| Bentonite is wildly distributed in China and provided properties with modifiability, inflation and adsorption. In terms of “silicate influence” in non-wood pulping mills, a type of novel, efficient and recycling silicate removal agent called Aluminum salts modified bentonite was applied in green liquor desilication in this thesis. The demerits of low silicate efficiency, apparent p H fluctuation and long time to precipitate for traditional silicate removal agent were improved. In addition, the processes of preparation, desilication, desorption and regeneration were comprehensively introduced. Regarding as the background of “Twelfth five years plan of Energy Saving and Emission Reduction”, the paper did not only introduce an integrated application of the adequate mineral resources in China, but would realize the energy saving and emission reduction of enterprises in future. Besides, the performances of recycling and economic processes will catch a goal of the environment protection and the production had a broad market prospect.(1) Process and mechanism of preparation of Aluminum salts modified bentoniteA variety of novel and efficient silicate removal agent was prepared by adding the modifier of Aluminum salts as th e Na-bentonite used as the raw material. The agent called Aluminum salts modified bentonite had a desirable adsorption on silicate in green liquor which could exceed 90% in maximum. Primarily, the preparation process of Aluminum salts modified bentonite wa s investigated in the term of the silicate adsorption and fluctuation of p H of green liquor. The Scale of Na-bentonite and Aluminum salt was 1 to 4. The adding order was that the Na-bentonite was mixed with the water initially, then, the Aluminum salt was put into. The loading of the water was 66 m L of 1g Na-bentonite. The stirring time and speed were 2h and 1200r/min, respectively. Contact temperature was at room temperature. After stirring, the mixture was settled for 12 h and separated. The sediment was dr ied at 105℃ in the oven and ground before screened with 200 mesh screen. The well-prepared Aluminum salt modified bentonite was characterized by SEM, XRD, TG-DSC, FTIR and particles distribution. Therefore, the preparation mechanism could be deduced that t here were a number of Na atoms and hydroxyl groups on the surfaces and layers of Na-bentonite. Blending the Na-bentonite and water resulted in the layer space increased as the water availed to open the layer of the bentonite and benefit to lead the Aluminu m salt into the layer. After mixing, Aluminum salts was added, and due to that the attraction between hydroxyl groups and Aluminum atoms was stronger than that of Na atoms, the Aluminum atoms commenced to replace the Na atoms which were freed from the original system. Furthermore, the ploy-hydroxyl-aluminum was produced by hydrolysis of Aluminum salt. Over 2 hours stirring, the Aluminum atoms had completely superseded Na atoms and the surface of bentonite had distributed many Al-OH groups. Finally, the Aluminum salt modified bentonite was accomplished.(2) Process and mechanism of desilication of Aluminum salt modified bentoniteThe optimum process parameters of desilication were the loading of Aluminum salt modified bentonite was 16g/L, contact time was 10 min, the temperature was 90 ℃ and the equilibrium time was 5min.The kinetic model, isothermal model and thermodynamic model of silicate adsorption were indicated that the whole adsorption process was suitable to the pseudo-second-order kinetic model embracing physical and chemical adsorption. The maximum adsorption capacity was 252.5252mg/g coinciding with the experimental adsorption capacity of 238.4411mg/g. Moreover, the entire adsorption also was suitable to the Freundlich model in which 1/n was smaller t han 1, which means the adsorption reaction was easy to occur and the adsorbent could offer a high absorbing ability. After a brief thermodynamic analysis, it was concluded that the absorbing reaction was spontaneous without any extra energy. The physical adsorption of Aluminum salt modified bentonite relied on the expanded gaps of bentonite, but the performance of physical adsorption was poor. The chemical adsorption was predominant and the mechanism could be depicted that the existence form of silicate was Si-OH groups in the green liquor(alkali solution) and there were a number of Al-OH groups on the surface of modified bentonite, and when the modified bentonite was added into the green liquor, Al-OH reacted with the Si-OH called adsorption reaction. So, the silicon element could be absorbed on the bentonite. While the reaction was finished, the modified bentonite was filtered with the silicon element. Eventually, the purpose of desilication was successfully obtained.(3) Process and mechanism of flocculating desilication of Aluminum salt modified bentoniteThe flocculating desilication with the flocculants such as Aluminum salt modified bentonite, CPAM and Ca O had an excellent silicate removal rate arriving at the summit of 99.9%. The other merits of the flocculants were reducing the p H fluctuation and accelerating the settlement of silicate. Thus, the flocculation desilication measure was considered as a novel, efficient and environmental process. The optimum conditions were the loading of Aluminum sa lt modified bentonite was 16g/L, the dosage of CPAM was 15 m L per 1 L green liquor, the consumption of Ca O was 15g/L, stirring time was 10 min and the clarifying time was 5 min. In the section of flocculating desulication, the modified bentonite connected with silicate sufficiently in the green liquor. The CPAM was in the shapes “ring and tail” regarded as the function of “bridge”. It relinked the dispersed flocculates into aggregation. The equivalent point was achieved by adding a small quantity of flocculants and the flocculants gathered into flakes. The pieces of flakes were settled and segregated with the supernatant resulting in advancing the speed of settling and shortening the clarifying time.(4)Process of desorption and regeneration of Aluminum salt modified bentoniteThe desorption process can be described that 2 mol/L sodium hydroxide was added into an recycling applied Aluminum salt modified bentonite, and the mixture was heated for 4 min by microwave in the high level. The silicate connected on t he modified bentonite was freed. After filtration and desorption, the modified bentonite was left on the filter net. The silicate was in the filter liquor. The operation of cooling, filtration and drying was introduced for separating out silicate. The modified bentonite after desorbing was regenerated with the preparation process. Considering of the cost and the equipment consumption, it suggested that the modified bentonite could be recycled for 2 or 3 times. Additionally, desorption kinetic model elucidat ed that the process of desorbing silicate under high level of microwave was suitable to the pseudo-second-order kinetic model, the electromagnetic field facilitated the physical and chemical desorption. The maximum desorption material capacity was 129.870 mg/g coincided with the experimental capacity of 120.948 mg/g. Lastly, the desorption was characterized by SEM, FTIR and energy element spectrum was concluded that the main composition of desorption material was silicate. |
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