Preparation Of Activated Red Mud And Its Performance And Mechanisms For Phosphate Adsorption

Red mud is a solid waste from alumina production. The amount of red mud ranges from 0.3 to more than two tons per ton of alumina production. With the development of aluminum industry and the reduction of available bauxite grade, the generation of red mud is growing in the world. Therefore, it is quite important to treat and utilize the red mud with reasonable methods.Combination of neutralization activation and HCl activation was used to improve the phosphate adsorption performances of red mud in this paper. The phosphate adsorption performances and related mechanisms of acid-activated neutralized red mud(Aa N-RM) were systematically investigated. The powdered Aa N-RM was further granulated, and the phosphate adsorption performances and related mechanisms of granular Aa N-RM in the batch and fixed-bed experiments were investigated.The results indicated that the combination of neutralization activation and HCl activation was effective to improve the adsorption capacity of red mud. The constructed RSM model was highly reliable, and the fitted quadratic equation model effectively reflected the relationship between various factors and response value during the activation process. The maximum adsorption capacit y of acid-activated neutralized red mud(AaN-RM) was 170.75 mg/g with a HCl concentration of 10.20 mol/L, an activation temperature of 41.00 oC, and an activation time of 5.60 h, which increased by 10.53 and 6.62 times compared with the raw red mud and neutralized red mud, respectively.The influence of Aa N-RM dosage, adsorption temperature, init ial solution p H, initial phosphate concentration and competing ions on the phosphate adsorption of Aa N-RM was investigated with batch experiments. The results illustrated that the optimal Aa N-RM dosage, adsorption temperature and initial solution p H were 0.5 g/L, 50 oC and 4.5, respectively. With the increase of initial phosphate concentration the phosphate adsorption capacity of AaN-RM firstly increased and then tended to stable. Carbonate ions competed the effective adsorption sites on Aa N-RM surface with phosphate, which reduced the phosphate adsorption capacity of Aa N-RM. The whole adsorption process was well described by pseudo second-order kinetic model and Langmuir-Freundlich isotherm, which suggested that the phosphate adsorption was governed by heterogeneous processes. The phosphate complexes of Fe-P, Al-P, Fe-P-H3PO4 and Al-P-H3PO4 were formed on AaN-RM surface through the ion exchange, precipitation and surface deposition mechanisms. XPS analysis of P 2p peak showed that 59.78% of the phosphate was adsorbed through the ion exchange and precipitation with strong chemical bonds, and 40.22% was adsorbed through the surface deposition with weak chemical bonds.A new granular Aa N-RM was successfully prepared with powdered Aa N-RM, powdered straw, and hydroxypropyl methylcellulose(HPMC) for effective phosphate adsorption. The excellent phosphate adsorption capacity was mainly attributed to the high specific surface area, which was caused by complex mineralogy of iron and aluminum, and pore formation due to the addition of powdered straw during preparation of granular Aa N-RM. The influence of preparation parameters of granular Aa N-RM on phosphate adsorption performances was also systematically investigated. The results demonstrated that the mass ratio of different ingredients, sintering temperature, and sintering time greatly affected the characteristics of granular Aa N-RM. When the mass ratio of powdered straw increased from zero to 33%, the total pore volume of granular Aa N-RM increased from 0.0056 to 0.0375 cm3/g, however, the number of effective chemical sites reacting with phosphate reduced when the mass ratio of powdered straw was too high. With the increase of sintering temperature and sintering time, the polymerizability of granular AaN-RM was significantly improved, but too high sintering temperature and too long sintering time resulted in the conversion of part Fe Cl3 2H2 O to Fe OCl and Fe Cl2, and part Al(OH)3 to Al2O3, which resulted in a weaker attraction on phosphate onto granular Aa N-RM. With the consideration of strength and phosphate adsorption capacity of granular Aa N-RM, the optimal mass ratio of powdered Aa N-RM, powdered straw and HPMC was 71:22:7, and the sintering temperature and sintering time were 225 oC and 30 min, respectively.In order to better understand the phosphate adsorption performances of granular Aan-RM, the influence of operational parameters on the performances of granular Aan-RM and the possible adsorption mechanisms were investigated in batch experiment. The results demonstrated that adsorption temperature and initial solution pH significantly influenced the phosphate adsorption performance of granular Aan-RM. The maximum phosphate adsorption capacity reached 120.68 mg/g with an adsorption temperature of 40 oC and an initial solution p H value of 6.0. The whole adsorption process was well described by n_th-order kinetic model and Langmuir-Freundlich isotherm. The XPS analysis of P 2p peak on granular Aan-RM after phosphate adsorption demonstrated that 79.01% of phosphate was adsorbed through the precipitation and ion exchange mechanisms with strong chemical bonds, and 20.99% of phosphate was adsorbed through the surface deposition mechanism with weak chemical bonds.The potential of granular Aa N-RM to adsorb phosphate in a fixed-bed column was firstly investigated in this study. The results demonstrated that the granular Aa N-RM had excellent adsorption performances in the fixed-bed column. The phosphate adsorption capacity of 84.80 mg/g and the number of bed volume processed higher than 495 were much higher than those previously reported. The saturated granular Aa N-RM was successfully regenerated with 0.5 mol/L HCl solution. The phosphate desorption efficiency was 92.30% in the first adsorption-desorption cycle, which still maintained 83.45% in the fifth cycle with a low weight loss of 3.57%. Moreover, it was found that the phosphate adsorption capacity of granular Aa N-RM increased with the increase of bed depth and initial phosphate concentration, while it decreased with the increase of flow rate. The optimal empty bed contact time and initial solution p H were 24.17 min and 5.52, respectively. Additionally, the modeling results of breakthrough curves showed the superiority of a 5-9-1 feed forward artificial neural network(ANN) compared with Adams-Bohart model and Thomas model, and the ANN can be effectively used for the design and optimization of adsorption system.