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Vol. 8. Issue 6.
Pages 5728-5735 (November - December 2019)
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Vol. 8. Issue 6.
Pages 5728-5735 (November - December 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.09.040
Open Access
Immobilization forms of ZnO in the solidification/stabilization (S/S) of a zinc mine tailing through geopolymerization
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Qian Wana,b,c, Feng Raob,c,d,
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fengrao@umich.mx

Corresponding author.
, Shaoxian Songc, Yimin Zhanga
a School of Resources and Environmental Engineering, Wuhan University of Science and Technology, Hubei Provincial Engineering Technology Research Center of High Efficient Cleaning Utilization for Shale Vanadium Resource, Wuhan, 430081, China
b School of Zijin Mining, Fuzhou University, Fuzhou, 350108, China
c School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan, 430070, China
d CONACYT Instituto de Investigación en Metalurgia y Materiales, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, 58030, Mexico
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Tables (3)
Table 1. Chemical composition of the mine tailings.
Table 2. Chemical composition of the metakaolin.
Table 3. The atomic percent of main element in the geopolymers before and after leaching (The error in elemental quantitation is ±0.1 atomic %).
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Abstract

Solidification/stabilization (S/S) of a zinc mine tailing through geopolymerization has been studied. The mechanical property and microstructure of the mine tailings based-geopolymers were investigated through compressive strength measurements, FTIR, NMR and SEM characterizations. The stabilization of zinc in the geopolymers was analyzed through toxicity characteristic leaching procedure (TCLP) and the immobilization forms were characterized through XPS. With the content of metakaolin in raw materials increased from 0 to 50%, the amount of geopolymer gel in the binders and the compressive strength of geopolymer increased. As a ratio of metakaolin to mine tailings of 1:1, the mine tailings promoted the formation of silica-rich gel and the compressive strength of geopolymer was 30.1 MPa which was similar to pure metakaolin-based geopolymer. The immobilization efficiency of Zn in the geopolymers is correlated to their mechanical property. Zinc is immobilized in the geopolymers through physical encapsulation and adsorption of the leached Zn2+ by geopolymer gel.

Keywords:
Mine tailings
Geopolymerization
Solidification/stabilization
Immobilization
Full Text
1Introduction

In extractive industry, mine tailings are increasingly disposed due to steadily rising tonnage of low-grade ore. In China, approximately 10 billion tons of mine tailings were deposited until 2009, with more than 1 billion tons per year of mine tailings produced (1.581 billion tons in 2011) [1]. Moreover, until 2016, the ratio in reuse of the mine tailings was less than 17% [2]. Mine tailings cause several environmental problems. The stockpiling of mine tailings that include residual reagents and heavy metals in a slurry form can contaminate the water resources and soil through broad leakage [3,4]. In arid area, the dry mine tailings raise into dust easily and pollute the air [5]. Therefore, the management of mine tailings has long been a problem for extractive industry and the reuse of mine tailings is studied extensively [6]. The technologies explored for the reuse usually are the backfilling of mining tunnels, production of construction materials and reclamation of mining sites [7–10], which can potentially consume the huge quantity of mine tailings economically. The solidification of the mine tailings is the key in exploring these technologies [11].

In recently years, some researchers explored geopolymerization reactions to consolidate mine tailings [12,13]. They used mine tailings as the raw materials, and added alkali activators to induce geopolymerization reactions; then the mine tailings were consolidated after curing. For instance, copper mine tailings-based bricks were prepared through geopolymerization at varies alkali concentrations, water contents, forming pressure and curing temperature. The compressive strength of the bricks at optimal synthesizing procedure was 33.7 MPa [14]. Jiao et al. [15] synthesized vanadium mine tailings-based geopolymers after a roasting pretreatment of the mine tailings mixed with sodium hydroxide, of which the highest compressive strength was 55.7 MPa. Kuranchie et al. [16] studied the effects of the setting time, curing temperature and activator content on the synthesis of iron mine tailings-based bricks through geopolymerization. A compressive strength of 50.53 MPa was obtained for the bricks at optimal synthesizing regime. These studies provide an alternative approach to consolidate the mine tailings waste into construction bricks, in which the residual reagents and heavy metals might be stabilized after the geopolymerization reactions [17,18]. However, the completion of mine tailings-based materials into large-scale application requires more studies on 1) economical consolidation regime without such energy consuming procedures as pretreatment, calcination and applied high pressure; and 2) the stabilization forms and mechanism of heavy metals in the geopolymer bricks.

The present work studies the solidification/stabilization (S/S) of a zinc mine tailing, which has main components of andradite, calcite, quartz and ZnO. Metakaolin is used as additive when the adjustment of the mine tailing-based geopolymer microstructure is required. It has more consistent chemical compositions than other materials like fly ash and slag, which results in more consistent and predictable product [19]. The mechanical properties and microstructure of the mine tailing-based geopolymers, and stabilization forms of zinc in the geopolymers are discussed.

2Experimental2.1Materials

The mine tailings were collected as a slurry from from Chenzhou mill, China. The slurry was dried to a constant weight in 105 °C before the geopolymer synthesis. Table 1 gives the chemical analysis of the mine tailings measured by X-ray fluorescence spectroscopy (XRF, PANalytical Axios mAX instrument), in which the main components of CaO, Fe2O3, SiO2, ZnO, Al2O3 were observed. In consideration of the low accuracy of XRF, the chemical titration analysis of the mine tailings was performed and gave the zinc content of 2.1%. Fig. 1 showed the X-ray diffraction (XRD, Brucker D8) pattern of the mine tailings with high amounts of quartz (card 99-0088), calcite (card 99-0022) and andradite (card 76-0874). The size distribution of the mine tailings was measured by a Shimadzu SALD-1100 laser diffraction analyser (Japan) with d50 and d85 of 23.7 and 48.6 µm, respectively. Kaolinite used in the experiments was purchased from Hubei Chemicals, China with 50% of particle size below 4 µm. Metakaolin was prepared through calcination of kaolinite at 800 °C in air for 6 h. Its chemical analysis by XRF is given in Table 2, which showed the mass ratios of SiO2 and Al2O3 were close to theoretical ratio for pure metakaolin [20]. Sodium silicate (Na2SiO3, ACS reagent grade) purchased from Sinopharm Chemical Reagent, China was used as the alkali activator in the geopolymerization process. The deionized water was used in all experiments.

Table 1.

Chemical composition of the mine tailings.

Component  CaO  Fe2O3  SiO2  ZnO  Al2O3  K2CuO 
wt%  29.9  11.9  38.4  2.96  12.65  1.67  0.21 
  As2O3  PbO  P2O5  MnO  TiO2  SO3   
  0.11  0.237  0.845  0.621  0.598  0.527   
Fig. 1.

X-ray diffractogram of the mine tailings.

(0.12MB).
Table 2.

Chemical composition of the metakaolin.

Component  SiO2  Al2O3  K2Fe2O3  TiO2  MgO 
wt%  52.8  43.7  1.2  0.6  0.5  0.2 
2.2Methods

Fig. 2 shows the diagram of the geopolymer preparation process. In a typical synthesis of the mine tailings-based geopolymers, 444 g raw material was mixed with alkaline solution that was prepared with 122 g Na2SiO3 and 216 mlH2O, making a solid to liquid ratio of 2.05. The mixture of raw material and alkaline solution was poured into a cubic steel mold (50 × 50 × 50 mm) and vibrated on a vibration table for 3 min to liberate the air bubbles. After that, the mold was sealed for the curing process. It was cured at 60 °C for 6 h and subsequently cured at room temperature for 7 days. The only variable in the syntheses was the contents of metakaolin in the raw materials, which were from 0 to 50%.

Fig. 2.

The diagram of the geopolymer preparation process.

(0.24MB).

The mine tailings-based geopolymers were characterized by scanning electron microscopy (SEM, JEOL JSM-5610LV) and Fourier transform infrared spectroscopy (FTIR, Nexus JSM-5610) for the morphology and microstructures. The compressive strength data of geopolymers was obtained in a mechanical tester (Hangzhou Xingo Technology, EHC-1300) according to ASTM C109 [21]. For each binder, three specimens were tested and the average value was used. 29Si nuclear magnetic resonance (NMR) spectra of the geopolymers were obtained in a Bruker AVANCE III NMR spectroscopy operating at 79.49 MHz. The geopolymer specimens were crashed to powder condition and packed into 7 mm ZrO2 rotors. Spectra were acquired at spinning speeds of 5 kHz with peak positions referenced to an external standard of tetramethylsilane (TMS) and recorded with 5 s delay time. The excitation pulse for 29Si was 6 μs with a recycle time of 5 s. The 29Si solid-state NMR spectra were deconvoluted using Seasolve PeakFit™ software with GaussAmp function [22]. The chemical states of Zn in the geopolymers were determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi). An Al Kα X-ray source (1486.6 eV) was used and operated at the voltage of 15 kV. The spectra were obtained at the 30 eV pass energy with a 0.1 eV energy step size. Binding energies were corrected with respect to the C 1s peak at 284.8 eV.

The leaching of zinc from the mine tailings-based geopolymers was analyzed through the toxicity characteristic leaching procedure (TCLP) tests according to US EPA Method 1311 [23]. The extraction fluid of acetic acid solution was prepared by adding 5.7 ml acetic acid in 1 L deionized water. The geopolymers were crushed into powder with particle size less than 48 μm and mixed with the extraction fluid for a liquid to solid ratio of 20:1. After agitating for 20 h, the leachate was obtained by filtration. The zinc concentration in the leachate was determined in an atomic absorption spectrophotometer (AAS, GBC Avanta M).

3Results and discussion3.1Solidification of zinc mine tailing through geopolymerization

Fig. 3 shows the compressive strength of the mine tailings-based geopolymers synthesized with various additions of metakaolin. For the geopolymer synthesized without metakaolin, the compressive strength was 1.1 MPa. It suggests that the mine tailings can be activated by Na2SiO3 in geopolymerization and form consolidated binders. With increasing the contents of metakaolin to 50%, the compressive strength of the geopolymers increased steadily to 30.1 MPa, indicating that the addition of metakaolin enhances the mechanical properties of the geopolymers. In addition, the same dosage of metakaolin, Na2SiO3 and water in the synthesis of geopolymer with 50% mine tailings was used to synthesize geopolymer (Si:Al = 1.5) in the previous research [24]. Without adding the mine tailings, the compressive strength was 32 MPa, which is similar to the geopolymer with 50% of mine tailings. It indicates that if the amount of mine tailings is less than 50%, the compressive strength of geopolymer with mine tailings is almost the same as the pure metakaolin-based geopolymer.

Fig. 3.

Compressive strength of the mine tailings-based geopolymers synthesized with different contents of metakaolin.

(0.13MB).

Fig. 4 presents the FTIR spectra of the mine tailings-based geopolymers synthesized with various additions of metakaolin. The absorption bands at 3430, 1642, 727 and 471 cm−1 corresponds to the OH- stretching vibrations, HOH, SiO and OSiO bonds, respectively [24]. The band at 1450 cm−1 is attributed to the stretching mode ofbonds in CO32− groups [25]. The intensity of it increased with the less metakaolin involved, suggesting that more calcite remains in the gel after the geopolymerization. The band around 1000 cm−1 was due to the SiOT (T is tetrahedral Si or Al) asymmetric stretching band in geopolymer [26]. With the increasing content of metakaolin from 0 to 50%, intensity of the band increased, indicating metakaolin promotes the formation of geopolymer gel.

Fig. 4.

FTIR spectra of geopolymers synthesized with various contents of metakaolin.

(0.19MB).

Fig. 5 shows the SEM images of the geopolymers synthesized with various content of metakaolin additive. In the geopolymer synthesized with the content of metakaolin at 50%, the formed geopolymer gel is enough to encapsulate the incompletely reacted crystals in mine tailings to form a compact structure. Thus, a satisfactory compressive strength is achieved. In the geopolymer synthesized with the content of metakaolin decreasing to 30%, the formation of geopolymer gel reduces and a porous structure is formed. In the geopolymer synthesized without metakaolin, 100% mine tailings involving the synthesis results in a significant decrement of geopolymer gel and the plentiful crystals, which are not agglutinated by geopolymer gel. Thus, the geopolymer shows a loose and heterogeneous structure.

Fig. 5.

SEM images of geopolymers synthesized with different content of metakaolin.

(0.78MB).

In studying microstructure of geopolymer, 29Si NMR spectroscopy can provide the various types of the cross-linked SiO4 tetrahedra with Al tetrahedral in geopolymer, namely Q4(4Al), Q4(3Al), Q4(2Al), Q4(1Al), Q4(0Al) [27]. The resonance of the different SiO4 tetrahedra locates at approximately −84, −89, −93, −99 and −108 ppm, respectively [28]. In addition, the andradite in mine tailings shows the peak at −84 ppm, which overlaps with Q4(4Al) [29], and quartz in mine tailings shows the peak at −108 ppm, which overlaps with Q4(0Al) [30]. Fig. 6 gives the 29Si NMR spectra and their deconvolution of geopolymers synthesized with different contents of metakaolin. As the content of metakaolin decreasing from 50 to 0%, the total percentages of Q4(3Al), Q4(2Al), Q4(1Al) decrease from 72.74 to 54.08%. It indicates that the decreased amount of metakaolin reduces the formation of geopolymer gel. But the percentages of Q4(4Al) and Q4(0Al) increase from 23.19 and 4.05% to 37.57 and 8.34%. Considering the reduction of geopolymer gel, it suggests that the more crystalline andradite and quartz is in the geopolymers. Interestingly, the percentages of Q4(3Al), Q4(2Al) and Q4(1Al) was 17.03, 17.27 and 19.78% in the geopolymer without metakaolin, with the content of metakaolin increasing to 50% the percentages of Q4(3Al), Q4(2Al) increased to 28.62 and 29.09% but Q4(1Al) decreased to 15.05%. It indicates that the mine tailings promote the formation of silica-rich gel, but due to the reduce of geopolymer gel, it hardly increases the compressive strength. Furthermore, the Q4(3Al), Q4(2Al) and Q4(1Al) of geopolymer synthesized in previous work (the same dosage of metakaolin, Na2SiO3 and water of geopolymer with 50% of mine tailings, but without mine tailings) was 33.24, 20.77 and 9.77% [24]. It also confirms that the more silica participated in the geopolymer gel with 50% of mine tailings than the geopolymer gel without mine tailings. However, there is no obvious change in compressive strength, which may be due to the positive effect of silica-rich gel formation that is offset by the negative one of a large amounts of inert minerals in tailings involving the geopolymerization. These results suggest that the mine tailings can be consolidated through geopolymerization. As the amount of metakaolin was 50%, the compressive strength of geopolymer was close to the pure metakaolin-geopolymer. Therefore, in the case of ensuring the same mechanical property, this method not only partly uses the solid waste instead of metakaolin without pre-treatment, but also reduce the energy consumption in the calcination of the whole raw materials.

Fig. 6.

29Si NMR spectra and their deconvolution of the geopolymers synthesized with different contents of metakaolin.

(0.39MB).
3.2Immobilization of Zn through geopolymerization

Fig. 7 showed the results of TCLP tests of zinc from the mine tailings-based geopolymers synthesized with different contents of metakaolin. Zn concentration in the leachate and the pecentage of leached Zn to total Zn in mine tailings are 2.7 ppm and 0.91% respectively, when the geopolymer synthesized with 50% metakaolin in raw materials. As the content of metakaolin decreased from 40 to 0%, the concentration of Zn increases from 12.7 to 58.2 ppm which are 3.55 and 9.76% to the total Zn in mine tailings. These results indicate that Zn was effectively immobilized in mine tailings-based geopolymers with the content of metakaolin at 50%, but the immobilization efficiency of Zn decreases when the content of metakaolin decreases. The immobilization of Zn is positively related to mechanical property of the geopolymers, suggests that the immobilization of Zn in geopolymer may due to the physical encapsulating.

Fig. 7.

TCLP tests of Zn from the mine tailing-based geopolymers synthesized with different contents of metakaolin.

(0.17MB).

To determine the immobilized form of Zn in the geopolymers, XPS was performed and the detailed XPS spectra of Zn 2p in mine tailings and the geopolymers with different content of metakaolin before (50 and 0%) and after leaching (50 L and 0%L) were shown in Fig. 8. In mine tailings, Zn 2p3/2 centered at 1022 eV is ascribed to ZnO [31]. It suggests that the main phase of Zn in mine tailings is ZnO. In the geopolymers, the Zn 2p3/2 still centered at 1022 eV indicates that ZnO is not reacted in the geopolymerization and still remains. In addition, the peak at 1022.7 eV is ascribed to Zn2+[32], which is not obvious in the geopolymers before leaching. But after leaching, the intensity of this peak increases, it indicates that the ZnO was partly dissolved in the leachate and the formed Zn2+ is adsorbed by the geopolymers.

Fig. 8.

Zn 2p XPS spectra of mine tailings and the geopolymers before and after leaching.

(0.15MB).

Table 3 shows the atomic percent of main element in the surface of geopolymers before and after leaching, which is obtained by XPS. The content of oxygen decreases in both of geopolymers after leaching, it indicates the oxide in mine tailings is dissolved in leachate. The content of Al and Si increases slightly, it suggests the geopolymer gel is hardly affected by the leachate. And due to the dissolution of oxide minerals, the percentage of geopolymer gel in geopolymers increases. The content of Ca and Zn increases significantly, it means that the calcium and zinc compound is dissolved in the leachate, and the Ca2+ and Zn2+ are adsorbed on the surface by the geopolymers. It is good agreement with the literature that the geopolymer has an excellent adsorption for metal cations [33]. In addition, the increase of Zn in geopolymer synthesized with 50% metakaolin is 3 times but in geopolymer synthesized without metakaolin it is less than 2 times; this may be due to the more formation of geopolymer gel in the former. These results suggest that a part of leached Zn2+ is immobilized in geopolymer through adsorption, and the more geopolymer gel formed, the more cations can be adsorbed.

Table 3.

The atomic percent of main element in the geopolymers before and after leaching (The error in elemental quantitation is ±0.1 atomic %).

  50%  50%L  0%  0%L 
Al  8.33%  8.85%  2.67%  3.09% 
Si  14.72%  16.25%  8.06%  10.38% 
Ca  0.47%  1.36%  1.88%  2.50% 
76.34%  73.12%  86.58%  82.50% 
Zn  0.14%  0.42%  0.81%  1.53% 
4Conclusion

  • 1)

    Mine tailings containing andradite, calcite and quartz can be consolidated through geopolymerization. The compressive strength of the tailings-based geopolymers increases from 1.1 to 30.1 MPa with the content of metakaolin increasing. The compressive strength of geopolymer with 50% mine tailing is similar to the pure metakaolin-based geopolymer, which is due to the promotion of silica-rich gel formation by silicate minerals in mine tailings.

  • 2)

    Zn in mine tailings is effectively immobilized in the geopolymers when the content of metakaolin at 50%. With the content of metakaolin decreasing from 50 to 0%, the concentration of leached Zn increases from 2.7 to 58.2 ppm and the percentage of leached Zn to total Zn increases from 0.91 to 9.76%.

  • 3)

    The ZnO in mine tailings is nonreactive in geopolymerization, which is immobilized in mine tailings-based geopolymers through physical encapsulation. With the formation of geopolymer gel increasing, the efficiency of immobilization increases.

  • 4)

    A small amount of ZnO in geopolymers is dissolved to Zn2+ in acid leachate, and the Zn2+ is adsorbed by the geopolymer gel. The amount of adsorbed Zn2+ increases with the content of geopolymer gel increasing.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgement

This study was financially supported by the Consejo Nacional de Ciencia y Tecnología (CONACyT) of Mexico under the Grant No. 270186, the Natural Science Foundation of Hubei Province of China under the Grant No. 2016CFA013 and the Wuhan Science and Technology Bureau of China under the Project No. 2016070204020156, for which the authors are grateful.

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