A Novel Method To Produce Highvalue Added Catalyst From Mn-rich Slag, And Its Utilization In Efficient Low-temperature Selective Catalytic Reduction Of NO_x

At present, the utilization of Mn-rich slag in China has serious limitation, and the electroplating wastewater treatment is also very difficult. Meanwhile, the Si O2 and metal oxides in Mn-rich slag and the heavy metals in wastewater are available resources. Thus, we propose a concept that utilize Mn-rich slag and wastewater to synthesize derived supported NO_x catalyst. Firstly, Mn-rich slag was activated by acid/alkali methods to form active matrix material. Then, Cr-containing wastewater was added for necessary structure control to form high efficiency catalyst. The performance was further improved by synthesis conditions optimization. Additionly, the formation process of Mn-rich slag and Cr-containing wastewater was discussed by process analysis methods. Moreover, the key mechanism of this catalyst for NO_x removal was investigated by evaluation of catalystic performances and establishment of simulation system. The main research processes and conclusions were as follows:(1) Surface activation of Mn-rich slag enhanced NO_x catalytic activity Since the performance of Mn-rich slag was limited by its structure of silica coated metal oxides, Mn-rich slag was immersed in acid or alkali to reorganize the surface structure and optimize the catalytic performance. The result indicated that nitric acid could leach metal ions and precipitate the metal ions on Mn-rich slag surface. Thus, NO removal of catalyst increased from 40% to 88% at 200 oC by 1 mol/L HNO3 activation; Treatment of 0.1 mol/L hydrofluoric acid had 5%-12% promotion to catalytic performance in the whole temperature range. However, the activity decreased after treatment of 1 mol/L of hydrofluoric acid, because 44% Si dissolved from Mn-rich slag and the structure was destroyed; On the other hand, treatment of ammonia solution made Si, Al, Ca in Mnrich slag partially being hydrolyzed. As a result, part of metal oxieds were exposed on surface, and the activity of catalyst improved from 10% to 50% at 100 oC. Above results showed that suitable proportion and structure of surface were very important to the NO catalytic performance of Mn-rich slag derived catalyst.(2) Mn-rich slag derived catalyst had high catalytic acitivity of NO_x-SCR Cr-containing wastewater modified the surface structure of ammonia activated Mn-rich slag, and formed catalyst with good NO catalytic performance. Catalysts were prepared by four methods, including physical mixing, citrate method, impregnation and coprecipitation method. Co-precipitation method proved to be most beneficial for solidliquid combination in Mn-rich slag derived catalyst system. When the temperature increased, the catalytic activity improved from 47% to 93% at 100-200 oC. Furthermore, microwave hydrothermal method had positive effects on the catalytic performance and the catalyst composite prepared in this way could almost totally remove NO at 150 oC. Besides, Si O2 network skeleton was found to be a very important support in catalyst, NO removal of catalyst without Si O2 decreased to lower than 40%; Ammonia activation was beneficial for combination in formation of Mn-rich slag derived catalysts, the catalyst without ammonia activation only removed 27% of NO at 150 oC. Thus, the efficient structure in Mn-rich slag derived catalyst was resulted from the supported structure of Si O2, the surface of which was highly dispered by Mn, Cr et al. oxides.(3) Formation of highly efficient Mn-Cr structure on the surface was the key to excellent catalytic performance of Mn-rich slag derived catalyst Gas product analysis showed that NO was mainly decomposed to N2 after the catalytic reaction. And in this process, the structure of catalyst was changed, part of Mn2O3 convert to lower valence as Mn3O4. Meanwhile, the NO3- remained in the catalyst lead to the decrease of adsorb sites and caused decrease of NO removal. These phenomena illustrated that the high valence Mn and strong adsorption capacity of NO were important to the catalytic performace of Mn-rich derived catalyst at low temperature. NH3-TPD showed that the derived catalyst with Cr modification containing 0.99 mmol/g weak acidic sites, which was 0.74 mmol/g more than the catalyst without Cr. What’s more, the removal was promoted from 55% to 98% at 150 oC, because the increase of acid sites improved the adsorption capacity of ammonia at low temperature. Moreover, the XRD and HRTEM results suggested that the Mn species on catalyst mainly existed in amorphous phase and dispersed very well, which were benefit for the lessen of agglomeration and the addition of active sites. In addition, the result of XPS showed that the Mn4+ ratio on catalyst surface improved from 21% to 50% with the Cr modification and formed Mn4+-Cr3+ structure. Because of the above structural advantages, Mn-rich derived catalyst easily oxidated NO to NO2, NO2-, NO3-, which could react with NH3 rapidly to achieve high removalof NO.Mn-rich slag could form high value-added catalyst for NH3-SCR at low temperature by activation of surface and modification of Cr-containing wastewater. The present work thus provided a new strategy to the recovery of Mn-rich slag and Crcontaining wastewater with high value-added products.