Research On Continuous High Concentration Stable Sand Release Model Of Vertical Sand Silo

With the implementation of a sustainable development strategy in China,green mining technology has become the main focus of technical innovation in mining.The backfill mining method,due to its safety,high recovery ratio,and lower levels of damage to the surrounding environment,has meant that more mines have begun to build filling systems so as to implement backfill mining methods.As an integral part of a filling system,vertical silos can load,store,and discharge tailing slurry.The discharge of tailings from vertical silos is important,and its underflow concentration and slurry flow regime affect the efficiency and cost of filling.However,existing methods of tailing discharge face several problems,for example,multiple silos work simultaneously to alternate discharge tailings;discharge underflow needs secondary slurrying;and the underflow concentration fluctuates significantly and is difficult to control.Decreasing the number of working vertical silos and maximising their efficiency can not only save considerable infrastructure funds and the cost of water and electricity,but also improve the uniformity and density of the fill under working conditions.Therefore,research on vertical silos is important.A continuous,high-concentration,stable model,based on a settlement model,was proposed.Continuity is reflected by cancelling the complex work modes in which multiple silos work simultaneously and discharge tailing slurry alternately.Instead of this work mode,a balanced condition between the inflow and the discharge of water and tailings is found in each silo which allows it to fill large goaf volumes continuously.The high concentration here refers to increasing the volume concentration of underflow in existing tailing discharge models about 54%.Stability implies that the underflow concentration fluctuates as little as possible during tailings discharge.Large fluctuations in the concentration of tailings discharges due to the application of high-pressure flow and water for secondary dredging of a discharge outlet blocked by tailings are thus avoided.The research was concentrated on the simulation and solution of dynamic settlement of tailings in the silo and the solution of the continuous,high-concentration,stable model.In this study,the tailings from the first filling station in Dahongshan copper mine in Yuxi,located in Yunnan Province,China were taken as the experimental material.Solid flux function,effective bulk stress,and critical compression ratios were obtained experimentally through the use of batch settling and separation trials run in a centrifuge.Taking a vertical silo with a diameter of 9 m as the geometrical model,the dynamic and static settlements of tailings in the vertical silo were simulated.In the dynamic settlement of the tailings,volume concentrations of the underflow for four different cumulative tailings heights were evaluated and the variation of slurry concentration with the height of the silo was simulated.Next,the cumulative heights of tailings required for different underflow concentrations and the variation of slurry concentration with soil height were acquired by running the proposed continuous,high-concentration,stable model.The results were all close to the simulated results,which verified the accuracy of the model,and then,the theoretical results were applied to the tailing filling and discharge system.The industrial testing of the model was carried out in a vertical silo(diameter:9 m).The accuracy was also evinced by comparison of the results of the industrial test and the model.The key conclusions were as follows:(1)Fluent software was used to simulate the static settlement model.The simulation of the static settlement of tailings is performed in two steps:the first simulated the process from transporting the tailing slurry until overflow through the overflow ports on the top of the silo;and the second step involved the modelling the accumulation of the tailings in the slurry from the cone bottom to the top of the silo,layer-by-layer;until the whole silo had been filled.In the first step,four simulation schemes were formulated,for which the feeding flow rates were:200,250,300,and 350 m3/h,to obtain the distribution,and state of settlement,of the tailings with different grain sizes under each of the four conditions.The second step formulated three schemes,in which the volume fractions of overflow tailings were:0,0.04,and 0.06 to acquire the distribution and state of accumulation of the tailings with different grain sizes.In addition,the dynamic settlement of the tailings in the silo was simulated using the same software.In the simulation,four accumulated heights of tailings were selected to record the corresponding underflow volume concentration and the variation of slurry concentration with changing height of the silo based on the pre-set monitoring curves.(2)In existing tailings discharge systems,feeding,settlement,and discharge are alternately conducte.d in multiple vertical silos.Differing from this mode of operation,a continuous,high-concentration,stable tailings discharge model is established on the basis of the mass balance between the tailings and water.(3)Six groups of whole tailing slurry with the mass concentrations of 15 to 40%were prepared to conduct batch settling experiments.During the experiments,data including the rate of settlement and time taken for the tailing slurry to reach different concentrations were recorded.On this basis,the solid fluxes of the tailing slurry with different concentrations were calculated.The equation governing solid flux behaviour is regressed by using the volume concentration of the slurry as the abscissa and the solid flux value as the ordinate.According to the tailings settlement curve,the critical compression volume concentration was computed,that is,the volume concentration at the interface between the compression zone and the settlement zone in the silo.By carrying out seven groups of separation experiments on a centrifuge,the rate of rotation of which was set to between 400 to 1,700 rpm,the volume concentrations of different slurrys and the corresponding effective solid stresses were calculated.By applying the volume concentration as the abscissa and the effective solid stress as the ordinate,the equation of the effective solid stress was derived by regression.The equations for the solid flux and the effective solid stress are two important parameters in the continuous tailings discharge mathematical model.The critical compression volume concentration serves as the discriminant value of the tailings height.When the volume concentration of the tailings exceeds its critical value,the particles in the tailings are in contact with each other and an effective solid stress is generated.(4)The discriminant ?0<?1 for the formation of self-flow of the slurry solution at the tailing discharge outlets is proposed.It is a comparison between the yield stress in the slurry and the maximum principal stress on the arch foot of the tailings.If the former is less than the latter,the slurry solution is able to flow automatically over the conical base of the silo;otherwise,it cannot flow unaided.(5)The equations governing the solid flux and the effective solid stress are derived by carrying out settling experiments and separation experiments using the centrifuge.Then,they are substituted into the proposed continuous,high-concentration,stable tailings discharge model to solve the model using mathematical software.In this way,the underflow volume concentrations corresponding to different accumulation heights and the variation of the volume concentration of the slurry with silo height are obtained.The comparison of the results and the simulated results using Fluent showed that they were sufficiently similar to verify the correctness of the model.The application of the model to an industrial test displays favourable effects,which validates the theoretical efficacy of the model.(6)By substituting different underflow volume concentrations(in increments of 0.05 in the range 0.52 to 0.58)in the proposed model,the authors calculated the height of the tailings when dynamic equilibrium was reached in the silo.Based on these data,the relationship between the underflow volume concentration and the height of the tailings was explored,and was found to be an exponential function.Then,the relevant equation was obtained by regression.The application of the proposed continuous,high-concentration,stable tailings discharge model for vertical silos effectively overcomes the technical problems facing existing tailing discharge systems in mines.These problems include:low discharge concentration,large concentration fluctuations,and the necessity of frequently applying high-pressure flow and water.It greatly improved the work efficiency of vertical silos as it reduced the number of working vertical silos,omitted the process of completely discharging and charging the silos,and simplifying the preparation of slurry materials.With these advantages,the model guaranteed the filling efficiency and quality.Industrial tests showed that the model worked and that the underflow volume concentration met the requirements for mine production.Therefore,it avoided the waste of resources and equipment and saved water and electricity.The research provided a theoretical basis,and technical guidance,for the design of continuous tailings discharge and filling systems.