Numerical-Experimental Study And Parameter Optimization Of Red Mud Separation Process For Alumina Thickener

Red mud separation is one of the most important procedures in Alumina refinery industry, and it has significant impact on the quality and the yield of alumina products.Problems were frequently occurred in the solid-liquid separation process due to the degradation of the bauxite’s grade in recent years, which make thickeners’capacity and stability becoming the bottleneck in alumina refinery industry. Therefore,a comprehensive study of the thickener and red mud separation process will provide great benefit for improving capacity, optimization as well as predictive design of the thickener.For the purposes of promoting the flocculation reaction rate, a high rate converging base thickener on an industry scale was chosen as the research object, and the solid-liquid separation process inside the thickener was investigated through numerical calculation, experimental study as well as theoretical analysis techniques respectively. The main detailed work and findings of this research were expatiated hereunder:(1)The domestic researches of separation thickener and red mud separation process was rare and relevant publications was lack of details due to confidential issue.Therefore a three-dimensional numerical model was established which can be applied to investigate the red mud separation process of the thickener.The model was validated through the on-site data.(2) For the purpose of improving the red mud concentration distribution and the flocculation reaction rate, the best volume fraction of solid concentration was proposed as the evaluation indicator and a novel self-dilute inlet pipe was also created and optimized based on the self-defined evaluation factor using the numerical simulation technique. The optimized self-dilute inlet pipe was introduced to a alumina refinery enterprise.The numerical simulation results indicated that the feedwell’s average solid concentration was decreased from71.8g·1to56.8g·1, and the best flocculation volume fraction was increased from7%to21% after introducing the self-dilute inlet pipe.The self-dilute inlet pipe can dramatically improve the flocculation reaction rate in the feedwell.The on-site modification result reported that the solid concentration of the underflow was increased by2.4%while the amount of overflow’s float and the flocculants dose was decreased by2%and8.1%respectively1.635million Yuan can be saved from the capital cost of the enterprise annually.(3) An experimental thickener model was established. Tracer-Respond as well as Finite Granules Trace Method was proposed and used to investigate the flow patterns of the feedwell qualitatively and quantitatively. The swirling flow, dead zones and "short circuit" were detected during the experiment, and the buff ring can minimize the "short circuit"’s occurrence dramatically. The feedwell’s aspect ratio have significant effect on its flow patterns.The dimensionless mean residence time was increased along with the feedwell’s aspect ratio increase.(4) A transient numerical model was established which can be used to investigate the mixing process of bauxite slurry. The effective flow regime volume fraction ζeff was introduced as the evaluation criteria, non-ideal flow models,non-linear regression and residence time distribution’s moments were combined as the analysis method to investigate the flow distribution of the feedwell under different structural and process parameters.The simulation results indicated that the feed flows inside the feedwell belong to highly dispersed flow scheme.Under such condition, the AMD model parameter Pe is between0.21to3.21, and the feed flow inside the feedwell can be depicted as1.19—3.44mixing tanks connected in series.The structural and process parameters have dominate influence on feed flow’s distribution of the feedwell. The dead zone and plug flow will convert into each other when altering the feedwell’s diameter and the buff ring’s position; Feedwell’s height and the feed inlet rate can lead a conversion between the dead zone and the mixing flow of the feedwell;Inlet pipe’s position can also lead a conversion between the dead zone,plug flow and mixing flow in the feedwell.(5)According to the orthogonal experimental design and the evaluation criteria ζeff, five factors which influence the feed flow’s distribution of the feedwell were simulated and optimized. Among which, the feedwell height (D) and the inlet feed rate(A) are most significant factor, while the inlet pipe’s position(C) is significant and the feedwell’s diameter(B) is less significant, the buff ring’s position gives no significant impact. The optimum factor scheme is A5B1C5D1E1, and the simulation results show that the ζeff can be increased up to90%compared that of the base case’s67%.The optimum scheme can provide a better flow environment and promote the flocculation reaction rate.(6)Based on the red mud’s settling and thickening mechanism, a one-dimensional mathematical model was established to depict the red mud’s thickening-compression process.The calculation of predictive solid flux of the thickener was realized by Matlab programming, and the model was validated through on-site data. Combined with traditional feed material properties characterization techniques,thickener’s capacity can be predicted rapidly, which will benefit the control, design and the digitization for the thickening process of the thickener.