Recycling and detoxification process of waste tire ash via acid leaching and neutralization

The possibility of a novel process of recycling and detoxification for waste tire ash, which contains hazardous Zn and Al, using acid leaching and neutralization was investigated. The waste tire ash mainly contains Zn, Al, Fe, Ca and Si. The leached contents of Zn, Al, and Fe with 1 mol/L of sulphuric acid (H2SO4) and hydrochloric acid (HCl) are almost same and the highest, while those of Ca with HCl is high and with H2SO4 is almost zero due to the formation of gypsum (CaSO4·2H2O). The optimum leaching condition is that waste tire ash was treated with 1 M H2SO4 for 1 h at 60 °C (solid-liquid ratio is 125 g/L). The elution of Zn and Al from the residue via H2SO4 leaching are 0.7 mg/L and 0.15 mg/L, respectively, which are lower than Japanese elution standard (Zn: 1 mg/L, Al: 0.2 mg/L), while those from raw ash are 3.3 mg/L and 1.4 mg/L. Zn-type layered double hydroxide (Zn-LDH) can be synthesized from leached liquor at 40 °C for 1 h by neutralization at pH 7. These results suggested that detoxified ash and environmental friendly material can be obtained from waste tire ash using H2SO4 acid leaching and neutralization.


Introduction
In 21 st century, the efficient utilization of resources and energy is becoming more essential.Among various industrial wastes, much attention has been paid to combustion ashes containing heavy metals, and research on their treatment is becoming an important field from both viewpoints of environmental protection and resource recovery.
Waste tires have a disposal problem in many countries for decades.Many waste tires are currently stockpiled in various countries all over the world, and are dangerous because they pose potential environmental problems for fire hazardous and providing mosquitoes breeding grounds.The landfill disposal of waste tires is becoming unacceptable because of the shortage of available landfill sites.As illegal dumping of waste tires is becoming an public concern all over the world, their disposal and treatment come under the spotlight.In Japan, nearly 1 million tons of waste tires have been discharged annually in the past decade [1], more than 4.4 million tons of spent tires are stockpiled in United States [2] and over 3 million tons of passenger tires are generated in Europe [3].The most popular utilization of waste tires is thermal recovery, given that the combustion heat of tires (7.2-8.5 kcal/kg) is much higher than that of other solid residues such as plastic-derived fuel or municipal solid wastes and is comparable to that of C-grade heavy oil (9.2 kcal/kg).In the United States, Canada, Germany, the United Kingdom, and Japan, waste tires have been used as a supplemental fuel for cement kilns and in paper mills.As automobile tires contain acceptable amounts of heavy metals such as zinc, iron, and aluminium, these metals are eventually concentrated in the combustion ashes, especially in fly ash.In view of resource recycling, waste tire fly ash would be a promising secondary source for these valuable metals.
In general, automobile tires consist of rubbers, carbon black, metal codes, additives compounds and bead wires [4].The codes and additives contain iron, copper, zinc and aluminium [5].Zinc is added as zinc oxide (ZnO) to the rubber compound for tires as a vulcanizing agent.The average value of ZnO is 1.2 % for car tire (0.4 to 9 %) and 2.1 % (1.2 to 3 %) for truck tires [6,7].On combustion, the ZnO significantly concentrate within the ash.Therefore, variable Zn in the ash is desired to extract to be used for some applications, especially using acid leaching as conventional method [5,8].However, these process discharged acid hazardous wastewater or hazardous residue, meaning that not eco-friendly.
Layered double hydroxide (LDH) is a nonstoichiometric compound represented by the general formula Here, M 2+ is a divalent metal ion, such as Ca 2+ , Mg 2+ or Zn 2+ , and M 3+ is a trivalent ion, such as Fe 3+ , Al 3+ or Cr 3+ .By replacing a part of the divalent metal ions with trivalent metal ions, a positively charged hydroxide basic layer and a laminated structure having an anion in the intermediate layer to compensate for the charge, are adopted.The amount of positive charges in the basic layer results in an anion exchange capacity and the anion in the intermediate layer is an inorganic anion, such as Cl -, NO3 -, OH -, CO3 2-or SO4 2-.
In this study, we attempted to extract hazardous contents of Zn and Al from waste tire ash using acid leaching to synthesize a material including Zn layered double hydroxide (Zn-LDH) by neutralizing extracted solution.The novelty of this study is eco-friendly recycling process to discharge no hazardous wastes by converting extracted element into functional material, Zn-LDH.

Extraction using acid leaching
To obtain non-hazardous ash by the extraction of Zn and Al from the ash, the ash (0 -20.0 g) was added to 40 mL of HCl solution with 0 -18 M, and shaken for 60 min at room temperature.After shaking, filtration was conducted, and then the extraction amounts of Zn, Al, Fe and Ca (the four major elements in the ash) in the solution from the ash were investigated using an atomic absorption spectrometer (AAS, AAnalyst200, Perkin Elmer).The residue after filtration was dried, and analyzed using a powder X-ray diffraction device (XRD, MiniFlex600, Rigaku).The surface structures of the ash and the residue following acid extraction were analyzed by a scanning electron microscope (SEM, JEOL, JSM-6510A).To examine the effect of temperature and reaction time, the ash (25 g) was added into 1 M H2SO4 solution (200 mL), and stirred for 120 min with a hot magnetic stirrer at 20, 40 and 60 o C.During stirring, a part of slurry was collected and filtrated, and the concentrations of extracted Zn, Al, Fe and Ca in the filtrate were determined using AAS.
Elution test was done for the ash and the residues after acid leaching with 1 M H2SO4 or 1 M HCl. 2 g of the raw ash or the residues was added into 20 mL of distilled water in 50 mL tube, and the tube was shaken for 6 h at room temperature.After shaking, filtration was conducted, and then the eluted amounts of Zn and Al in the solution from the ash was investigated using AAS.

Synthesis
The obtained mixed acid solution (50 mL) after leaching with 1 M H2SO4 for 60 min at 60 o C (125 g/L) was neutralized to pH 7 by adding 1 M NaOH solution, and stirred with a hot stirrer at 40 °C.After 60 min stirring, the slurry was filtered, dried, and washed with distilled water to obtain a product.The mineralogical phases of the product were identified using XRD, and the surface morphologies of the product were analyzed using an SEM.

Results and discussion
Figure 2 shows the extraction amounts of Zn, Al, Ca and Fe from the ash using 0 -18 M acid solution of HCl and H2SO4.In the case of HCl extraction, Zn, Al and Ca increased up to 1 M HCl and remained constant thereafter, while the concentration of Fe increased above 8 M HCl.In the case of H2SO4, the concentrations of Zn and Al increased up to 1 M H2SO4 and then decreased.Ca and Fe were hardly eluted regardless of concentrations of acid solution.It may be considered that the surface of the ash was more covered by CaSO4 formed from Ca content in the ash and SO4 2-in H2SO4 solution with increasing H2SO4 concentration above 1 M. Table 1 and Figure 1 show the chemical and mineralogical compositions of raw ash and the residues after leaching with 1 M HCl and 1 M H2SO4.The H2SO4-treated ash contained less ZnO and Al2O3 than the raw ash, and the contents of ZnO and Al2O3 were lower and CaO content was higher than HCltreated ash.After leaching with HCl and H2SO4, peaks of zincite (ZnO) containing Zn and ettringite (Ca6Al2(SO4)3(OH)12•36H2O) containing Al in the raw ash were eliminated.The peaks of gypsum (CaSO4 • 2H2O) in the H2SO4-leached ash, which were not observed in the HCl-leached ash, are confirmed, due to the reaction between Ca 2+ extracted from the ash and SO4 2-in H2SO4 solution.
Figure 3 shows SEM photographs of (a) the raw ash and the residue leached with (b) HCl and (c) H2SO4.In the raw ash, agglomerates of some particles on the surface of the ash were detected (Fig. 3  (a)).The smooth surface of the residue after leaching with HCl was observed (Fig. 3 (b)), while needlelike crystals, which appear to be gypsum crystals, were observed on the surface of the ash leached with H2SO4 (Fig. 3 (c)).From these results, Zn and Al (hazardous elements) can be extracted from the ash using 1 M H2SO4 solution without dissolving Ca content (Non-hazardous elements).
Table 2 shows the elution of Zn and Al from the ash and the residue via acid leaching with HCl and H2SO4 in elution test.The elution of Zn and Al from the residue via H2SO4 leaching are 0.7 mg/L and 0.15 mg/L, respectively, which are lower than Japanese elution standard (Zn: 1 mg/L, Al: 0.2 mg/L), while those from raw ash are 3.3 mg/L and 1.4 mg/L.It is noted that the elution of Zn and Al from the residue via HCl leaching are 0.5 mg/L and 0.17 mg/L, respectively, which are also lower than Japanese elution standard.
Table 2. Elution of Zn and Al from waste tire ash, H2SO4-leached ash and HCl leached ash in elution test (mg/L).

Sample Zn Al
Raw ash 3.3 1.4 HCl-leached ash 0.3 0.17 H2SO4-leached ash 0.7 0.15 Figure 4 shows the extraction amounts of Zn, Al, Ca and Fe from the ash using 1 M H2SO4 by varying dosage from 0 to 500 g/L at 20 °C for 1 h.At low addition, Zn, Al, Fe and Ca were extracted, as addition increased to 125 g/L, Zn and Ca decreased while Al and Fe were almost constant.At 125 g/L, Ca was almost completely un-extracted, and above 125 g/L, the extraction of all metal ions decreased.Zn and Al were most extracted at 125 g/L addition.From these results, the optimum leaching condition is that waste tire ash was treated with 1 M H2SO4 for 1 h at 60 o C (solid-liquid ratio is 125 g/L).Contents of Zn, Al, Fe and Ca in the extracted solution and pH of the solution are shown in Table 3.  6 and 7 show the XRD patterns and SEM photo of the product from the extracted solution via neutralization.Zn-type layered double hydroxide (Zn-LDH), which is one of the environmental purification material, can be synthesized from extracted solution by neutralization at pH 7, and hexagonal plane crystals, which seems to be Zn-LDH, can be observed.These results suggested that detoxified ash and environmental friendly material can be obtained from waste tire ash using H2SO4 acid leaching and neutralization.

Conclusion
In this study, hazardous contents of Zn and Al was extracted from waste tire ash using acid leaching to synthesize a material including Zn-LDH by neutralizing the extracted solution was investigated.As a result, it was possible to extract Zn and Al from the ash using H2SO4 without extracting Ca, and the optimum leaching condition is that waste tire ash was treated with 1 M H2SO4 for 1 h at 60 o C (solidliquid ratio is 125 g/L).The elution of Zn and Al from the residue via H2SO4 leaching are lower than Japanese elution standard.Zn-LDH can be synthesized from the extracted solution by neutralizing at pH 7.These results suggested that detoxified ash and environmental friendly material can be obtained from waste tire ash using H2SO4 acid leaching and neutralization.

Figure 2 .
Figure 2. Extraction of (a) Zn, (b) Al, (c) Ca and (d) Fe from the ash using HCl and H2SO4.

Figure 3 .
Figure 3. SEM photographs of (a) the raw ash, the residue leached with (b) HCl and (c) H2SO4.

Figure 5
Figure 5 shows the concentrations of (a) Ca 2+ and Zn 2+ , and (b) Al 3+ and Fe 3+ extracted from the ash in 1 M H2SO4 solution (125 g/L) during stirring at 20, 40 and 60 °C.While Ca was hardly extracted at any temperature, extracted Zn increased with increasing temperature and remained constant after 60 minutes.Extracted Al was almost constant regardless of temperatures, and extracted Fe increased with increasing temperature.

Figure 4 .
Figure 4. Extraction of Zn, Al, Fe and Ca from the ash using 1 M H2SO4 solution by varying dosage from 0 to 500 g/L at 20 °C for 1 h.

Figure 8
Figure8shows the summery for the proposed process of this study.Waste tire ash, including zincite (ZnO), magnetite (Fe3O4), quartz (SiO2), gypsum (CaSO4‧2H2O) and ettringite (Ca6Al2(SO4)3(OH)12‧36H2O), was leached with 1 M H2SO4 solution at 60 o C for 1 hour (125 g/L) to extract Zn 2+ , Al3+ and Fe 3+ in the solution.The residue contains SiO2 and Fe3O4 remained in the ash and CaSO4•2H2O formed from Ca content in the ash and SO4 2-in the solution, which is non-hazardous residue with lower elution level of Zn and Al.Extracted elements in the solution can be converted into the product including Zn-LDH as functional material by neutralizing to pH 7 with NaOH.

Figure 6 .
Figure 6.XRD patterns of the product from the extracted solution via neutralization.

Figure 7 .
Figure 7. SEM photo of the product from the extracted solution via neutralization.

Figure 8 .
Figure 8. Proposed process of this study.

Table 3 .
Zn, Al, Fe and Ca contents in extracted solution and pH of the solution.