
Figure 1 shows the evolution of the final pH after extraction. Figure 2 and 3 show the leaching eficiency in hnction of the "acid dose". Acid dose (AD) is the ratio of added Hfions to the amount of fly ash (mol H+/kg fly ash)
0
,
,
I
1
,
,
,
2
4
6
8
10
12
14
@ , * ~ ,, 16
1E
20
acld dose (mol/kg)
Figure 1: Final pH in function of the acid dose (molkg), L/S=10 Vkg, time=3hrs.
Between acid dose 0 and 4 mol/kg the pH decreases rapidly. Between acid dose 4 en 12 mol/kg the pH decreases slowly. The added acid is neutralised by the dissolution of basic metal salts. During the strong decrease between acid dose 0 and 4, there is only little dissolution of metal salts. The added acid is not neutralised, so the pH decreases.
60
60
40
40
Pb I
20
20
0
2
4
6
8
10
12
14
16
Acld dose (rnollkg)
Figure 2 Leaching efficiency (%) of Pb, Cu, Cd and Zn in function of the acid dose (moVkg)
18
I0
2
0 0
0
L 2
4
1
6
, 8
,
,
,
,
,
I4
16
18
10
12
jzo 0 20
A d d do80 (rnollkg)
Figure 3 Leaching efticienq (YO) of Al. Ca, Fe, Mn and Mg in function of the acid dose (moVkg)
Both Zn and Cd have a good leaching efficiency even at low acid doses. Pb and Cu however can only be leached out at high acid doses.
528
The chloride saltsof both Cu and Pb have a low solubility, but at high CI--concentration both metals can form complexes resulting in a higher solubility. The CI-concentration is only at high acid doses high enough for the complexation. Also the lower pH gives a better solubility. Ca ,Al and Mn leach gradually until almost complete removal is established. Mg and Fe only leach for 50%. These 5 metals are not important for neither toxicity nor possibility for recovery, but they play an important role in the leaching process because of their high amounts in fly ash. In general the metals can be divided in 3 groups according to their leached amount (mg leached/kg fly ash): 1. Ca, Al, Na, K 2. Zn, Mg, Fe, Pb 3. Mn, Cu, Cd An optimal acid dose for a maximum leaching efficiency of Pb, Cu, Zn and Cd with a minimal cost is situated at 6 moVkg for a liquid solid ratio of 10 Vkg. This means a leaching solution with a HCI concentration of 0.6 mol/l. 2.3.2. Leachability in function of liquid to solid ratio
The influence of the liquid solid ratio is measured by changing the volume of the leaching solution while the acid dose is kept constant. This means that the same amount of acid is added to a changing volume of water. This experiment is carried out for several acid doses. The leaching time was 3 hours. Figure 4 shows the final pH and figures 5 and 6 show the leaching efficiency in fimction of the liquid solid ratio.
0' 0
' 10
2
20
"
30
40
"
50
60
'
70
'
80
"
90
100
ilquld solld ratio (Vkg)
Figure 4 :Final pH in function of the liquid solid ratio (mg) for different acid doses (moVkg).
The final pH stays constant for acid doses 0, 2 and 6 moVkg. Acid dose 10 moVkg gives an increasing pH in fknction of the liquid solid ratio. The most abundant metals present are almost completely leached out at an acid dose of 10 which results in some unused acid. The resulting pH increases therefore with increasing liquid solid ratio and depends on the composition of the used fly ash.
529
11 0
"
10
10
"
30
40
"
50
60
"
70
80
' 90
0' 0
1.1 100
"
10
llquld solld rallo (I/kp)
10
'
30
1
40
1
SO
"
'
I0
70
80
' 90
10 100
llquld solld ratlo (I/kg)
Figure 5 :Leaching efficiency (%) of Zn, Pb, Cu and Cd in function of the liquid solid ratio (vkg) for an acid dose of 0 movkg.
Figure 6 :Leaching eaciency (YO) of Zn, Pb, Cu and Cd in function of the liquid solid ratio (vkg) for an acid dose of 10 movkg.
The metals can be divided into 2 groups. The first group comprises the metals that show a constant leaching efficiency in function of liquid solid ratio. The concentration of the metals in the leaching solution decreases because of the dilution at higher liquid solid ratios. A second group of metals possesses an increasing leaching efficiency in function of the liquid solid ratio. The concentration decreases but not so fast as the concentrations of group 1. The increasing leaching efficiency points to a solubility restriction. When the volume of the leaching solution increases, more metal can be dissolved before the solubility product of the metal salt is reached. Table 2 shows the distribution of the metals over both groups in function of the changing acid dose. Table 2 Distribution of the metals over 2 groups in function of the behaviour of their leaching efficiency in function of the liquid solid ratio.( l=constant leaching - efficiency in function ofL/S, i=increasing leaching kfficiency in function of US)
I
Na AD=OIl A D = AD=61 D = l 1
K
1 2 1 1
Mg Mn 2 2 1 1 1 1 1 1 1
Cd Pb 2 2 1 1 1 1 1 1 1
Al 2 2 1 1
Ca 2 2 1 1
Fe 2 2 1 1
Co 2 2 1
Ni 2 2 1
Zn 2 2 1
Cr 2 2 2
1
1
1
1
Cu 2 2 2 2
If one wants to remove as much heavy metals as possible, a high leaching efficiency is needed, thus a high liquid solid ratio. When recovery of heavy metals is in view, a high concentration is recommended, thus a low liquid solid ratio. Most of the time a compromise between recovery and removal of heavy metals is needed. Table 3 shows the optimal liquid solid ratio for Zn as a function of the used acid dose. Each time an acid dose is searched where both the concentration and the leaching efficiency are as high as possible. Also the concentration of the leaching solution is included. Table 3 Optimal liquid solid ratio (Vkg) for various acid doses (movkg). AD (rnoVkg) L/S (vkg) concentration (moVI) 2 13.6 0.15 9.1 0.65 6 10 6.5 1.50
530 2.3.3. Leachability in function of the extraction time During this experiment the extraction time varies from 1 minute to 24 hours with a constant acid dose and liquid solid ratio. The experiment is repeated for different liquid solid ratios and acid doses. Figure 7 shows the final pH for an acid dose of 0 moYkg and figure 8 for an acid dose of 2 moVkg
'i : 7l 8
+
..
f
.
::I:::
.
10
100
x US=100 1000
10000
100000
llrne (sac)
Figure 7 :Final pH in function of the extraction time (sec.) for various liquid solid ratios (Ukg) and for an acid dose of 0 moykg.
1 :I 10
i, 100
,
,
1000
10000
1
x USrlOl 100000
Urns (800)
Figure 8 :Final pH in function of the extraction time (sec.) for various liquid solid ratios (Ukg) and for an acid dose of 2 moUkg.
For acid dose 0 moVkg the final pH increases until a maximum is reached after 3 hours. At short extraction times the pH increases also with increasing liquid solid ratio. This increase disappears when the maximum pH is reached. For acid dose 2 moVkg the final pH increases in function of the extraction time and stays constant in function of the liquid solid ratio. The experiments of the previous paragraphs were carried out during 3 hours, thus the combined influence of extraction time and liquid solid ratio is eliminated. The metals can be divided into 3 groups according to the behaviour of their leaching efficiency and concentration in the leaching solution in function of the extraction time. The first group holds the metals with a constant leaching efficiency in function of the extraction time. The concentration decreases in hnction of the liquid solid ratio while the leaching efficiency remains constant. Fot these metals there is no solubility restriction and the decrease in concentration is only caused by dilution at higher liquid solid ratios. The second group holds the metals with a decreasing leaching efficiency in function of the extraction time, as shown in figures 8 and 9. This evolution is the strongest with a liquid solid ratio of 5 I/kg. The concentration decreases in function of the liquid solid ratio while the leaching efficiency increases. The metals are influenced by a solubility restriction.
53 1
. us=s + LIS-25
US=50
lo
6tmUS=75
x L/SIlOO 0 10
-
* U6=50
. . . ..
1,
,+
100
I
,
1000
i
-i
+++++++
us-75
+
A
10000
100000
lime (sac)
Lima (sac)
Figure 8 :Leaching efficiency (%) of Zn in function of the extraction time (sec.) for various liquid solid ratios (lkg) an for an acid dose of 0 mol/kg.
Figure 9: Concentration (mmol/l) of Zn in the leaching solution in function of the extraction time (sec.) for various liquid solid ratios (lkg) and for an acid dose of 0 molikg.
The last group contains the metals which show an increasing leaching efficiency, as shown in figures 10 and 11. Several metals have a minimal solubility in function of pH. Because the pH increases during the process, the metal salts reach a minimum solubility. When the pH increases hrther, more metal salts can dissolve which results in an increasing leaching efficiency. Table 4 shows the distribution of the metals over the three groups,
00"cmd.IIWI
'
5
(nmol I)
I]
us=5
'+ us=x
5-X.
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i
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+A+++++. .
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.
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1
0.01 0
0
ia
i , 100
i
. ..
l
..
:*tat*::
j
1000
10000
;
~oopoo
lime (sac)
Figure 11 :Concentration (mmoUI) of Cr in the leaching solution in function of the extraction time (sec.) for various liquid solid ratios (l/kg) and for an acid dose of 2 moUkg.
AD=O molkg AD=2 moUkg
I Na, K, Fe
Ca, K, Na, Ni, Fe, Cr
I1 Mg, Ca, Mn, Ni, Cu, Zn, Cd, Pb Zn, Cu, Al, Pb, Cd, Co, Mn
I11
Al, Cr
Mg
532 3. ADAPTION OF THE PROCESS FOR FLY ASH TREATMENT IN FUNCTION OF METAL RECOVERY 3.1. Introduction The concept for the treatment of fly ash by acid leaching has to include the possibility for metal recovery from the leachate. First priority is given to the recovery of Zn, Cd, Pb and Cu. One of the parameters that effects the potential for recovery is the concentration of the metals in the solution. Table 5 shows the concentration of the 4 mentioned metals after optimization of the one step leaching procedure in hnction of pH and liquid to solid ratio. Except for Zn we notice too low concentrations of dissolved metals to perform metal recovery with sufficient efficiency by most methods. Many more recovery technics with high efficiency are available when the concentration of the metals exceeds a treshold of 0.5 g/l.
Table 5 Concentrations of metals in the leachate (AD = 6 mol/kg, L/S = 9.1 Vkg) Parameter Concentration mgll Zn 806 Cd 21 Pb 18 cu I To facilitate the metal recovery there is a need for additional interventions to increase the metal concentration. Two possibilities have been examined. A first possibility is the addition of additives to improve the leaching efficiency. Secondly, one can increase the concentration by recirculating the leachate 3.2. Increasing of the leaching efficiency by means of EDTA addition
.it
+
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+
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+
00
+ 101
20
+: : I .+. +. .+ 10
+ + + +
+
+
l.1 t
i
. . . .
.
'
0 g EDTA
+ lopEDTA
0 0
2
4
6
1
.p + lo 0 EDTil
10
Add do..
Figure 12 :Leaching efficiency (%) of Cu in function of the acid dose (molkg) with and without the addition of EDTA
12
14
16
11
10
(rnollkp)
Figure 13 :Leaching efficiency (%) of Pb in function of the acid dose (molkg)with and without the addition of EDTA
533 A difficulty in the leaching process is the restricted solubility of some metal salts. Pb and Cu for example can only leach out at high acid doses. A possibility to increase the solubility is the addition of a complexing reagent. EDTA is a good example and is used in this research. The addition of EDTA gave for most of the metals an increase in leachability. Especially the results for Cu and Pb are good and they can now be leached out at low acid doses as shown in figures 12 and 13. However more research is needed. 3.3. Influence of recirculation on the metal concentration A second method to increase the metal concentration is the re-use of the leachate in successive extraction steps. The number of successive extraction steps, and so the extent for the increase of the metal concentration is determined by the recirculation factor. The parameters that influence this factor depends on the process principle. Recirculation factors have been calculated for two different concepts.
3.3.1. Concept 1 The flow sheet ofthe first concept is shown in Figure 14. The leachate is directly recirculated to the extraction unit after acidification with fresh acid scrubber solution. In this case the recirculation factor depends on both the amount of acid scrubber solution needed to reach the required acid dose for extraction and the solubility of the metals in the chloride-solution.
Flu. G u I
flue G u
Figure 14 Process flow of the first concept
534 In this concept the pH value in the acid scrubber circuit has to be kept as low as possible. In practice the pH value of the acid flue gas scrubber solution is limited to pH 0.5 -0.3 due to corrosion problems [2]. By using NaOH in the acid scrubbing unit for partial neutralization (and minimization of water consumption) pH-control of the acid scrubber solution is possible. When assuming an initial pH for the extraction equal to 1 (0.1 mol H+/I) and considering a minimum acid dose of 4 mol H+/kg fly ash to reach sufficient leaching efficiency (although AD = 6 is better) , the liquid solid ratio must be equal to 40. From the leaching test we learned that in this case the pH of the extraction solution is equal to 4. Using the following equation we can calculate for this concept a recirculation factor between 3 and 5.
with Ca Ci Cr
:
: acid concentration (mol H+/I) in the acid flue gas scrubber solution : initial acid concentration (mol H+/I) of the extraction step : final acid concentration after extraction (mol H+/I)
The effect on the metal concentration in the solution for the metal recovery is shown in Table 6. As a result we notice a rather small effect on the concentration. Table 6 Effect of direct recirculation of the leachate on the metal concentration Parameter Leachability in one Recirculation factor Final concentration step extraction L/S = 40, initial pH = 1 final pH = 4 3-5 15 - 25 mg/l cu 5 mg/l Pb 13 mg/l 3-5 39 - 65 mg/l Zn 960 mg/l 3-5 2 880 - 4 800 mg/l Cd 12 mg/l 3-5 36 - 60 mg/l 3.3.2. Concept 2 Minimizing the water consumption in the scrubber unit is possible by recirculation of the scrubber solution. Since the pH value in the unit is limited to 0.5 - 0.3 partial neutralisation is required. The recirculation factor is in this case determined by the chloride concentration. In some incinerators (e.g. MSWI Iserlohn, Germany) the water consumption in the acid scrubber unit kan be restricted down to 100 Vton waste by neutralisation with sodium hydroxide. This corresponds to a maximum chloride concentration of 100 gA. In a second concept the extraction of fly ash is totally integrated in the scrubber unit. By using the fly ash basicity, the consumption of neutralizing agent can be minimized. No additional neutralization is needed in a concept which integrates fly ash extraction and electrolytic chlorine recovery in one process (see Figure 15).
535
I
I
L
Ll
Figure 15 Process flow of the second concept Experiments in a bench-scale electrolyzer performed at the Karlsruhe Nuclear Research Centre proved the technical feasibility of this electrochemical recovery process [2]. Critical factor for the recirculation in this concept however is the solubility of the metal chlorides. One must avoid precipitation in the scrubber solution due to accumulation of the metals during the recirculation. Most critical elements are Cu and Pb. The recirculation factor in function of the solubility of these two elements has been calculated for two different process conditions. In the first example pH value in the acid scrubber circuit is kept on 0.3 which corresponds with a concentration of 0.5 mol HCIA In the second example the pH-value is kept on 1 . To reach sufficient leaching efficiency a minimum acid dose of 4 mol Hf/kg fly ash is required (final pH after extraction equal to 4). The results are shown in Table 7. In both situations different parameters are most critical. Solubility of PbC12 is limiting the recirculation if the pH value in the scrubber unit is kept equal to 0,3. In the second situation (pH = 1) Pb becomes more soluble due to the decrease of the chloride concentration. Cu, however becomes less soluble, since solubility of Cu is mainly depending on the formation of chloride complexes. As a consequence the recirculation factor of the second process is depending on the solubility of Cu.
536 Table 7 Calculation of the recirculation factor as a function of solubility Example 1 Parameter Maximum Leachability solubility LIS = 10 [HCl] = 0.5 M initial pH = 0.3 final pH = 4 Pb 423 mg/l 31 mg/l 6.5 mg/l cu 3 llOmg/l Example 2 Parameter Maximum Leachability solubility LIS = 40 initial pH = 1 [HCI] = 0.1 M final pH = 4 Pb 1 378 mg/l 13 mg/l cu 410 mg/l 5 mg/l
Recirculation factor 13.6 478 Recirculation factor 106 82
Effects on the metal concentration in the solution are summerized in Table 8. We notice a significant increase of the concentration, which gives much more perspectives in view of recovery potential. However more research is needed to examine the chlorine recovery from this specific solution. Table 8 Influence of the integration of fly ash treatment in the acid scrubber unit on the metal concentration in the solution prepared for recovery Metal concentration in solution prepared for recovery Parameter Example 1 Example 2 Recirculation factor = 82 Recirculation factor = 13.6 1 066 mgll Pb 423 mg/l cu 89 mg/l 410 mg/l Cd 313 mg/l 984 mg/l Zn 11 000 mg/l 79 000 mg/l 4. CONCLUSIONS
In view of the reduction of municipal waste incineration residues concepts are developed which link fly ash extraction, using the acid flue gas scrubbing solution, at metal recovery. To reach this goal the leaching process must be optimised in terms of leaching efficiency and final concentration of metals in the leachate. One of the most important factors of the leaching process is the solubility of the metal salts. This is shown by examining the role of pH, liquid solid ratio and extraction time. Especially the experiment with varying liquid to solid ratio showed the importance of the solubility. When the liquid solid ratio increases the leaching efficiency increases since more metal salts can dissolve before solubility restriction is reached. By decreasing the pH, the solubility and so the leaching efficiency of the metal salts are increased. Several metals show a decreasing leaching efficiency as a function of the extraction time due to a increasing pH value.
531
Since the process links extraction at metal recovery a high concentration of dissolved metals is also needed. First priority is given to the recovery of Zn, Cd, Pb and Cu. Especially Cu(I1) and Pb give problems in view of potential recovery because of the limited solubility of their salts. The solubility of these elements can be improved by the use of complexing reagents. Positive results are noticed with EDTA. The increase of the final metal concentration in the solution can also be obtained in a total different way By using the fly ash extraction as a neutralising step in the acid scrubber unit and in combination with an electrolytic chlorine recovery process the leachate can be recirculated several times. During the recirculation metals are accumulated. The recirculation factor depends on the solubility of the metal salts. Using this concept the metal concentration of the most critical metals (Cu and Pb) can be increased up to 0.5 - 1 g/l which facilitates their metal recovery. 5. REFERENCES
1 Vehlow, Braun, Horch, Merz, Scneider, Stieglitz and Vogg, Semi Technical demonstration of the 3R-process, Waste Management & Research No. 8 (l990), 461-472. 2 Volkmann, Vehlow, Vogg, Improvement of flue gas cleaning concepts in MSWI and uti-
lization of by-products, Studies in Environmental Science 48, Waste Materials in Construction, Elsevier (1991), 145-152 3 Laethem, Elslander, Kinnaer, Geuzens, Geintegreerde venverking van reststoffen van huisvuilverbrandingsinstallaties,VITO (l993), 120.
525
Integrated treatment of MSWI-residues Treatment of fly ash in view of metal recovery B. Laethema, P. Van Herckb, P. Geuzensa, C. Vandecasteeleb aFlemish Institute for Technological Research, Department of Environment, Boeretang 200, 2400 Mol, Belgium buniversity of Louvain, Department of Chemical Engineering, de Croylaan 46, 3001 Heverlee, Belgium Abstract To minimize the production of residues in municipal waste incineration, concepts are developed, which link fly ash extraction and inertizing at metal recovery To facilitate the possibility for metal recovery an extended study has been performed on the leaching behaviour of the fly ash in function of pH, liquid to solid ratio and time Additional interventions, both on the leaching process and the total concept of fly ash treatment, are examined in order to optimize leaching efficiency and final concentrations 1. INTRODUCTION
Up till now the focus within the environmental research field on municipal solid waste incineration (MSWI) has been laid at first on the maximal reduction of toxic components in the flue gas. Because of a more stringent emission-regulation new flue gas cleaning techniques have been developed. As a consequence higher amounts of residuals are produced. To minimize the amount of residues new concepts of gas cleaning in waste incineration are needed. MSWI-fly ash is one of the most critical products produced during the flue gas cleaning process. Due to the high concentration and mobility of several heavy metals the reuse of MSWI-fly ash as a secondary building material is prohibited in Flanders. At this moment only disposal in a covered landfill is allowed. Within the scope of a project on the treatment of MSWI-fly ashes, by order of the Public Waste Company for Flanders (OVAM), a concept is proposed for inertizing fly ash and metal recovery. The basic concept for the treatment of fly ashes is similar to the German 3Rprocess (3R-proces = Rauchgas-Reinigung mit Ruckstandsbehandlung, which means flue gas purification including residue treatment) [I]. In this process, developed by the Nuclear Research Centre in Karlsruhe, an acid extraction is carried out by which the soluble heavy metals 2re leached from the fly ashes to a certain extent, using the acid flue gas scrubbing solution. The contemporary method for the removal of the heavy metals from the extraction solution is a hydroxide precipitation. In the following text the leaching behaviour of fly ash is discussed in function of the development of a concept which integrates the recovery of heavy metals from the extraction solution.
526 2. LEACHING PROPERTIES OF MSWI-FLY ASH
The basic principle of the 3R-process is the extraction of fly ash with the solution from the acid flue gas scrubber. In addition the recovery potential of the metals is examined. To this goal different leaching procedures can be used. To optimize the leaching procedure an extensive study on the composition and the leachability of the fly ash has been carried out. First priority has been given to the influence of pH, extraction time and the liquid solid-ratio on the leaching behaviour of the fly ash. 2.1. Material and methods Samples of MSWI fly ash were obtained from the Houthalen Waste Incineration Facility, a municipal solid waste facility with a annual capacity of 98 000 ton. The fly ash was collected by a classic electrofilter. The chemical composition of the fly ash was measured by inductively coupled emission spectrometry (ICP-MS) after destruction with aqua regia. The extracts obtained from the leaching tests were analyzed by the same method. Simple batch extractions on lab scale were used to characterize the leaching properties. To simulate the acid flue gas scrubber solution, and since the main input of acidity into the scrubber unit is represented by absorbed HCI, synthetic solutions of HCI were used as extraction medium. 2.2. Results of elemental analysis Table 1 shows results of the elemental analysis on the incinerator fly ash samples. Results
are compared with literature data of the elemental analysis of the fly ash used during experiments with the German 3R-process [I]. We note a generally good agreement among the element composition of the fly ash used in both experments with the exception of Ca, Fe, Cr and Sn. Especially the difference in the amount of Ca in the fly ash is important since Ca is the most important element with respect t o neutralization capacity and leaching behaviour. Table 1 Composition of the fly ashes (mg/kg d.m.) Parameter This work Ref [I] Parameter Al 85 000 a2 000 Cr Ca 187 900 89 000 cu Fe 11 250 30 000 Ni Pb 13 000 16 700 Mg As 115 100 Sn Cd 305 280 Zn
This work 120 810
215 3 250 500 1 1 970
Ref [I] 810 1100 140
5 300
1800 16 000
2.3. Results of the leaching tests
2.3.1. Leachability in function of pH Influence of pH on the leaching of several elements has been measured during a set of extraction procedures with different additions of acid. Each extraction is carried out at a liquid solid ratio of 10 (L/S=lO Vkg) during 3 hours.
527
Figure 1 shows the evolution of the final pH after extraction. Figure 2 and 3 show the leaching eficiency in hnction of the "acid dose". Acid dose (AD) is the ratio of added Hfions to the amount of fly ash (mol H+/kg fly ash)
0
,
,
I
1
,
,
,
2
4
6
8
10
12
14
@ , * ~ ,, 16
1E
20
acld dose (mol/kg)
Figure 1: Final pH in function of the acid dose (molkg), L/S=10 Vkg, time=3hrs.
Between acid dose 0 and 4 mol/kg the pH decreases rapidly. Between acid dose 4 en 12 mol/kg the pH decreases slowly. The added acid is neutralised by the dissolution of basic metal salts. During the strong decrease between acid dose 0 and 4, there is only little dissolution of metal salts. The added acid is not neutralised, so the pH decreases.
60
60
40
40
Pb I
20
20
0
2
4
6
8
10
12
14
16
Acld dose (rnollkg)
Figure 2 Leaching efficiency (%) of Pb, Cu, Cd and Zn in function of the acid dose (moVkg)
18
I0
2
0 0
0
L 2
4
1
6
, 8
,
,
,
,
,
I4
16
18
10
12
jzo 0 20
A d d do80 (rnollkg)
Figure 3 Leaching efticienq (YO) of Al. Ca, Fe, Mn and Mg in function of the acid dose (moVkg)
Both Zn and Cd have a good leaching efficiency even at low acid doses. Pb and Cu however can only be leached out at high acid doses.
528
The chloride saltsof both Cu and Pb have a low solubility, but at high CI--concentration both metals can form complexes resulting in a higher solubility. The CI-concentration is only at high acid doses high enough for the complexation. Also the lower pH gives a better solubility. Ca ,Al and Mn leach gradually until almost complete removal is established. Mg and Fe only leach for 50%. These 5 metals are not important for neither toxicity nor possibility for recovery, but they play an important role in the leaching process because of their high amounts in fly ash. In general the metals can be divided in 3 groups according to their leached amount (mg leached/kg fly ash): 1. Ca, Al, Na, K 2. Zn, Mg, Fe, Pb 3. Mn, Cu, Cd An optimal acid dose for a maximum leaching efficiency of Pb, Cu, Zn and Cd with a minimal cost is situated at 6 moVkg for a liquid solid ratio of 10 Vkg. This means a leaching solution with a HCI concentration of 0.6 mol/l. 2.3.2. Leachability in function of liquid to solid ratio
The influence of the liquid solid ratio is measured by changing the volume of the leaching solution while the acid dose is kept constant. This means that the same amount of acid is added to a changing volume of water. This experiment is carried out for several acid doses. The leaching time was 3 hours. Figure 4 shows the final pH and figures 5 and 6 show the leaching efficiency in fimction of the liquid solid ratio.
0' 0
' 10
2
20
"
30
40
"
50
60
'
70
'
80
"
90
100
ilquld solld ratio (Vkg)
Figure 4 :Final pH in function of the liquid solid ratio (mg) for different acid doses (moVkg).
The final pH stays constant for acid doses 0, 2 and 6 moVkg. Acid dose 10 moVkg gives an increasing pH in fknction of the liquid solid ratio. The most abundant metals present are almost completely leached out at an acid dose of 10 which results in some unused acid. The resulting pH increases therefore with increasing liquid solid ratio and depends on the composition of the used fly ash.
529
11 0
"
10
10
"
30
40
"
50
60
"
70
80
' 90
0' 0
1.1 100
"
10
llquld solld rallo (I/kp)
10
'
30
1
40
1
SO
"
'
I0
70
80
' 90
10 100
llquld solld ratlo (I/kg)
Figure 5 :Leaching efficiency (%) of Zn, Pb, Cu and Cd in function of the liquid solid ratio (vkg) for an acid dose of 0 movkg.
Figure 6 :Leaching eaciency (YO) of Zn, Pb, Cu and Cd in function of the liquid solid ratio (vkg) for an acid dose of 10 movkg.
The metals can be divided into 2 groups. The first group comprises the metals that show a constant leaching efficiency in function of liquid solid ratio. The concentration of the metals in the leaching solution decreases because of the dilution at higher liquid solid ratios. A second group of metals possesses an increasing leaching efficiency in function of the liquid solid ratio. The concentration decreases but not so fast as the concentrations of group 1. The increasing leaching efficiency points to a solubility restriction. When the volume of the leaching solution increases, more metal can be dissolved before the solubility product of the metal salt is reached. Table 2 shows the distribution of the metals over both groups in function of the changing acid dose. Table 2 Distribution of the metals over 2 groups in function of the behaviour of their leaching efficiency in function of the liquid solid ratio.( l=constant leaching - efficiency in function ofL/S, i=increasing leaching kfficiency in function of US)
I
Na AD=OIl A D = AD=61 D = l 1
K
1 2 1 1
Mg Mn 2 2 1 1 1 1 1 1 1
Cd Pb 2 2 1 1 1 1 1 1 1
Al 2 2 1 1
Ca 2 2 1 1
Fe 2 2 1 1
Co 2 2 1
Ni 2 2 1
Zn 2 2 1
Cr 2 2 2
1
1
1
1
Cu 2 2 2 2
If one wants to remove as much heavy metals as possible, a high leaching efficiency is needed, thus a high liquid solid ratio. When recovery of heavy metals is in view, a high concentration is recommended, thus a low liquid solid ratio. Most of the time a compromise between recovery and removal of heavy metals is needed. Table 3 shows the optimal liquid solid ratio for Zn as a function of the used acid dose. Each time an acid dose is searched where both the concentration and the leaching efficiency are as high as possible. Also the concentration of the leaching solution is included. Table 3 Optimal liquid solid ratio (Vkg) for various acid doses (movkg). AD (rnoVkg) L/S (vkg) concentration (moVI) 2 13.6 0.15 9.1 0.65 6 10 6.5 1.50
530 2.3.3. Leachability in function of the extraction time During this experiment the extraction time varies from 1 minute to 24 hours with a constant acid dose and liquid solid ratio. The experiment is repeated for different liquid solid ratios and acid doses. Figure 7 shows the final pH for an acid dose of 0 moYkg and figure 8 for an acid dose of 2 moVkg
'i : 7l 8
+
..
f
.
::I:::
.
10
100
x US=100 1000
10000
100000
llrne (sac)
Figure 7 :Final pH in function of the extraction time (sec.) for various liquid solid ratios (Ukg) and for an acid dose of 0 moykg.
1 :I 10
i, 100
,
,
1000
10000
1
x USrlOl 100000
Urns (800)
Figure 8 :Final pH in function of the extraction time (sec.) for various liquid solid ratios (Ukg) and for an acid dose of 2 moUkg.
For acid dose 0 moVkg the final pH increases until a maximum is reached after 3 hours. At short extraction times the pH increases also with increasing liquid solid ratio. This increase disappears when the maximum pH is reached. For acid dose 2 moVkg the final pH increases in function of the extraction time and stays constant in function of the liquid solid ratio. The experiments of the previous paragraphs were carried out during 3 hours, thus the combined influence of extraction time and liquid solid ratio is eliminated. The metals can be divided into 3 groups according to the behaviour of their leaching efficiency and concentration in the leaching solution in function of the extraction time. The first group holds the metals with a constant leaching efficiency in function of the extraction time. The concentration decreases in hnction of the liquid solid ratio while the leaching efficiency remains constant. Fot these metals there is no solubility restriction and the decrease in concentration is only caused by dilution at higher liquid solid ratios. The second group holds the metals with a decreasing leaching efficiency in function of the extraction time, as shown in figures 8 and 9. This evolution is the strongest with a liquid solid ratio of 5 I/kg. The concentration decreases in function of the liquid solid ratio while the leaching efficiency increases. The metals are influenced by a solubility restriction.
53 1
. us=s + LIS-25
US=50
lo
6tmUS=75
x L/SIlOO 0 10
-
* U6=50
. . . ..
1,
,+
100
I
,
1000
i
-i
+++++++
us-75
+
A
10000
100000
lime (sac)
Lima (sac)
Figure 8 :Leaching efficiency (%) of Zn in function of the extraction time (sec.) for various liquid solid ratios (lkg) an for an acid dose of 0 mol/kg.
Figure 9: Concentration (mmol/l) of Zn in the leaching solution in function of the extraction time (sec.) for various liquid solid ratios (lkg) and for an acid dose of 0 molikg.
The last group contains the metals which show an increasing leaching efficiency, as shown in figures 10 and 11. Several metals have a minimal solubility in function of pH. Because the pH increases during the process, the metal salts reach a minimum solubility. When the pH increases hrther, more metal salts can dissolve which results in an increasing leaching efficiency. Table 4 shows the distribution of the metals over the three groups,
00"cmd.IIWI
'
5
(nmol I)
I]
us=5
'+ us=x
5-X.
LlS-50
xu(
US176 -2
x x
us-100
#
3-
21-
.
i
.I#*
+A+++++. .
..
*.
n
+
1+
.
m
a
LIS=50
xx
0.02
.
x
1
0.01 0
0
ia
i , 100
i
. ..
l
..
:*tat*::
j
1000
10000
;
~oopoo
lime (sac)
Figure 11 :Concentration (mmoUI) of Cr in the leaching solution in function of the extraction time (sec.) for various liquid solid ratios (l/kg) and for an acid dose of 2 moUkg.
AD=O molkg AD=2 moUkg
I Na, K, Fe
Ca, K, Na, Ni, Fe, Cr
I1 Mg, Ca, Mn, Ni, Cu, Zn, Cd, Pb Zn, Cu, Al, Pb, Cd, Co, Mn
I11
Al, Cr
Mg
532 3. ADAPTION OF THE PROCESS FOR FLY ASH TREATMENT IN FUNCTION OF METAL RECOVERY 3.1. Introduction The concept for the treatment of fly ash by acid leaching has to include the possibility for metal recovery from the leachate. First priority is given to the recovery of Zn, Cd, Pb and Cu. One of the parameters that effects the potential for recovery is the concentration of the metals in the solution. Table 5 shows the concentration of the 4 mentioned metals after optimization of the one step leaching procedure in hnction of pH and liquid to solid ratio. Except for Zn we notice too low concentrations of dissolved metals to perform metal recovery with sufficient efficiency by most methods. Many more recovery technics with high efficiency are available when the concentration of the metals exceeds a treshold of 0.5 g/l.
Table 5 Concentrations of metals in the leachate (AD = 6 mol/kg, L/S = 9.1 Vkg) Parameter Concentration mgll Zn 806 Cd 21 Pb 18 cu I To facilitate the metal recovery there is a need for additional interventions to increase the metal concentration. Two possibilities have been examined. A first possibility is the addition of additives to improve the leaching efficiency. Secondly, one can increase the concentration by recirculating the leachate 3.2. Increasing of the leaching efficiency by means of EDTA addition
.it
+
+ '
+
+ +.
+
00
+ 101
20
+: : I .+. +. .+ 10
+ + + +
+
+
l.1 t
i
. . . .
.
'
0 g EDTA
+ lopEDTA
0 0
2
4
6
1
.p + lo 0 EDTil
10
Add do..
Figure 12 :Leaching efficiency (%) of Cu in function of the acid dose (molkg) with and without the addition of EDTA
12
14
16
11
10
(rnollkp)
Figure 13 :Leaching efficiency (%) of Pb in function of the acid dose (molkg)with and without the addition of EDTA
533 A difficulty in the leaching process is the restricted solubility of some metal salts. Pb and Cu for example can only leach out at high acid doses. A possibility to increase the solubility is the addition of a complexing reagent. EDTA is a good example and is used in this research. The addition of EDTA gave for most of the metals an increase in leachability. Especially the results for Cu and Pb are good and they can now be leached out at low acid doses as shown in figures 12 and 13. However more research is needed. 3.3. Influence of recirculation on the metal concentration A second method to increase the metal concentration is the re-use of the leachate in successive extraction steps. The number of successive extraction steps, and so the extent for the increase of the metal concentration is determined by the recirculation factor. The parameters that influence this factor depends on the process principle. Recirculation factors have been calculated for two different concepts.
3.3.1. Concept 1 The flow sheet ofthe first concept is shown in Figure 14. The leachate is directly recirculated to the extraction unit after acidification with fresh acid scrubber solution. In this case the recirculation factor depends on both the amount of acid scrubber solution needed to reach the required acid dose for extraction and the solubility of the metals in the chloride-solution.
Flu. G u I
flue G u
Figure 14 Process flow of the first concept
534 In this concept the pH value in the acid scrubber circuit has to be kept as low as possible. In practice the pH value of the acid flue gas scrubber solution is limited to pH 0.5 -0.3 due to corrosion problems [2]. By using NaOH in the acid scrubbing unit for partial neutralization (and minimization of water consumption) pH-control of the acid scrubber solution is possible. When assuming an initial pH for the extraction equal to 1 (0.1 mol H+/I) and considering a minimum acid dose of 4 mol H+/kg fly ash to reach sufficient leaching efficiency (although AD = 6 is better) , the liquid solid ratio must be equal to 40. From the leaching test we learned that in this case the pH of the extraction solution is equal to 4. Using the following equation we can calculate for this concept a recirculation factor between 3 and 5.
with Ca Ci Cr
:
: acid concentration (mol H+/I) in the acid flue gas scrubber solution : initial acid concentration (mol H+/I) of the extraction step : final acid concentration after extraction (mol H+/I)
The effect on the metal concentration in the solution for the metal recovery is shown in Table 6. As a result we notice a rather small effect on the concentration. Table 6 Effect of direct recirculation of the leachate on the metal concentration Parameter Leachability in one Recirculation factor Final concentration step extraction L/S = 40, initial pH = 1 final pH = 4 3-5 15 - 25 mg/l cu 5 mg/l Pb 13 mg/l 3-5 39 - 65 mg/l Zn 960 mg/l 3-5 2 880 - 4 800 mg/l Cd 12 mg/l 3-5 36 - 60 mg/l 3.3.2. Concept 2 Minimizing the water consumption in the scrubber unit is possible by recirculation of the scrubber solution. Since the pH value in the unit is limited to 0.5 - 0.3 partial neutralisation is required. The recirculation factor is in this case determined by the chloride concentration. In some incinerators (e.g. MSWI Iserlohn, Germany) the water consumption in the acid scrubber unit kan be restricted down to 100 Vton waste by neutralisation with sodium hydroxide. This corresponds to a maximum chloride concentration of 100 gA. In a second concept the extraction of fly ash is totally integrated in the scrubber unit. By using the fly ash basicity, the consumption of neutralizing agent can be minimized. No additional neutralization is needed in a concept which integrates fly ash extraction and electrolytic chlorine recovery in one process (see Figure 15).
535
I
I
L
Ll
Figure 15 Process flow of the second concept Experiments in a bench-scale electrolyzer performed at the Karlsruhe Nuclear Research Centre proved the technical feasibility of this electrochemical recovery process [2]. Critical factor for the recirculation in this concept however is the solubility of the metal chlorides. One must avoid precipitation in the scrubber solution due to accumulation of the metals during the recirculation. Most critical elements are Cu and Pb. The recirculation factor in function of the solubility of these two elements has been calculated for two different process conditions. In the first example pH value in the acid scrubber circuit is kept on 0.3 which corresponds with a concentration of 0.5 mol HCIA In the second example the pH-value is kept on 1 . To reach sufficient leaching efficiency a minimum acid dose of 4 mol Hf/kg fly ash is required (final pH after extraction equal to 4). The results are shown in Table 7. In both situations different parameters are most critical. Solubility of PbC12 is limiting the recirculation if the pH value in the scrubber unit is kept equal to 0,3. In the second situation (pH = 1) Pb becomes more soluble due to the decrease of the chloride concentration. Cu, however becomes less soluble, since solubility of Cu is mainly depending on the formation of chloride complexes. As a consequence the recirculation factor of the second process is depending on the solubility of Cu.
536 Table 7 Calculation of the recirculation factor as a function of solubility Example 1 Parameter Maximum Leachability solubility LIS = 10 [HCl] = 0.5 M initial pH = 0.3 final pH = 4 Pb 423 mg/l 31 mg/l 6.5 mg/l cu 3 llOmg/l Example 2 Parameter Maximum Leachability solubility LIS = 40 initial pH = 1 [HCI] = 0.1 M final pH = 4 Pb 1 378 mg/l 13 mg/l cu 410 mg/l 5 mg/l
Recirculation factor 13.6 478 Recirculation factor 106 82
Effects on the metal concentration in the solution are summerized in Table 8. We notice a significant increase of the concentration, which gives much more perspectives in view of recovery potential. However more research is needed to examine the chlorine recovery from this specific solution. Table 8 Influence of the integration of fly ash treatment in the acid scrubber unit on the metal concentration in the solution prepared for recovery Metal concentration in solution prepared for recovery Parameter Example 1 Example 2 Recirculation factor = 82 Recirculation factor = 13.6 1 066 mgll Pb 423 mg/l cu 89 mg/l 410 mg/l Cd 313 mg/l 984 mg/l Zn 11 000 mg/l 79 000 mg/l 4. CONCLUSIONS
In view of the reduction of municipal waste incineration residues concepts are developed which link fly ash extraction, using the acid flue gas scrubbing solution, at metal recovery. To reach this goal the leaching process must be optimised in terms of leaching efficiency and final concentration of metals in the leachate. One of the most important factors of the leaching process is the solubility of the metal salts. This is shown by examining the role of pH, liquid solid ratio and extraction time. Especially the experiment with varying liquid to solid ratio showed the importance of the solubility. When the liquid solid ratio increases the leaching efficiency increases since more metal salts can dissolve before solubility restriction is reached. By decreasing the pH, the solubility and so the leaching efficiency of the metal salts are increased. Several metals show a decreasing leaching efficiency as a function of the extraction time due to a increasing pH value.
531
Since the process links extraction at metal recovery a high concentration of dissolved metals is also needed. First priority is given to the recovery of Zn, Cd, Pb and Cu. Especially Cu(I1) and Pb give problems in view of potential recovery because of the limited solubility of their salts. The solubility of these elements can be improved by the use of complexing reagents. Positive results are noticed with EDTA. The increase of the final metal concentration in the solution can also be obtained in a total different way By using the fly ash extraction as a neutralising step in the acid scrubber unit and in combination with an electrolytic chlorine recovery process the leachate can be recirculated several times. During the recirculation metals are accumulated. The recirculation factor depends on the solubility of the metal salts. Using this concept the metal concentration of the most critical metals (Cu and Pb) can be increased up to 0.5 - 1 g/l which facilitates their metal recovery. 5. REFERENCES
1 Vehlow, Braun, Horch, Merz, Scneider, Stieglitz and Vogg, Semi Technical demonstration of the 3R-process, Waste Management & Research No. 8 (l990), 461-472. 2 Volkmann, Vehlow, Vogg, Improvement of flue gas cleaning concepts in MSWI and uti-
lization of by-products, Studies in Environmental Science 48, Waste Materials in Construction, Elsevier (1991), 145-152 3 Laethem, Elslander, Kinnaer, Geuzens, Geintegreerde venverking van reststoffen van huisvuilverbrandingsinstallaties,VITO (l993), 120.