solidification of hot dip galvanizing ash using different binders

Journal of Hazardous Materials 320 (2016) 105–113

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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Stabilization/solidification of hot dip galvanizing ash using different binders S. Vinter a , M.T. Montanes b , V. Bednarik a,∗ , P. Hrivnova a a

Department of Environment Protection Engineering, Tomas Bata University in Zlin, Faculty of Technology, Vavreckova 275, 760 01, Zlin, Czech Republic Ingeniería Electroquímica y Corrosión. Departamento de Ingeniería Química y Nuclear. Universitat Politécnica de Valencia (Polytechnic University of Valencia), Camino de Vera s/n, 46022, Valencia, Spain b

h i g h l i g h t s • A stabilization/solidification of zinc-containing hazardous waste is studied. • Portland cement and coal fly ashes are used as binders. • Statistical regression analysis is used for finding the best mixture composition.

a r t i c l e

i n f o

Article history: Received 6 May 2016 Received in revised form 3 August 2016 Accepted 7 August 2016 Available online 8 August 2016 Keywords: Hot-dip galvanizing ash Zinc Stabilization/solidification Leaching tests Statistical analysis

a b s t r a c t This study focuses on solidification of hot dip-galvanizing ash with a high content of zinc and soluble substances. The main purpose of this paper is to immobilize these pollutants into a matrix and allow a safer way for landfill disposal of that waste. Three different binders (Portland cement, fly ash and coal fluidized-bed combustion ash) were used for the waste solidification. Effectiveness of the process was evaluated using leaching test according to EN 12457-4 and by using the variance analysis and the categorical multifactorial test. In the leaching test, four parameters were observed: pH, zinc concentration in leachate, and concentration of chlorides and dissolved substances in leachate. The acquired data was then processed using statistical software to find an optimal solidifying ratio of the addition of binder, water, and waste to the mixture, with the aim to fulfil the requirement for landfill disposal set by the Council Decision 2003/33/EC. The influence on the main observed parameters (relative amount of water and a binder) on the effectiveness of the used method and their influence of measured parameters was also studied. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Metal industries, such as galvanizing, casting, smelting, and several others produce a large amount of waste containing high content of zinc. It is estimated that about 55% of zinc resources are used for galvanization. This technique is a multilevel process. As the first step, steel pieces are chemically degreased in a bath containing bacteria and surfactants to easily remove oil and fat, after that, the rust on surfaces is removed, and then they are put into a bath of molten zinc at 450 ◦ C. By this process, the pieces gain a thin layer of zinc (150 ␮m) on their surfaces. This procedure produces two types of waste. The first type of waste is a sludge, which is gen-

∗ Corresponding author. E-mail addresses: [email protected] (M.T. Montanes), [email protected] (V. Bednarik). http://dx.doi.org/10.1016/j.jhazmat.2016.08.023 0304-3894/© 2016 Elsevier B.V. All rights reserved.

erated after a pre-treatment of steel parts prior to dip into a bath of molten zinc. It contains about 65–75 wt.% of water and 25–35 wt.% of solids, mainly iron a zinc compounds. The second type of waste is hot-dip galvanizing ash collected from air filters above the bath with a molten zinc. This waste contains considerable amounts of zinc, its compounds and ammonium chloride [1–4]. The US Environmental Protection Agency (US-EPA) lists zinc as a priority pollutant because it is harmful for the environment and only 0.3 mg/kg/day is a reference dose for chronic oral exposure in humans [5]. Thus it is necessary to prevent the zinc ions from leaching to the environment [6]. One possibility is using the stabilization/solidification technique, which immobilize toxic metal into a matrix made from a suitable binder and is often used for the treatment of industrial hazardous waste [7]. The most common method of stabilization/solidification (S/S) uses Portland cement as the binder. Moon et al. [6] and also Trezza [8] reported that zinc reacts with cement clinker during

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hydration. Calcium hydroxide Ca(OH)2 is the main phase that fix zinc in cement matrices and the zinc solubility is controlled by calcium zincate (CaZn2 (OH)6 ·2H2 O). Zinc is also associated with increased formation of ettringite, which causes expansion and cracking of cement under some circumstances [9]. However, other studies [6,8,10] indicated that the ettringite, which is formed during the cement hydration process, affecting metal immobilization by replacing of Ca2+ ions with metal cations, e.g. Zn2+ . Formation of a low-soluble calcium zincate was also considered as the cause of zinc immobilization in calcium systems [11] and interference with the cement hydration resulting in decrease of compressive strength and retardation of hardening process [8]. Similar mechanisms was observed while fly ash was used as the binder [12,13]. Many authors have also reported that the pH value had a significant effect on fixation of heavy metals in the solidified waste [14–16]. The zinc in aqueous systems prevails in the form of Zn2+ cations in pH <8.5, in the range of pH values between 8.5 and 11.5 forms low soluble zinc hydroxide, and in the pH >11.5 it is dissolved as zincate anions and can be easily leached out from the cement-based solidified waste [17]. Thus, the waste with a high zinc content could be intractable by this way. The goal of this study is optimizing the stabilization/solidification process of hot-dip galvanizing ash using three different binders to find the optimal ratio between the addition of a chosen binder, ash and water, considering the environmental safety of the solidified waste landfill disposal and minimization of the operational and landfilling costs. Very few studies considered the effect of variability in water and binder proportions [18]. The approach presented in this paper uses different water and binder contents and the statistical regression analysis method of finding the best mixture composition for the S/S treatment of the waste.

2. Experimental 2.1. Waste The sample of waste was collected from the factory Galvanizadora Valenciana, S.A located in Spain near Valencia, where galvanized steel pieces are produced. The waste is a dust collected by air filters at the treatment of hot gasses generated above the surface of molten zinc in the bath at about 450 ◦ C. It contained zinc in a form of metallic zinc, zincite (ZnO), and chlorides which are in the form of simonkolleite Zn5 (OH)8 Cl2 ·H2 O and ZnCl2 . 2.2. Binders The waste with high content of zinc was treated using three types of binders and their combination. The chemical composition of selected binders is summarized in Table 1. Ordinary Portland Cement (OPC) II/B – S was obtained from the company CEMMAC Inc. (Hornie Srnie, Slovakia). The second binder was fly ash (FA) and it was obtained from heating plant in the town of Otrokovice (Czech Republic) and the last binder was a by-product of fluidizedbed combustion of coal (FBC) and it was collected in heating plant in the town of Zlin (Czech Republic).

Table 1 Chemical composition of binders (wt.%) by energy-dispersive x-ray fluorescence.

OPC1 FA2 FBC3

Al2 O3

SiO2

K2 O

CaO

TiO2

Fe2 O3

SO3

9.88 33.4 23.3

32.5 56.1 29.4

6.33 3.14 3.43

45.5 2.04 28.7

0.02 1.01 0.39

0.87 3.98 3.15

4.96 0.36 11.6

1–ordinary Portland cement, 2–fly ash, 3–fluidized-bed combustion ash.

2.3. Stabilizing and solidifying of hot-dip galvanizing ash The solidifying mixture was prepared from the waste, different type of binder and water. The binder and the waste were blended for 5 min then water was added and the mixture was blended for about 10 min until it had a pasty structure. After that, the mixture was poured into plastic forms with the length of 92 mm, width 43 mm, and height 50 mm. The forms were covered with plastic covers and the samples were cured at ambient conditions for 28 days. After that, the solidified samples were evaluated after standard curing age of 28 days according to EN 197-1 [19]. After this time of curing the samples were dried and according to EN 12457-4 the evaluating parameters in the leachate were determined [20]. The experimental work was designed considering the following parameters: • The relative amount of hot-dip galvanizing ash in the solid components of the mixture, ranged from 45 to 90 wt.%, and it was balanced with cement or different binder. • The relative amount of water added to the mixture, ranging from 10 to 40 wt.% • Ordinary Portland Cement with 65–79% of clinker, 21–35% of slag and ratio SiO2 /Al2 O3 = 3.29. • Fly ash with 56% SiO2 and ratio SiO2 /Al2 O3 = 1.68. • Fluid product with a ratio SiO2 /Al2 O3 = 1.26. The nomenclature for the mixtures was following B-X-Z, where B was the type of a binder used, X was the relative amount of water added to the mixture and Z was the relative amount of binder added in the solid components of the mixture. B was different for every binder added to the mixture, so that BI refers to cement, BII refers to fly ash, BIII refers to product of fluidized-bed combustion of coal. For example, the ratio BI-19-10 was prepared from the addition of 19 wt.% relative amount of water and balanced with solid components from which, the cement addition was 10 wt.% and the waste addition was 90 wt.%. That means this mixture was made from 8.1 g of cement, 72.9 g of waste, and 19 mL of water. 2.4. Acid digestion For the determination of total zinc concentration in dry matter of the ash, mineralization method in sulphuric acid was used. The process was performed in the following way: 5 g of sample was weighed and put into a beaker with 60 ml of 0.5 M H2 SO4 . The mixture was stirred for 15 min, then the solution was poured into 250 ml flask, completed with distilled water and homogenized. After that, the liquid phase was filtered through a 0.45-␮m filter and subjected to the chemical analysis. 2.5. Leaching tests Leaching tests using distilled water to evaluate the zinc concentration, retention capacity, the concentration of chlorides, and the concentration of dissolved substances were performed in triplicate. Solidified samples were evaluated after 28 days of curing. Leaching tests were performed using distilled water at the solid/liquid ratio 1/10 by weight. The leaching procedure consisted of the following: 50 g of pulverized sample (untreated or solidified waste) of particle size less than 10 mm were mixed with 500 ml of distilled water during 24 h under permanent shaking at room temperature on a vibrating shaker at the shaking frequency 150 rpm. Afterwards, the liquid phase was filtered through a 0.45-␮m filter, and then was analysed by atomic absorption spectrophotometry. The European regulation 2003/33/EC establishes the criteria for the acceptance of waste at landfills, which are based on EN 124574 leachability test [20,21]. Table 2 shows the leaching limit values

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Table 2 Relevant leaching limits for landfill disposal. Indicator

Leaching limit values (mg/L) Landfill for inert waste (class I)

Zinc Chlorides Dissolved solids pH

0.4 80 400

Landfills for non-hazardous waste (class IIa and IIb)

Landfills for hazardous waste (class III)

20 and 5 1,500 8,000 and 6,000 ≥6

20 2,500 10,000

for zinc, chlorides, and dissolved substance at different-types of landfills (for inert, non-hazardous and hazardous waste). The untreated waste and selected solidified sample were also evaluated using Toxicity Characteristic Leaching Procedure (TCLP) US-EPA Method 1311. Extraction fluid # 2 with pH 2.88 ± 0.05, liquid/solid ratio 20:1 and extraction time 18 h were used. 2.6. Determination of chlorides The concentration of chlorides (Cl− ) was determined by argentometric titration as follows: the exact amount of sample (10 ml) was taken and then 3 drops of 5% K2 CrO4 as the indicator were added. Then this solution was titrated by 0.05 M AgNO3 till the first permanent colour change from yellow to red-brown was observed. The concentration of chlorides was determined by this equation: cCl− = MCl .VAgNO3 .cAgNO3 .fD .

1 Vsample

(1)

Where cCl - is concentration of chloride (mg/L); MCl − is molecular weight of Cl− ; VAgNO3 is the endpoint volume of the volumetric solution of AgNO3 (mL); cAgNO3 is the exact concentration of AgNO3 (mol/L); fd − dilution factor; Vsample – volume of sample analysed (L).

2.9. Atomic absorption spectrophotometry The concentrations of the zinc in leachates obtained using water and acid-digestion of the original hot-dip galvanizing ash and all types of binders, were determined by GBC 933A atomic absorption spectrophotometer (GBC 933 AA, GBC Scientific Equipment Pty. Ltd., Australia) using the air-acetylene flame. Instrumental parameters were adjusted according to the manufacture’s recommendations for high concentration: spectral bandwidth of 0.2 nm, wavelength of 307.6 nm and an operating lamp current of 5 mA. The calibration solution was prepared from ZnSO4 ·7H2 O and the range of calibration was from 0.1 g/L to 5 g/L. 2.10. X-ray fluorescence spectrophotometry (XRF) The elementary composition of the solids was analysed by energy-dispersive X-ray fluorescence spectrophotometry using an ElvaX equipment (Elvatech Ltd., Ukraine) equipped with Ag X-ray tube. The settings were following: voltage on X-ray tube was 10 kV, current was 64 ␮A, and time for spectre of light elements was set to 100 s. Samples were analysed as powders in a special sample containers made from polypropylene and with a diameter of 30 mm and height of 20 mm. 2.11. pH value measurement

2.7. Determination of dissolved substances The procedure was carried out by the modified method ASTM D5907-13 [22]. The concentration of total dissolved substances was determined by drying of 10 mL samples of the waste leachate at 105 ◦ C to the constant weight. 2.8. Weight, volume increase and retention of zinc The weight and volume increase was measured for calculating the landfilling cost and the value of the retention of zinc was calculated to eliminate the dilution effect. The weight increase was determined using this equation: m =

mf − min min

× 100

(2)

where m is the weight increase, in mg; mf is the weight after 28 days of curing, in mg: min is the real amount of waste put into the mixture. The value of V was counted in a similar manner, but the density for each sample had to be counted for the calculation of volume increase. The retention of zinc was calculated using this type of equation: ret.(%) =

mt − ml × 100 mt

(3)

where mt is the total amount of zinc used in a sample, in mg; ml is the total amount of zinc in a leachate after 28 days of curing, in mg. Furthermore, the retention of chlorides was counted in a similar way.

pH value was measured on equipment pH meter InoLab 730 equipped with glass-electrode Sentix 81 (WTW, Germany). It was calibrated using the buffer solutions of pH = 4 and pH = 7 (WTW, Germany). 2.12. Statistical evaluation Data were evaluated by the statistical program Statgraphics Centurion XVI® using the multilevel factorial design. The binder and water contents were set as independent variables and the stabilisation/solidification efficiency was observed as the concentrations of zinc, chlorides and dissolved solids in the leachates as dependent variables. The significant parameters were determined using pareto charts and then the optimal mix composition was calculated from the regression model. 3. Results and discussion 3.1. Characterization of untreated waste First of all, the elementary composition of untreated waste was measured by XRF analysis that it had following settings: voltage on X-ray tube was 14 kV, current was 4 ␮A, and time for spectre of light elements was set to 100 s. It was determined that the waste contained mainly Zn and Cl. Other elements detected were Ca, Fe, S, Al, Pb. The measured spectrum of light metals is shown in Fig. 1. Secondly, the maximum leached zinc concentration and the concentration of chlorides in the original waste were determined by acid digestion. The measured value for zinc concentration was 5.5 g/L (275 g kg−1 ) and for chlorides the concentration was 7.6 g/L

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200 180

Cl

160 count rate [s-1]

140

Zn Kα

120 100 80 60

Zn Kβ

40

Al

Ca

20 0

0

2

Fe

4

6

8 E [keV]

10

12

14

Fig. 1. XRF spectrum of untreated waste (light elements).

Table 3 Measured parameters for the cement addition to the mixture. sample

pH

dissolved solids

zinc

chlorides

[1]

[g/L]

[g/L]

[g/L]

BI-19-10 BI-19-20 BI-19-30 BI-19-40

7.0 8.8 9.5 10.0

36.2 37.3 33.1 31.0

0.224 0.051 0.151 0.671

13.7 12.4 10.1 7.32

BI-24-10 BI-24-20 BI-24-30 BI-24-40

7.1 8.8 9.7 10.3

26.3 32.8 29.4 27.9

0.942 0.059 0.075 0.594

9.70 11.0 8.82 7.50

BI-29-10 BI-29-20 BI-29-30 BI-29-40

7.1 8.1 8.8 9.2

29.0 27.3 30.0 25.2

0.604 0.440 0.402 0.288

9.61 10.1 9.11 7.21

BI-34-10 BI-34-20 BI-34-30 BI-34-40

7.2 8.4 9.1 9.4

29.0 28.7 24.5 23.9

0.480 0.493 0.161 0.156

9.70 9.26 7.94 7.94

(382 g kg−1 ). In addition, the original waste was leached in a distilled water. Thus, the total concentration of dissolved substances in this medium was determined to 39.7 g/L, the zinc concentration was 4.6 g/L (46 g kg−1 ), and the concentration of chlorides was 23.8 g/L (238 g kg−1 ) in the leachate. The concentration of zinc in the TCLP extract was 6.2 g/L (124 g kg−1 ).

waste was 23.8 g/L. Thus, the average decrease was approximately 2 times. This could not be applied for dissolved substances because the decline was not too high. The stabilization/solidification into cementitious matrices was influenced by presence of chlorides and zinc ions in the waste. It has been reported that it slows the hydration process of cement and it also lowered the final strength of solidified monoliths [23]. If the hydration process is imperfect, the small solidified particles of waste are not encapsulated enough to fix the zinc in matrix or other components. 3.3. Solidification using fly ash A series of 16 samples of different composition was prepared. The addition of binder was in a range from 10 to 40% related to weight of whole mixture. The lowest concentration of zinc was determined in a mixture with 10% cement addition and 10% water addition. The value was 0.55 g/L and that is significantly worse than for the samples prepared with cement and coal fluidized-bed combustion ash. It could be caused by lack of sufficient amount of CaO in composition of fly ash. Other parameters measured were following: the lowest concentration of dissolved substances was determined in a mixture with 30% addition of cement and 40% addition of water. Also the concentration of chlorides was the lowest in the mixture of 40% cement addition and 40% water addition to the mixture. As a result, it could be concluded that the solidification/stabilization using fly ash showed high concentration of all parameters in a leachate. The measured parameters are summarized in Table 4.

3.2. Solidification using Portland cement 3.4. Solidification using fluidized-bed combustion ash A series of 16 solidified samples were prepared with a different addition of cement to the solidifying mixture. As aforementioned the addition was 10, 20, 30, and 40% to the solid part of a mixture. The measured parameters are summarized in Table 3. The lowest concentration of zinc in the leachate was 51 mg/L. It was for 20% cement addition and 19% water addition to the mixture. This zinc concentration was the lowest from all binders used. Other parameters measured with the lowest concentration values were following: 23.9 g/L of dissolved substances in a ratio BI-34-40. It was for the mixture with the highest addition of water and binder. On the other hand, the lowest concentration of chlorides in a leachate was 7.2 g/L for the ratio BI-29-40. So it could be concluded that higher addition of water means that the concentration of dissolved substances and chlorides decreased significantly. For comparison, the total concentration of chlorides in the leachate of untreated

The addition of fly ash from fluidized-bed combustion of coal to the mixture showed promising results due to composition that ensure all main components are in an equal amount. Aforementioned, it contains 29.43% SiO2 , 28.67% CaO, 23.30% Al2 O3 . So that the ettringite and other insoluble forms of zinc were generated in more effective way than in fly ash, but still the most effective treatment was the cement addition. The addition of 30 wt.% fluid product and 40% water showed the lowest concentration of zinc, e. i. 251 mg/L. It was the third best value determined in the solidified samples, but also this ratio showed low concentration of other observed parameters. Thus, it could be considered as the best decrease of pollutants in the solidified samples because other binders did not show this slump in all parameters. In addition, the lowest concentration of dissolved substances was 12.7 g/L and it

S. Vinter et al. / Journal of Hazardous Materials 320 (2016) 105–113 Table 4 Measured parameters for the fly ash addition to the mixture. sample

pH

dissolved solids

[1] BII-10-10 BII-10-20 BII-10-30 BII-10-40

6.8 6.8 6.8 7.0

BII-20-10 BII-20-20 BII-20-30 BII-20-40

3.6. Retention of zinc and chlorides

zinc

chlorides

[g/L]

[g/L]

[g/L]

36.1 35.2 29.3 22.7

0.552 0.584 0.618 0.653

10.6 10.1 8.82 12.4

7.0 7.0 7.0 7.4

37.4 29.4 26.1 25.4

2.20 1.86 1.75 0.990

11.0 10.1 7.50 10.6

BII-30-10 BII-30-20 BII-30-30 BII-30-40

7.1 7.2 7.2 7.4

31.5 24.6 23.6 22.0

1.90 1.36 1.16 0.753

10.6 10.1 7.94 10.1

BII-40-10 BII-40-20 BII-40-30 BII-40-40

7.1 7.1 7.2 7.1

27.9 18.3 15.5 18.6

1.36 1.11 1.06 0.985

11.0 10.1 7.50 7.10

Table 5 Measured parameters for the fluidized combustion of coal addition to the mixture. sample

109

pH

dissolved solids

zinc

chlorides

[1]

[g/L]

[g/L]

[g/L]

BIII-30-20 BIII-30-30 BIII-30-40 BIII-30-50

7.6 8.1 8.6 9.1

12.7 26.3 17.5 21.3

0.471 0.265 0.311 0.633

11.9 8.82 7.50 7.06

BIII-40-20 BIII-40-30 BIII-40-40 BIII-40-50

7.2 8.1 8.6 9.1

23.3 22.2 16.0 21.9

0.306 0.251 0.415 0.646

8.82 7.50 10.1 7.94

was determined in the 20 wt.% cement addition and 30 wt.% water addition. For comparison, it was the lowest value from all binders used. The lowest concentration of chlorides was 7.1 g/L in a ratio BIII-30-50. All measured parameters are summarized in Table 5.

3.5. The weight and volume increase The weight and volume was calculated for each prepared sample. Firstly, the density was measured, and then it was compared with determined density of original sludge. All samples prepared with the addition of coal fluidized-bed combustion ash had higher density than original waste, but for the fly ash addition the density values were mainly lower. The density values of solidified samples by cement fluctuated around the value of original waste or they were lower. All solidified samples showed increasing tendency as shown in Fig. 2. The increasing values of volume and weight also means that the greater amount of waste should be disposed of in landfill. Thus, the samples, which had lowest increase of volume, will be most suitable ones from the economic point of view. One tonne of used cement cost 120 D , other binders were obtained for free from the mentioned companies. As an illustration, the treatment of one tonne of original waste will cost 13.3 D using the best determined ratio for the lowest value of the concentration of zinc in a leachate. Since other binders were free of charge, the only fee for land disposal had to be paid. For example, one tonne of solidified/stabilized waste cost 112 D in the Czech Republic.

The percentage of pollutants retention was calculated for the purpose of eliminating the dilution effect that might take place when the binders are mixed with the original waste. The retention of original waste was 83.3%. The retention of zinc in the solidified samples of original waste using different binders ranged from 90.3 to 98.3% as is shown in Fig. 3. The lowest effectiveness demonstrated the addition of fly ash, but it differs from other binders that had increasing tendency when the amount of fly ash was raised in the mixture. The highest retention of zinc from all binders showed the cement addition at 30%, in the similar way behaved the coal fluidized ash, only the difference was that the highest retention of zinc was determined at 20%. The tendency for these two binders was similar, after reaching the highest value of the retention of zinc the significant decline was observed. Other parameter observed was the retention of chlorides in the solidified samples. The original waste had 38% retention. The important difference was the effectiveness of the retention. The retention of zinc was above 90%, but for chlorides the retention was from 39 % to 61% as shown in Fig. 4. The highest retention rate demonstrated cement addition. It remained steady around 60%. In contrast, the steep decrease was observed for the addition of fly ash and coal fluidized bed of combustion after they peaked their highest retention value. Moreover, it can be suggested for these two that if the relative amount of binder is increased the retention of chlorides decreased. 3.7. pH influence on S/S process Overall, pH values ranged from 6.8 to 10.3 for all samples prepared. The original waste had 6.44 pH value. The solidified cement samples had the lowest zinc concentration at 8.8. This confirms that the leachability of zinc ions was controlled by Zn(OH)2 precipitation. Moreover, the solidified samples made from coal fluidized-bed combustion ash showed 8.1 pH value as the best optimum to achieve low zinc concentration, but in the leachate prevailed zinc in a form of Zn2+ . So that the concentration was higher than for cement samples. An exception was the addition of fly ash to the mixture. The best pH value was 6.8, which showed the Zn2+ form in the leachate. Because of the lowest value of pH, the zinc concentration was the highest of all prepared samples. The dependency of measured values of zinc concentration on pH values is shown in Fig. 5. The three best samples with the lowest zinc concentrations in the leachate (BI-19-20, BI-24-20 and BI-24-30) were also evaluated using TCLP test. Observed zinc concentrations in these TCLP extracts were 1560, 2020 and 208 mg/L, respectively. All these values were higher than those from water leaching test, which again confirmed the influence of pH value on the metal leaching. Notice that the lowest zinc concentration in the TCLP extract showed the sample BI-24-30, which was also better with regard to chlorides and dissolved solids, than other two samples. It could indicated different mechanism of the zinc immobilization in this case. The lowest concentration of chlorides in the water leachate was achieved using the binders with high content of CaO (OPC, coal fluidized-bed combustion ash). The measured pH value was 9.2. Furthermore, the pH values for the lowest concentration using the fly ash and combination of binders were around 7.0. However, the concentration of chlorides varied far less with pH value, than the concentration of zinc. As a conclusion, zinc has different forms in solutions depending on pH values. From this point view, the best ratios had pH values from 8.8 to 9.7, which is in good agreement with theory of forming insoluble Zn(OH)2 described in the literature [17]. In the pH values below this range, zinc prevails in a form of zinc cations and

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cement 160

fly ash

coal fluidized-bed combuson ash

- - - = ΔV ___ = Δm

140

Δm, ΔV (%)

120 100 80 60 40 20 0 10

0

20

30

40

50

60

relave amount of binder (wt. %) Fig. 2. Effect of the relative amount of binder on the volume and weight increase.

cement

fly ash

coal fluidized-bed combuson ash

99 98

Retenon of zinc (%)

97 96 95 94 93 92 91 90 89 0

5

10

15

20

25

30

35

40

45

Relave amount of binder (wt. %) Fig. 3. Effect of the relative amount of binder on the retention of zinc.

its concentration is higher. On the other hand, in a highly alkaline environment, soluble zincate ions are formed and the zinc leachability increases again. Furthermore, to compare the results with pH values, some ratios had pH values between the demanded range, but the concentration of zinc in leachates were several times higher. It could be caused by insufficient stabilization/solidification of prepared monoliths where the zinc ions are near the surface of monolith and they can be easily washed off.

3.8. Criteria for landfill disposal As can be seen from the results, the solidified samples prepared with either type of binder did not meet the criteria for hazardous waste landfills set by the EU Council Decision 2003/33/EC [21]. The limit value for zinc concentration in the water leachate at liquid/solid ratio 10 L/kg, which is 20 mg/L, was exceeded at

least 2.5 times. The limit values for chlorides and dissolved solids were, in the best cases, exceeded 3 and 1.3 times, respectively. On the other hand, aforementioned data showed a steep decrease of zinc concentrations, up to 2 orders of magnitude, as the leaching test of the untreated waste showed the zinc concentration about 4600 mg/L. Compared to the total content of zinc, the solidified samples showed the retention of zinc up to 98%. Thus, the stabilization/solidification could be considered as a partially successful treatment of this toxic waste. For its use in the real waste treatment, it should be improved to meet the regulatory criteria, e.g. by a secondary barrier, which is currently under research in our lab.

3.9. The evaluation using statistical analysis The evaluation using statistical software should provide the best optimized ratio of used measured data. Thus, this optimiza-

S. Vinter et al. / Journal of Hazardous Materials 320 (2016) 105–113

cement

fly ash

111

coal fluidized-bed combuson ash

65

Retenon of chlorides (%)

60

55

50

45

40

35 0

5

10

15

20

25

30

35

40

45

Relave amount of binder (wt. %) Fig. 4. Effect of the relative amount of binder on the retention of chlorides.

Fig. 5. The dependency of zinc concentration on pH values.

Fig. 6. Pareto charts for concentrations of zinc, chlorides and dissolved solids in leachate of waste solidified by cement.

tion observes the interactions of factors to response variables. The analysis of each variable are showed by pareto charts (Figs. 6–8) that indicates increase (−) or decrease (+) of the variable response. Moreover, they indicate if the effect was significant or not. If the

bar crossed the vertical line is significant at a confidence level 95% (p-value lower than 0.05). The desirability function was counted from each design and it was based on values of the response variables. In this study, an optimal process of S/S involves minimum

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Fig. 7. Pareto charts for concentrations of zinc, chlorides and dissolved solids in leachate of waste solidified by fly ash.

Fig. 8. Pareto charts for concentrations of zinc, chlorides and dissolved solids in leachate of waste solidified by fluidized-bed combustion ash.

zinc concentration, concentration of dissolved substances and concentration of chlorides. Thus, the best ratio determined using the software for the cement addition was BI-34-29, that should assure the concentration of zinc in a leachate equals to 110 mg/L, concentration of dissolved substances 27 g/L and concentration of chlorides 8.9 g/L. These values are still above the limits for landfill disposal, but the treatment showed capability of cement to immobilize zinc in a cementitious matrix. As can be seen in Fig. 6, pareto charts showd that the cement and water addition had significant decreasing effect on concentrations of chlorides and dissolved solids, but not on the concentration of zinc. The overall desirability was 0.73. The best ratio determined for the addition of fly ash was BII10-25. This should assure that the zinc concentration in a leachate equals to 693 mg/L, concentration of chlorides 9.5 g/L and concentration of dissolved substances 29.3 g/L. As is shown in Fig. 7. the significant decreasing effect had fly ash addition and also factor AA on concentration of zinc. For the concentration of dissolved substances had a significant decreasing effect water and fly ash addition to the solidifying mixture, whereas the concentration of chlorides was influenced only by factors AA and BB. For comparison, the best ratio found from data was BII-10-10 due to low zinc concentration. The overall desirability was 0.65. For the last binder the best ratio was BIII-39-28. The statistical software evaluated that in the leachate of this sample could be 251 mg/L of zinc, 8.6 g/L of chlorides, and 21.5 g/L of dissolved substances. From the view of pareto charts the increasing significant effect had coal fluidized-bed combustion ash addition and factor BB on zinc concentration as is shown in Fig. 8. Other observed parameters were not influenced by the addition of ash neither by the addition of water. The overall desirability was 0.73.

4. Conclusions 1. The highest decrease of zinc concentration was achieved by addition of cement to the mixture. The concentration of zinc plummeted from 5.5 g/L to 51 mg/L. Also the addition of coal fluidized-bed combustion ash showed a promising result because the zinc concentration decreased to 251 mg/L.

2. The addition of fly ash showed neither the considerable decrease of zinc concentration in a leachate or of concentration of chlorides in a leachate. 3. The concentration of chlorides and dissolved substances in a leachate were approximately two times lower than in original waste. 4. The retention of zinc in the original waste was 83%. Thus, the treatment showed positive effect on all solidified samples of original waste, because the retention of zinc was ranged from 90.3% to 98.3%. The similar tendency was observed for the concentration of chlorides. 5. Using statistical software determined that the best ratios from all measured data were for the first binder following: 34% wt. water addition and 29% wt. cement addition; for the second binder: 10% wt. water addition and 25% wt. fly ash addition; for the last binder: 39% wt. water addition and 28% wt. coal fluidized-bed combustion ash addition. 6. Using the measured data, the best ratios seemed to be 24% wt. water addition and 30 % wt. cement addition; 10% wt. water addition and 10% wt. fly ash addition; 40 % wt. water addition and 30% wt. coal fluidized-bed combustion ash addition. For comparison, these ratios were fairly similar to those found by the statistical analysis, except the dosage of fly ash, which differed considerably. The statistical method could provide a more objective basis for a design of the S/S treatment on smaller scale experimental work and with potentially better results, but is not yet ready-to-use and should be further developed and validated.

Acknowledgement This work was supported by the Internal Grant Agency of Tomas Bata University in Zlin (project numbers IGA/FT/2015/012 and IGA/FT/2016/012).

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