| Large amounts of tailings were discharged during the mining process.Once these tailings cannot be used for gob backfilling,they should be stored in tailing ponds on the surface.Traditional tailings pond is a hidden danger for the dam breach.Contaminants which include sulfur,arsenic,heavy metals and so on in the discharged tailings can cause environmental pollution,too.The technology of consolidated and cemented tailings emission disposal has the advantages of safety,environmental protection,economy and land resource conservation.It also is a new technology that is expected to solve the above two major hazards.This article is funded by the National Fund Project(No.51674263).Research on pollutant migration behavior was carried out during the process of consolidated and cemented tailings emission disposal.This paper firstly establishes the technical route which considered the heavy metal leaching in all links of tailings consolidation and emission,using the methods such as theoretical analysis,laboratory tests and numerical simulation.Then,this article studied the evolution of physical properties of consolidated and cemented tailings,the multi-field coupling theory in tailings emission disposal and the corresponding tailings emission disposal technology.It also explored the leaching and migration behavior of consolidated and cemented tailings in different stages,different external conditions and different material ratio conditions.The main research is as follows:(1)An experimental study relating to the physical property evolution in tailings emission disposal was carried out.The chemical composition and particle size of the tailings were determined.Rheological properties of tailings slurry,initial setting time and final setting time of slurry consolidation,uniaxial compressive strength,the shear strength of whole tailings induration,pore structure parameters and permeability was tested.The mathematical model of the hydration reaction degree was established and the mathematical model for the evolution of the porosity and permeability coefficient with the hydration reaction degree was deduced,based on the experimental data.The microstructure change rule of the consolidated and cemented tailings was revealed by the SEM scanning.The study shows that: the main factor that affect the rheological properties of the slurry is the slurry concentration.The effect of cement content on the rheological properties of the slurry is less important.The uniaxial compressive strength increased with the increase of cement content,slurry concentration and curing age.Among them,the cement content is the main factor and the slurry concentration is the minor factor.The cohesion increased with the increase of cement content,slurry concentration and curing age.The friction angle increased with the increase of cement content,which changed from 5° to 35°.With the increase of slurry concentration and curing time,the porosity decrease,the total pore volume decrease,average pore diameter decrease,the pore structure fractal dimension increases.The pore structure is more complex and its space occupied capacity increases.(2)Experimental study on leaching regularity of heavy metal and disintegration characteristics of consolidated and cemented tailings were carried out.The water bleeding law of at the early stage of tailings discharged and the disintegration law of consolidated and cemented tailings after long-term pile-up were studied.The law of heavy metal leaching at different stages after tailings were studied,the solidification effects of heavy metals in consolidated and cemented tailings were clarified.The mechanism of disintegration of consolidated and cemented tailings was revealed.The main results are as follows: The main factor affecting the water bleeding is the slurry concentration,and the effect of the cement content and type on the bleeding volume is small.The content of heavy metal in the bleeding water with different cement content is basically the same.Slurry concentration has little effect on disintegration of consolidated and cemented tailings,but cement content has a great effect on it.3% is the critical cement content of disintegration of the consolidated and cemented tailings.Below this value,the consolidated and cemented tailings will disintegrate in water and the amount of heavy metals leaching will increase sharply.when the cement content is greater than 3%,the disintegration will not occur and the amount of heavy metal leaching is greatly reduced.Therefore,whether the consolidated and cemented tailings will disintegrate is particularly important for heavy metals solidification.With the increase of the cement content,the amount of heavy metal leached decreases.The effect of heavy metal solidification was revealed: the addition of cement in water bleeding stage can not reduce heavy metal’s leaching.But it can significantly reduce the amount of heavy metal leaching in the long run.The mechanism of consolidated and cemented tailings’ disintegration was obtained.Matrix suction cause the water immersed.Water infiltration will cause the original gas to be squeezed and discharged.The disturbance formed by the gas discharged and the extrusion of the original gas between the pores by the infiltration of water will break the linked of particle and then cause the disintegration.(3)This paper described the coupling theory of mechanical-chemical-hydraulicheavy metal leaching and migration in the process of consolidated and cemented tailings emission disposal.The mathematic models for heavy metal leaching-migration behavior under the effect of multi-field coupling in different stages,different external conditions and different material ratio conditions were established.The mathematic models were used to describe water bleeding and solute migration behavior under initial gravity and chemical reaction.It is also used to describe the behavior of seepage and solute transport under the chemical field in unsaturated areas and saturated areas.It revealed the mechanism of heavy metal leaching in the process of consolidated and cemented tailings emission disposal.It elucidated the mechanism of the solute migration in the consolidated and cemented tailings.The main conclusions are as follows:(1)This paper established a deformation model of consolidated and cemented tailings under stress-chemical coupling in the early stage of tailings emission disposal.The calculation formula for the deformation was deduced as well.(2)The mechanism of heavy metals leaching was revealed.The water between the pores of consolidated and cemented tailings will contact with the tailings.Due to the concentration gradient of heavy metal between matrix and the water in the pores,heavy metal will diffuse into the pore water and cause the leaching of heavy metals.A model for heavy metal leaching in consolidated and cemented tailings was established,and an expression to describe the leaching rate was obtained.(3)It clarifies the mechanism of heavy metal migration between pores.Heavy metal leached into pores migrated due to convection,mechanical dispersion,molecular diffusion.In this process,due to the multi-field coupling effect of mechanical-chemical-hydraulic,the pore volume decreases,the permeation performance decreases and the solute diffusion capacity weakens,which in turn causes changes in the migration ability of heavy metals.(4)The leaching-migration behavior of heavy metal in the process of consolidated and cemented tailings emission disposal under multi-field coupling has been studied.The mathematical model is embedded into Comsol software to simulate and study heavy metal leaching-migration behavior at different stage and different condition.The main results are as follows:(1)In the stage of tailings sedimentation and water bleeding,the porosity,water storage coefficient and permeability coefficient are decreased with the development of cement hydration.The pore water pressure increased,stress on the matrix reduced and the volume decreased.These change rules are same when cement content changes.In the early time the tailing emission disposal,solute concentration in the tailing is high.With the leaching of heavy metals,solute concentration increased further.The content of heavy metals is little in the soil below and the pore water will take away some of the solute.As a result,its content will decrease.Due to the time for tailings sedimentation and water bleeding is short,the water in the tailings has not transferred as well as the heavy metal.All in all,the heavy metal migration is not significant in this stage.(2)The chemical reaction changed the matrix properties of the tailings after long time discharged.As a result,the discharged tailings’ porosity,water storage coefficient,permeability coefficient,heavy metal leaching rate and solute diffusion coefficient reduced significantly.The results also show that these reduction increased with the cement content increased.Beside,the seepage characteristic and the solute transport characteristics of the tailings changed with the cement content change: with the increase of cement content,the permeation slow down,the range and size of effective saturation decrease and the change of pore water pressure slow down.Obviously,the consolidated and cemented tailings emission disposal can significantly reduce the seepage behavior of discharge field.(3)The leaching and migration behavior of heavy metal have significant differences in saturated and unsaturated regions.When there is a water head in an unsaturated region,heavy metal in discharge tailings begin to leach out,and then,migrated to the soil.The range of heavy metal migrated to the soil increases with the duration of water head,and decreases with the increase of cement content.It can be seen that the permeation and solute migration in the discharge tailings are reduced,and the leaching and migration of heavy metals are effectively controlled due to the increase of the cement content.When it comes to the saturated region,the heavy metal in the discharge tailings have fully leached.The range of the heavy metal is relatively large.Due to the water head is large and lasts for a long time,when the cement content is low or the cement is not mixed,the diffusion range of heavy metal in the saturated area is greater than that in the unsaturated area.However,the range of heavy metal migrated from the discharge tailings to the soil is significantly reduced.The leaching and migration of heavy metal in discharge tailings can also be significantly controlled with the increase of the cement content.Therefore,the consolidated and cemented tailings emission disposal can effectively control the leaching of pollutants in the discharge tailings,and it has a better blocking and regulating effect on the pollutants migration from the discharge tailings to soil. |
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