Mechanism And Process Simulation For Chemical Clogging Of The Tailings Dam

The safety and stability of the tailings dam are very critical for the ecosystem of the surroundings and the life and fortune of the people. However, clogging of the tailings dam will bring about difficulty for water drainage to have great influence on the stability and safety of the tailings dam. Therefore, the mechanism and control of the clogging are of important value in solving this problem. Financially supported by"A Model of CoupledDeformation-Seepage-Chemical Processes in Variably Saturated Media"(National Natural Science Foundation of China, Grant No. 10572090), this dissertation has studied the main clogging materials, the mechanism of the clogging, and the interaction between the clogging and seepage by carrying out the field investigation and laboratory sand column experiments and using analysis methods such as ICP-AES, XRD and SEM-EDS, and the main research results are as follows.(1) The results of field investigation on Lixi tailings dam of Jinduicheng Molybdenum Group Mining Corporation show the water types of the samples from the tailing pond and the effluent of the radial wells are SO₄^2-—Ca²⁺ and SO₄^2--HCO₃^-—Ca²⁺. The pH of the samples ranges from 6.8 to 7.5. Iron is the dominant element in the clogging materials, and the contents of two clogging samples are 54.35% and 40.24%, respectively. The analysis results show that the clogging materials are a mixture of iron hydroxide and its converted products, and the clogging materials commonly exist in an amorphous form with a cluster microstructure when viewed under SEM.(2) The results of ferrous iron oxidation experiments show that the ferrous iron oxidation follows first order kinetics, and the oxidation process is strongly dependent on pH, a higher pH resulting in a higher oxidation rate. Moreover, results also indicate that a higher concentration of ferrous iron causes a higher oxidation rate.(3) Columns were packed with different clean quartz sands including 5# sand (particle size 0.20-0.50 mm), 6# sand (particle size 0.10-0.20 mm), and 8# sand (particle size 0.076-0.15 mm), respectively. The ferrous iron solution was continuously pumped using a peristaltic pump from the bottom to the top of the experimental column. The results of clogging experiments using quartz sand columns indicate that ferrous iron in the solution layer is oxidized and the clogging materials are gradually formed under the conditions of adjusting the pH of the solution layer as 6.8-7.5 and the natural oxygen supply. The clogging materials accumulated and clogged the outlet to cause the clogging according to the results of the experiments. When the inlet concentration of ferrous iron solution was 100 mg/L, clogging completely occurred for 5# sand column experiment, 6# sand column experiment, and 8# sand column experiment operated for 20, 23, and 26 days, respectively. When the inlet concentration of ferrous iron solution was 10 mg/L, clogging completely occurred for 5# sand column experiment, 6# sand column experiment, and 8# sand column experiment operated for 54, 51, and 77 days, respectively. The results show that the occurrence of clogging bears relation with the inlet concentration of ferrous iron solution, a higher concentration resulting in a shorter stage to occur clogging. Moreover, the occurrence of clogging is in connection with the properties of porous media. The change in the hydraulic conductivity of different clogging experiments has the similar trend that the hydraulic conductivity decreases significantly at the last stage.Furthermore, the clogging experiments of tailings sand column were carried out. When the inlet concentration of ferrous iron solution was 100 mg/L and 10 mg/L, clogging completely occurred for tailings sand column experiments operated for 42 and 105 days, respectively. Results have shown that there are two stages during the experiment, the buffer capacity of the tailings sand layer is gradually attenuated at the first stage, and then the ferrous iron is continuously oxidized into the clogging materials to cause the clogging at the second stage. The change in the hydraulic conductivity of tailings sand column experiments is similar to that of quartz sand column experiments.Clogging materials were analyzed after clogging experiments, and iron contents of clogging materials are higher than 37%. The clogging materials are found to exist in the loose cluster microstructure, and iron hydroxide may be the main initial compound according to the results of XRD and SEM.(4) The results of the tracer tests indicate that there are two stages for the change in the dispersion coefficient during the clogging experiment by alternately introducing the solution. The clogging materials accumulate in the porous media to result in stronger heterogeneity, so the dispersion coefficient increases at the first stage. When the accumulation of clogging materials is gradually relatively stable, and pore distribution becomes homogeneous, so the dispersion coefficient decreases at the second stage.(5) Based on the hypothesis that the ferrous iron has been oxidized into the iron hydroxide, a series of sand column experiments are carried out. The relationship between concentration and other parameters such as the hydraulic conductivity, the dispersion coefficient and so on is acquired. The flow and transport models during clogging process are coupled according to the sand column experiment results.(6) The chaos theory and fractal theory are applied to simulate the change in the concentrations of ferrous iron, total iron and filtered total iron of the effluent during the clogging experiments of 6# quartz sand column.