Study On Evolution Of Tailings Pipeline Flocculation Behavior And Its High-concentration Discharge Performance

Paste stockpiling technology is an important support of national green mine construction.As an emerging technology to ensure the quality of paste stockpiling,the flocculation in pipeline technology refers to a complex dynamic process which is used to treat the tailing slurry that fails to meet the stockpiling standard after thickening.Through mixing slurry with the new flocculant in the pipeline near the stockpiling discharge outlet,under the action of hydraulic shear breaking and mixing,the internal floc structure and dewatering capacity of slurry is optimized.After slurry is discharged,combining with gravity deposition,it dehydrates rapidly.Then the mass concentration,rheological properties,early sedimentation slope of concentrated slurry are improved,thus ensuring the quality of tailings stockpiling.At present,there is a lack of systematic research on the process of tailings pipeline flocculation and its high-concentration discharge performance(i.e.mass concentration,rheological properties,early sedimentation slope and their relationship).The traditional single flocculation theory shows poor applicability.The discharge performance of high-concentration slurry is the external performance of its floc behavior,and the floc behavior is the internal reason for high-conccntration slurry discharge performance.Therefore,it is of great significance to carry out the research on the evolution of floc behavior and high-concentration discharge performance in the process of tailings pipeline flocculation.In this paper,the process of flocculation in pipeline was divided into three stages:the primary floc breakage stage,the secondary floc re-growth stage and tailings deposition and dehydration stage.Based on the flocculant selection and the condition optimization of two-step flocculation,with the help of macro experiment,micro experiment and theoretical analysis,the paper studied the evolution of tailings pipeline flocculation behavior and its high-concentration discharge performance.Firstly,the influence of shear action in different stages of pipeline flocculation on the discharge performance of high-concentration tailings was investigated,and the optimal range of shear action was obtained;Then,the floc behavior patterns of the primary floc breakage stage and the secondary floc re-growth stage were explored respectively,and the internal reasons for the change of high concentration tailing discharge performance were explained;Finally,the concept of shear ratio coefficient was proposed,and the theoretical prediction models of high-concentration tailings discharge performance were established,which effectively analyzed the process pipeline flocculation.The research work could be summarized as follows:(1)The flocculant selection and the optimization of two-step flocculation conditions were completed.Through single flocculation experiment and two-step flocculation experiment,the optimal flocculant combination type was selected for the fine tailings of an iron mine in Canada:anion flocculant 923 VHM and cation flocculant 4800 SSH.A 5-factor and 3-level Box Behnken scheme was carried out to design 46 groups of two-step flocculation condition optimization experiments and establish a multiple quadratic regression model of underflow concentration.Based on the response surface method,the influence of interaction between different factors on underflow concentration was analyzed and the optimal experimental parameters were obtained.(2)The change of high-concentration discharge performance for fine tailings with shear in the process of flocculation in pipeline was investigated.The physical simulation of the process of flocculation in pipeline for fine tailings was carried out using a self-made small-scale experimental device.The product value of the velocity gradient and the residence time was taken as the shear action index.The mass concentration,rheological properties and early sedimentary slope of concentrated slurry with the shear action index were discussed to obtain the optimal range of shear action in the primary floc breakage stage and the sccondary floc re-growth stage,respectively.(3)The effects of shear on the micro characristics of the primary floes and the internal reasons for floc breakage behavior were analyzed.The variations of the average particle size,the strength factor,the micro structure and the Zeta potential of the flocculating slurry with the shear were investigated.The failure behavior of the primary floes and its internal causes were analyzed.Shear fai lure led to stnuctural fracture and surface peeling of floes and produced broken floes with reasonable size distribution.At the same time,it disturbed the particle diffusion layer,reduced the repulsion between broken floes and made the floes in an unstable state,which provided the best conditions for the occurrcnce of secondary flocculation.(4)The effects of shear on the micro characristics of the secondary flocs and the internal reasons for floc re-growth behavior were studied.The variation of the average particle size,the re-growth factor,the micro structure and the Zeta potential of the flocculating slurry with the shear were investigated.It is found that the flocs could re-grow completely.The re-growth behavior of secondary flocs and its internal causes were analyzed.Shear promoted the collision probability between floes and the secondary flocculant and formed dense secondary floc network structure by means of electric neutralization and disturbance to particle diffusion layer.This was the fundamental reason for the improvement of high-concentration tailings discharge performance.Considering the floc breakage and floc re-growth behavior,the hypothesis of shear breaking structure and shear promoting flocculation synergistic effect was put forward.(5)The theoretical prediction models for the discharge performance of concentrated slurry were put forward.The concept of shear ratio coefficient was proposed for the first time by constructing the relationship between the velocity gradient and residence time of shear breaking and promoting flocculation and the decomposition rate coefficient and aggregation rate coefficient of floc structure.Based on the relationship between the shear ratio coefficient and the floc structure parameters,a prediction model of the ultimate concentration was established;Combined with the flocculation kinetic equation,the yield stress prediction model was derived;Based on the model of plastic viscosity of non-Newtonian fluid,the prediction model of plastic viscosity was derived.Based on the relationship between the Reynolds number of the fluid and the rheological properties of the slurry,a prediction model of the sedimentation slope was established.Combined with the key points of shear ratio coefficient,the change rule of concentrated slurry discharge performance with shear ratio coefficient was analyzed,and the above theoretical models were verified.(6)Based on the engineering background of a tailings stockpiling project of Alcoa,aiming at the actual situation of large output and difficulty in dewatering of fine-grained tailings,a process of flocculation in pipeline was designed in a test area of tailings pond.The rheological properties and early sedimentation slope of high-concentration tailings were analyzed using the theoretical prediction models.By comparing the characteristics obtained from the two adding points,the reasonable adding position of secondary flocculant was determined.The practical application in the experimental area showed that the theoretical prediction models were consistent with the measured results.The treatment scheme of flocculation in pipeline could significantly improve the high-concentration tailings discharge performance,and provided a practical basis for systematically solving the problem of reasonable stockpiling for fine tailings.