Wide-temperature Range Modification And Commercial TiO₂ Extraction Of Spent Denitration Catalyst

Spent vanadium-titanium denitrification catalyst（V₂O₅-WO₃/TiO₂）belongs to hazardous solid waste.Because of its large amount and easy to cause secondary environmental pollution,it has always been a hot topic to carry out the regeneration as well as the extraction and reuse of valuable metals of spent vanadium-titanium denitration catalyst.At present,the operating temperature of vanadium-titanium denitrification catalyst is between 300～400℃.It is mainly suitable for the denitration of flue gas from coal-fired power plants.However,due to the low temperature of the flue gas emitted by the non-electric power industry（100～300℃）,the vanadium-titanium denitration catalyst has low denitration efficiency and cannot be applied.At the same time,the vanadium-titanium denitrification catalyst has poor SO₂ tolerance and is prone to poisoning and deactivation.In addition,the vanadium-titanium denitration catalyst with no regeneration value contains about 80%TiO₂,but the reported extraction methods are still difficult to obtain TiO₂ that meets the requirements of commercial technical indicators.In view of the above problems,this article mainly studied the regeneration and modification of spent vanadium-titanium denitrification catalyst,as well as the extraction process of commercial TiO₂,aiming to prepare a denitration catalyst with wide-temperature working window and excellent SO₂ resistance and studied its denitrification and anti-sulfur poisoning mechanisms,while efficiently extracting TiO₂ that meets commercial technical indicators.Project research is of important research value and significance.In this paper,the spent vanadium-titanium denitration catalyst was used as the research object.Based on the double-layer electric theory,the effective loading（adsorption）of vanadium and cerium ions on the catalyst was achieved by adjusting the zeta potential on the catalyst surface.The results showed that adjusting the p H of the impregnating solution could change the zeta potential of the catalyst surface,and also affected the loading of vanadium and cerium ions on the catalyst surface.When the p H was about 3,the zeta potential of the catalyst surface was negative,and VO₂⁺and Ce³⁺in the impregnating solution would be adsorbed on the surface of the catalyst through electrostatic action,making the loading of vanadium and cerium to reach maximum.The optimized conditions were:p H of 3.2,the concentration of ammonium metavanadate of 0.6%,the concentration of cerium nitrate hexahydrate of 200 g/L,the impregnation sequence of simultaneous impregnation,the liquid-solid ratio of 10:1 and the reaction time of 60 minutes.Through the loading modification of active component vanadium and cerium ions,the working temperature window of the modified catalyst was expanded from 300～400℃to 150～450℃.More interestingly,it was found that the vanadium-cerium modified denitrification catalyst had an unuausl resistance to SO₂ poisoning.When introduced 200 ppm SO₂ at 200℃,the denitration activity rose from 81%to 89%instead of falling.The denitration and anti-sulfur poisoning mechanisms of the vanadium-cerium modified catalyst were also studied.The results showed that the denitrification reaction on the vanadium-cerium modified catalyst mainly followed L-H mechanism,that is,while NH₃ was adsorbed on the acidic sites on the catalyst surface,NO was also adsorbed on the vanadium-cerium oxide over the catalyst surface and converted into active intermediates such as nitrate,nitrite and cis-N₂O₂^2-.These active intermediates underwent redox reactions with adsorbed NH₃ to generate N₂ and H₂O.At the same time,we found that the loading of vanadium and cerium could increase the number of acidic sites on the surface of the catalyst and enhanceed the oxidation-reduction performance of the catalyst,thus improving the denitration activity of the catalyst at low temperature and widening its working temperature window.Furthermore,the vanadium-cerium doping improved the SO₂ poisoning resistance of the denitrification catalyst due to the fact that Ce O₂,as an electron acceptor,reacted with SO₂to form Ce₂（SO₄）₃.The generated SO₄^2-increased the acidic sites on the catalyst surface,and Ce³⁺generated more oxygen vacancies on the catalyst surface,increased the amount of adsorbed oxygen on the surface,and promoted the conversion of NO to the SCR active intermediate product cis-N₂O₂^2-,thus improving the overall denitration activity of the vanadium-cerium modified catalyst.In addition,the formation and decomposition rate of Ce₂（SO₄）₃ on the denitration catalyst was the same.Therefore,in the presence of SO₂,the composition of the vanadium-cerium denitration catalyst remained unchanged during the entire SCR reaction process,which made it exhibit stable and efficient denitration activity.Finally,we studied the extraction of commercial TiO₂ from the denitration catalyst through the process of alkali roasting+acidification+ammonification hydrolysis,and the influence of various process parameters on the performance of TiO₂ products was also investigated.Studies have found that the Na OH/Na₂CO₃ compound ratio in the alkali roasting process had a greater impact on the conversion of TiO₂ to sodium titanate in the catalyst.The higher ratio of Na OH,the higher conversion rate of TiO₂（converted to sodium titanate）.In the next acidification process,sulfuric acid reacted with sodium titanate to obtain the soluble salt TiOSO₄,and then ammonia water was added to the leaching solution.The TiOSO₄solution was easily hydrolyzed to form a solid precipitate of metatitanic acid,while impurities such as Na,K and Al remained in the solution,thereby achieving the effective seperation of titanium and impurity ions.In addition,adding ammonia in the hydrolysis process to adjust the p H was beneficial to the TiOSO₄ hydrolysis reaction and greatly improved the recovery rate of TiO₂.Finally,TiO₂ products with high purity,high recovery rate and impurity content meeting commercial specifications were obtained through roasting.The optimized conditions were:alkali:catalyst of 1.4:1,Na OH:Na₂CO₃ of 1:0.4,calcination temperature of 500℃,calcination time of two hours,sulfuric acid concentration of 50%,liquid-solid ratio of 15:1,ammonia water dosage:catalyst of 15:1.After filtration,drying and calcined at 600°C for two hours,the purity of the extracted TiO₂ was high up to 96.44%,recovery rate was 83.5%,BET specific surface area was 106 m²/g and the impurity content of Na,K,S,P,Si,Al met the commercial titanium dioxide standards.