Abstract
Tungsten and vanadium was separated from Ti in waste selective catalytic reduction (SCR) catalyst by roasting with waste NaCl salts (WNCS). Results show that the leaching efficiency of tungsten and vanadium reaches 84.63% and 66.42%, respectively, under the optimal conditions (roasting temperature 900 °C; roasting time 3 h; mass ratio of WNCS to waste SCR catalysts 0.5; reaction temperature 80 °C; reaction time 60 min), and the loss rate of titanium is 1.3%. The waste NaCl salts and roasting temperature play a positive role in converting anatase TiO2 to rutile TiO2. And the rutile TiO2 is obtained. The valence of rutile TiO2 is quadrivalent, and the lattice oxygen and chemisorbed oxygen occupy 57.26% and 42.74%, respectively.
Selective catalytic reduction (SCR) method is one of the best available and popular ways to covert NOx to N2 from stationary sourc
NaCl is used in many industries and our daily life. At present, a large amount of by-product salt residues are produced in the production process of some industries, such as leather production, pharmaceutical industry and inorganic chemical industrie
Fortunately, some metals, such as V and W, can be leached out from minerals by roasting with NaCl or other sodium salts. Many researchers have focused on leaching vanadium from stone coal by roasting with NaC
V2O5 and WO3 exist in waste SCR catalyst, and there are reactions between W, V, and NaCl. The reaction products about W and V are soluble, while most of the titanates are not soluble. So it is easy to separate W and V from Ti. Though many researchers have conducted leaching experiments on waste catalyst with pure chemicals, some scholars put forward roasting with Na2CO
The waste honeycomb SCR catalyst was obtained from a coal-fired power plant in China, which has been used for about 3 years. The waste catalysts as feedstock were milled and screened less than 0.149 mm for analysis and experiment. The waste NaCl salt residue was collected from a chemical company, which was mainly composed of NaCl. The waste salt was also milled below 0.149 mm. The waste salt was the industrial by-product. Pulverized feedstock was dried at 105 °C in an oven over night to remove the residual moisture. Further pre-treatment process was not conducted for the waste catalyst and waste salt. For further research, the waste SCR catalyst and the waste NaCl salt were defined as WSCR and WNCS, respectively.
The whole process of the experiments included roasting stage, reaction stage and leaching stage. The WO3 and V2O5 from WSCR reacted with NaCl in WNCS. The main composition of WSCR and WNCS was analyzed by X-ray fluorescence spectrometer (XRF, ARL ADVANT XP+, Ther-mo Corporation, USA). The phases of WSCR and WNCS were identified by X-ray diffraction (XRD, X'Pert Pro, PANalytical B.V., Holand). The content of V and W in solution was analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-OES, Prodigy 7, USA). The valence of Ti and O was detected by X-ray photoelectron spectroscopy (XPS, QUANTUM 2000X, Physical Electronics Company, USA). The particle morphology of product was observed by scanning electron microscope (SEM, S-4800, Hitachi, Japan). The leaching efficiency of W and V was calculated according to their content in solution and WSCR.
The mixtures of WSCR and WNCS were roasted in a muffle furnace at a certain temperature for the setting time. In the roasting process, the effect of WNCS addition, roasting time and roasting temperature on the efficiency of the recovery of vanadium and W from the spent SCR catalyst was investigated. The calcined product was milled again before the leaching experiment. The main reactions in the roasting stage can be expressed as the following equations:
| y(V2O5)+2xNaCl+(x/2)O2→xNa2O·yV2O5+xCl2↑ | (1) |
| y(V2O5)+2xNaCl+xH2O→xNa2O·yV2O5+2xHCl↑ | (2) |
| y(WO3)+2xNaCl+(x/2)O2→xNa2O·yWO3+xCl2↑ | (3) |
| y(WO3)+2xNaCl+ xH2O→xNa2O·yWO3+2xHCl↑ | (4) |
where V2O5 is the vanadium bearing oxide in WSCR. For the products of xNa2O·yV2O5 and xNa2O·yWO3, the numerical values x and y are correlative with roasting condition.
The leaching experiment was conducted in a teflon beaker with a cover in the water bath. First 100 mL water was put in the teflon beaker. The mixture powder after roasting and milling was put in the teflon beaker when the settled temperature was reached. The reaction process was carried out in the water bath at a certain temperature with a magnetic rotor. When the reacting solution was cooled to room temperature after the reaction was finished, the filtration experiment was conducted to separate the solution and the residue.
According to
| Material | WNCS | WSCR | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Cl | Na | Mg | Si | Ti | W | V | Ca | Si | ||
| Content | 60.29 | 38.95 | 0.20 | 0.15 | 47.87 | 3.32 | 0.59 | 0.52 | 4.20 | |
Fig.1 XRD patterns of WSCR (a) and WNCS (b)
To selectively leach W and V from the spent SCR catalyst, water leaching was conducted after roasting with WNCS. The effect of roasting temperature during the roasting process was considered at first.
Fig.2 Effects of roasting temperature on leaching efficiency of W and V (roasting time=2 h, mass ratio of WCNS:WSCR=0.5, reaction temperature=60 °C, reaction time=60 min)
The XRD patterns of roasted products with different roasting temperatures from 600 °C to 1000 °C are shown in
Fig.3 XRD patterns of roasting products with different roasting temperatures
The influence of roasting time from 1 h to 5 h on metal leaching efficiency was studied and the results are shown in
Fig.4 Effects of roasting time on leaching efficiency of W and V (roasting temperature=900 °C, mass ratio of WCNS:WSCR=0.5, reaction temperature=60 °C, reaction time=60 min)
Fig.5 XRD patterns of roasting product with different roasting time
The experiments with different WNCS addition amounts were performed during the roasting process. The leaching efficiency of V and W was examined under different mass ratios of WNCS:WSCR (0.1‒0.9), as shown in
Fig.6 Effects of mass ratio of WCNS:WSCR on leaching efficiency of W and V (roasting temperature=900 °C, roasting time=3 h, reaction temperature=60 °C, reaction time=60 min)
Fig.7 XRD patterns of roasting product under different mass ratios of WCNS:WSCR
salt addition mass ratio.
The XRD patterns of roasted products with different WNCS additions are shown in
Although roasting experiments paly the main role in transforming metal oxides into soluble substances, the reactions between the roasting products and water are also necessary for leaching metals. The reaction temperature between roasting products and water is considered in the experiment. The metal leaching efficiency at reaction temperature from 50 °C to 90 °C is shown in
Fig.8 Effects of reaction temperature on leaching efficiency of W and V (roasting temperature=900 °C, roasting time=3 h, mass ratio of WCNS:WSCR=0.5, reaction time=60 min)
The reaction time from 30 min to 120 min was investigated in this part. As shown in
Fig.9 Effects of reaction time on leaching efficiency of W and V
(roasting temperature=900 °C, roasting time=3 h, mass ratio of WCNS:WSCR=0.5, reaction temperature=80 °C)
Considering the optimal conditions obtained above, the verification experiment was conducted. And the leaching efficiency of W and V is 83.96% and 66.75%, respectively, which is similar to the results above. Meanwhile, the Ti in solution was analyzed under the best leaching experiment conditions. About 98.7% of Ti remains in filter residue. The main composition in WCNS is NaCl, so it can be inferred that WO3 and V2O5 change into water-soluble salt after reaction with NaCl, as shown in
The XRD and SEM results of leaching filter residue are shown in
Fig.10 XRD pattern of filter residue
Fig.11 SEM images of rutile TiO2
The XPS analysis of leaching filter residue was conducted. As shown in
Fig.12 XPS spectra of rutile TiO2: (a) Ti and (b) O
The specific surface area, pore structure and pore volume were considered for the leaching filter residue, which are important factors for the material
The curves about adsorption-desorption isotherms and pore distribution of leaching filter residue are shown in
Fig.13 Adsorption-desorption curve (a) and pore size distribution (b) of leaching filter residue
1) The method of waste selective catalytic reduction (SCR) roasted with waste NaCl salts is investigated. Water leaching after roasting stage is conducted.
2) The leaching efficiency of W and V reaches 84.63% and 66.42%, respectively, under the optimal conditions (roasting temperature 900 °C; roasting time 3 h; the mass ratio of WNCS:WSCR=0.5; reaction temperature 80 °C; reaction time 60 min), and the loss rate of Ti is 1.3%.
3) The solution containing valuable metals (W and V) and stick structure of rutile TiO2 is obtained. The valence of rutile TiO2 is quadrivalent, and the lattice oxygen Oα and chemisorbed oxygen Oβ occupy 57.26% and 42.74%, respectively.
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