| Selective catalytic reduction(SCR)technology has become the current mainstream technology for flue gas denitration,which is widely used in our country.The vanadium-titanium SCR catalysts are universally applied in major coal-fired power plants due to their high efficiency.However,the catalysts deactivated after a long period of operation because of the influence of harmful components in the flue gas.And these catalysts cannot meet the denitration requirements after deactivation.In addition,some catalysts cannot be regenerated,for their severe deactivated activity or badly destroyed strength.The waste denitrification catalysts were collected from a coal-fired power plant.The recovery research was conducted,aiming to separate the main elements and get valuable products.The waste denitration catalysts were analyzed,and a complete recovery process route to separate and recover the valuable elements was put forward.Meanwhile,the products were prepared with the recovery elements.The method of separating vanadium with acid was put forward,and the way of leaching tungsten and silicon with sodium hydroxide solution was proposed.Then,the silicon was separated from tungsten in the alkaline leaching solution.Both the silicon and tungsten were precipitated and purified.Some measures were adopted with the alkali leaching residue,and the titanium dioxide was obtained.Meanwhile,the tungstic acid(tungsten trioxide)and silica were obtained.And continuous experiments and economic analysis were carried out on the basis of the laboratory results.The new denitration catalysts were prepared with the recycled products,and the catalysts show good denitrification activity.According to the experimental researches in the study,the following contents are summarized.The causes of catalyst deactivation were analyzed,and the catalyst and attached fly ash were characterized and analyzed through a series of characterization methods(XPS,XRD,SEM,BET,etc.).Some results were concluded as follows:①The content of active component(vanadium)decreased.②The proportion of low-priced vanadium in catalysts increased.③The content of alkali metals,SiO2 and other harmful elements increased substantially.④ The content of sulfate increased.Therefore,the main reasons for the catalyst deactivation are attributed to:the loss of active components and the increase of the low-priced vanadium proportion;the coverage of catalyst surface active sites with silica,sulfate,alkali metals or other heavy metals in the fly ash;the acid sites destroyed by sulfate,alkali metals and some heavy metals.The toxicity leaching experiments were performed on the waste catalysts.And the heavy metal leaching toxicity of the waste denitrification catalyst and their attached fly ash was investigated with sulfuric acid-nitric acid method.It was found that the leaching amount of arsenic from the waste denitrification catalyst and fly ash was 33.32 mg/L and 9.33 mg/L respectively,which are higher than the standard limit.Besides,a certain amount of vanadium could be extracted from the waste catalyst and attached fly ash,which indicating that both the waste catalyst and attached fly ash have a certain degree of danger to human and environment.Therefore,the waste catalysts and fly ash need to be disposed rationally.The vanadium in the waste catalysts was separated with acid,and different hydrochloric acid leaching conditions were considered.The leaching rate of vanadium can reach 71.54%under the optimal hydrochloric acid leaching conditions(hydrochloric acid concentration 8 mol/L;reaction temperature 90℃;liquid-solid ratio 10:1 mL/g;reaction time 80 min).Meanwhile,the leaching rate of tungsten was below 5%,and little titanium and silicon were extracted.Hence,the vanadium in the catalyst could be selectively separated effectively with hydrochloric acid.The leaching rate of vanadium in the compound acid leaching process can reach 91.36%with citric acid added.In addition,the mechanism of vanadium leaching part with hydrochloric acid was analyzed.Firstly,the thermodynamic analysis of the acid separation process of vanadium was conducted,which was considering the concentration-pH diagram,the potential-pH diagram,and the Gibbs free energy of the reaction.It was found that the vanadium oxide can be separated from other main components according to the thermodynamic analysis,and the form of V was predicted as V3+and VO2+in the solution.Secondly,the leaching solution was analyzed with ultraviolet full-wavelength scanning.It was also found that the form of vanadium in the hydrochloric acid leaching solution was V3+and VO2+,which was consistent with thermodynamic analysis.According to the form of vanadium in the leaching solution,the reaction mechanism and reaction equations were inferred.In addition,according to the metallurgical kinetic nuclear shrinkage model,the kinetics of the leaching process was studied.The kinetic process of leaching vanadium was conducted under the conditions of different hydrochloric acid concentrations(leaching temperature 90℃,liquid-solid ratio 10:1 mL/g)and different reaction temperature conditions(acid concentration 8 mol/L,liquid-solid ratio 10:1 mL/g).It was found that the vanadium leaching process was under the solid film diffusion control.The apparent reaction order and the apparent reaction activation energy were calculated as 1.07 and 20.1 KJ/mol respectively.And the kinetic equation of vanadium leaching under different reaction temperature conditions was as follows:1-2/3η-(1-η)2/3=1.15e[-2.01×10^4/RT]t.The alkaline leaching experiments were carried out on the acid leaching residue.The leaching rates of tungsten and silicon reached 92.57%and 82.37%respectively at the optimal reaction conditions(the concentration of sodium hydroxide solution 30%,the reaction time 2 h,the reaction temperature 110℃,and the liquid-solid ratio is 10:1 mL/g).According to the thermodynamic analysis and experimental exploration of the leaching process,the tungsten in the alkaline leaching was mainly tungstate.And the leaching tungsten mechanism was obtained.There were few negative effects on the crystal form of titanium dioxide during acid or alkaline leaching,and the crystal form of leaching residue was anatase titanium dioxide.The pilot-scale researches were conducted on the basis of the experiments,and good leaching results were achieved during the cycle process.Meanwhile,the acid and alkali were recycled as much as possible,which could save industrial costs.In the process of leaching tungsten with alkaline leaching,the reaction process was controlled by solid film diffusion under different reaction concentration conditions(reaction temperature 110℃,sodium hydroxide solution 200 mL)and different reaction temperature conditions(sodium hydroxide concentration 10 mol/L,liquid-solid ratio 10:1 mL/g).The apparent reaction order and the apparent reaction activation energy were 0.83 and 33.70 KJ/mol respectively.The results showed that the leaching processes of vanadium and tungsten with acid or sodium hydroxide solution are solid-film diffusion control processes under the experimental conditions,and the diffusion rate of the reaction products on the particle surface played a leading role on the reaction rate.The titanium,tungsten and silicon,separated from the catalyst during the experiments,were all prepared for corresponding products.The alkali-leached titanium slag is processed under the optimal conditions(the pH value of the titanium slag solution 2,the reaction time 60 min,the acid type hydrochloric acid,and the reaction temperature 60℃),and the TiO2 content in the product could reach 96.55%.The specific surface area of anatase titanium dioxide with good crystal form can reach 64.55 m2/g after surface treatment and roasting.Meanwhile,the titanium dioxide has good whiteness(94.25%)and oil absorption value(3.08 mL/g).The silicon in the alkaline leaching solution was precipitated under the optimal separation conditions(pH value 9,the hydrochloric acid concentration 6 mol/L,the aging time 2 h),and 99.2%silicon was precipitated.And the tungsten loss rate was only 0.12%at the optimal conditions.The separated silicon was purified under the best conditions(hydrochloric acid concentration 4 mol/L,liquid-to-solid ratio 10:1,reaction time 60 min).The high purity silica was obtained,and the purity of the dry product is as high as 97.55%.The specific surface area of the silica can reach 106.30m2/g,which has high whiteness value(97.72%)and oil absorption value(4.25 mL/g).The tungsten was precipitated after silicon removal.The tungsten precipitation rate can reach 87.22%under the optimal precipitation conditions(reaction temperature 90℃,theoretical addition of hydrochloric acid 40 times,hydrochloric acid volume dilution factor 3,reaction time 30 min).The purity of the obtained tungstic acid can reach 96.32%,and purity of 97.45%WO3 can be collected after the tungstic acid roasted.The tungsten in tungstic acid mainly existed in the form of W6+,the microscopic morphology was flake and rod-like structure.The crystal type of tungstic acid is WO3·H2O.Meanwhile,iron,arsenic,copper,phosphorus and sulfur were not detected in tungstic acid or tungsten trioxide.The recovered products,with good incators,are expected to be used in the fields of metallurgy,adsorption,catalysis and coatings.The denitration activity of the catalyst,prepared with titanium slag after alkali leaching and the recovered WO3,was higher than 92.21%at high temperature with the WO3 loading 7%.The catalyst,prepared with titanium slag,silica and recovered WO3,showed the highest denitration activity of 96.22%when the WO3 loading was 5%.At the same time,it was found that the crystal forms of prepared catalyst were all anatase TiO2,and the tungsten in the active component was demonstrated to be in the form of W5+ and W6+.In addition,1%V2O5 was loaded on the above catalysts,and the prepared vanadium tungsten titanium catalyst and vanadium tungsten titanium silicon catalyst maintain the denitrification activity above 98.24%and 99.35%respectively at the reaction temperature range(300-400℃).What is more,both the catalysts have a certain resistance degree to sulfur and water and 24h stability,which certified that preparing catalysts with the separated products was feasible.Continuous small tests were carried out for the overall process.The results showed that the overall metal separation efficiency was relatively high.The average purity of titanium dioxide,white carbon black and tungsten oxide obtained from the small tests was relatively stable(>93%),which showed the process was feasible.At the same time,the industrialized production of the project was estimated,and the fixed asset input,operation input and sales income were considered.The costs of each stage are obtained,and the cash flow statement of the program was obtained.The cumulative cash flow can reach 74.43 million yuan at the third year.Therefore,the recovery plan is feasible according to the economic accounting. |
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