Preparation Of V₂O₅-WO₃/TiO₂-SiO₂ Catalyst From Blast Furnace Slag And Its Performance For NH₃-SCR Of NO_x

The vanadium-based catalysts,especially V2O5/TiO₂ doped with WO₃ or MOO3,have achieved great success in selective catalytic reduction?SCR?with NH₃ for coal-fired flue gas denitration due to their high activity and durability toward SOx poisoning.Nevertheless,this kind of catalysts has relatively high price by using the Ti and V precursors,which consumes a large amount of high-purity TiO₂ and restricts their wide application in industrial boilers furnaces.On the other hand,titanium-bearing blast furnace slag?Ti-BFS?,which mainly consists of CaO,TiO₂,SiO₂ and Al₂O₃ with their contents of about 20-30%,is a massive solid waste from iron-steel industry.In China,it amounts 3.6 million tons per year,and could cause serious environmental problems if without efficient treatment and reuse.Considering the enriched Ti/Si sources in Ti-BFS,this study is devoted to developing low-cost V-W/Ti-Si catalysts with as well rather high efficiency from Ti-containing slag.The main focus in the thesis was laid on DeNOx performance of slag-based catalysts,but the information on synthesis,characteristics and potential applications of some slag-based products and residue were also addressed to outline a concept for full utilization of Ti-BFS.Through the adjustment of H₂SO₄ concentration and pH value in precipitation,a series of TiO₂-SiO₂ supports were obtained from Ti-BFS by conventional sulfate method,and they were further impregnated with V2O5 and WO₃ to prepare the SCR catalysts for NOx removal.It was revealed that SCR performance of the resulting V₂O₅-WO₃/TiO₂-SiO₂ catalysts is directly related to support structure and sensitive with the concentration of SiO₂ and unavoidable dopants in support composition.The incorporated SiO₂ up to 9.2%and the proper amount of Al₂O₃/Fe₂O₃/SO₄^2- unavoidable dopants in the slag-based supports are shown to have very positive impacts on NO conversion.While the former is responsible for the enhancement in the acidity and VO_x reducibility,the latter mainly resulted in a large surface area,well-dispersed active VOx species,sufficiently weak acid sites and high amount of Oads and V⁴⁺ species.These enabled slag-based catalysts to have good activity for catalyzing NO-NH₃ reaction in reference to catalysts synthesized from commercial TiO₂ and TiO₂-SiO₂ reagents.Nonetheless,too much SiO₂ and SO₄^2-dopant?about 3%?in the support caused agglomeration of TiO₂-SiO₂ particles,formation of strong acid sites and a high amount of Oads species to negatively impact the DeNOx activity,selectivity and lifetime of the prepared catalyst.In order to justify the feasibility of leaching process for Ti-BFS using waste acid with low concentration,different powdery TiO₂-SiO₂ supports and V2O5-WO₃/TiO₂-SiO₂ catalysts were prepared by mechano-chemical reaction using 30%H₂SO₄ aqueous and recycled H₂SO₄ aqueous.However,the more recycling times caused excessive SiO₂ and dopants in hydrolysis solution,which hampered the nanoporous structure of supports.For the catalyst using support made by recycling acid for two times,its pore size was rather heterogeneous and was mainly around 2 nm.This led to inhibit the mass transformation of the reactant molecules and the deposition of VOx on the surface catalyst,consequently causing an obviously poor DeNOx efficiency.Based on the above results,two monolith catalysts were produced by coating cordierite honeycomb substrate using powder catalysts on slag-based supports that were synthesized using fresh acid solution and the first-recycle filtration solution.Surprisingly,the latter exhibited the better NO reduction efficiency and stability in both powder and coated-monolith forms.It was acquired from the high acid strength and high porosity of such a support to facilitate the formation of abundant surface Oads and reduced V⁴⁺ species.Finally,hydrothermal processing of Ti-BFS has been performed to realize a high leaching yield of TiO₂ and diversify the slag-based products for a wide range of applications.Almost complete extraction of TiO₂ with a purity of 97%was achieved when Ti-slag reacted with concentrated acid in the acid/slag ratio?mass?of 2.0 at 150°C for 2 h.Meanwhile,this grade of TiO₂ has potential application in pigment,cosmetics,porcelains,and ceramic industries.The samples having 88-95%TiO₂with a recovery above 75%,which were obtained under low ratio of H₂SO₄/slag mass ratio or low acid concentration,can be adopted as the high-titanium slag feedstock for the production of metal Ti.Other TiO₂-based oxides including TiO₂-SiO₂,TiO₂-Al₂O₃ were also synthesized as a new pathway to fully utilize the valuable components in Ti-BFS.The TiO₂-SiO₂ binary oxide having 5.76%SiO₂ was proved to be fitted for SCR ofNOx by NH₃.The residue of CaSO₄.SiO₂ was characterized to show a fibrous structure with BET surface area of 80 m2/g,and it could be adopted as the green building materials?synthetic gypsum?,whereas SiO₂ existed in residue could positively enhance the gypsum properties.All three studied processes have proven their capacity to produce low-cost,stable and high-performance DeNOx catalyst from low-titanium slag.Of them,the mechano-synthesis is the promising approach to prepare SCR support and achieve the highest DeNOx efficiency.The major advantages are that the synthesized TiO₂-SiO₂ supports can be made under mild reaction conditions with diluted acid,and especially the waste sulfuric acid can be recycled to commence the next cycle of slag digestion.Overall,this thesis demonstrated the possibility of comprehensively utilizing Ti-BFS with the great economic and application interests.