| The ion-adsorption type rare earth ores in southern China are rich in medium and heavy rare earth elements and are an important strategic resource.According to the different outcrop situations of the bedrock,ion-type rare earth deposits can be divided into barefoot and full-cover types,with the latter accounting for over 80%.The bedrock of full-cover rare earth deposits is generally in a broken state.Currently,in situ leaching technology is mainly used to extract rare earth from iontype deposits.However,the leakage of the leaching solution from the broken bedrock of full-cover deposits results in a low liquid collection rate of less than 70%.To improve the liquid collection rate,this paper proposes a new and efficient liquid collection process.The process creates holes with increased permeability by using water jets to impact the sidewalls of the upper drainage holes,and creates artificial bedrock by cutting grooves and grouting the lower holes with water jets,thereby improving the liquid collection rate of in situ leaching through the "upward drainage and downward blocking" method.However,there are important technical issues during the implementation of this process.The strength of the ore in the deep holes is variable,and it is currently impossible to accurately predict the depth at which the high-pressure water jets erode the ore.Therefore,this paper adopts a method that combines theoretical analysis,remolded soil tests,and simulation experiments to accurately predict the depth of the high-pressure water jet erosion.The study focuses on the erosion mechanisms of fixed and moving high-pressure submerged water jets,providing a basis for the design and improvement of efficient liquid collection systems for iontype rare earth deposits.The main work and innovative achievements of this paper are as follows:(1)Through high-pressure submerged free water jet experiments,the distribution of stagnation pressure of the jet was obtained,and the velocity distribution law of the jet was studied using a single-phase flow model.The results show that the axial velocity of the jet and the range are inversely proportional,and there is a inflection point where the velocity drops sharply for the second time.In addition,as the velocity of the jet increases,the convergence of the jet becomes more obvious.Therefore,a high-speed thinning model was used to establish the relationship between fluid velocity and turbulent viscosity coefficient.Based on the high-speed thinning model,the calculation of the velocity of high-pressure submerged free water jets was realized.The average relative error between the calculation results of this model and the experimental results was no more than 12.2%.(2)The velocity of the fully developed cavitation zone of the jet was studied using a twophase flow model.A cavitation gas content calculation model was proposed based on the functional principle of a single cavitation bubble and the effect of turbulent flow in the jet boundary layer under time averaging.The velocity equation of the water-air homogeneous model was derived based on the differential model of jet velocity,momentum flux conservation,and the cavitation drag reduction effect of the jet.Combined with the stagnation pressure experiment,cavitation gas content,and water-air homogeneous model velocity equation,an equation group describing the transfer of jet velocity among various fluid elements was established for the calculation of the velocity of the fully developed cavitation zone of the jet.The calculation of the velocity of high-pressure submerged free water jets was divided into two stages according to the range.The velocity was calculated using the two-phase flow model of the jet in the first stage,considering the effect of cavitation drag reduction on velocity.The velocity was calculated using the single-phase flow model of the jet in the second stage,considering the effect of high-speed thinning on velocity.The method of segmented calculation can better simulate the variation of jet velocity along the distance.(3)A fixed high-pressure water jet excavation test was conducted to analyze the effect of jet pressure,nozzle diameter,and stand-off distance on the excavation depth of high-pressure fixed water jets.It was found that the process of high-pressure water jet impacting and breaking the soil can be divided into three stages: violent impact,block erosion,and surface erosion.The mechanism of fixed jet excavation was analyzed using erosion theory,which revealed a close correlation between the rate of soil failure in each stage and shear stress and critical shear stress.A modified erosion theory was proposed to calculate the rate of soil failure in each stage,with the key parameter being the modification coefficient of the erosion coefficient,which is related to the shear strength of the soil surface.The variation of excavation depth under time iteration was studied using a stepping algorithm,and a predictive model for the excavation depth of fixed high-pressure submerged water jets was proposed to provide theoretical support for the construction design of new efficient liquid collection technology with increased permeability.(4)A jet excavation test device was designed to control jet pressure and nozzle movement speed,and a high-pressure mobile water jet excavation test was conducted on soil to analyze the evolution characteristics of the erosion surface during the process.A slit wall jet velocity test device was designed to measure the velocity distribution of the moving water jet in the erosion groove,and the attenuation law of the velocity of high-pressure mobile water jet along the path was analyzed.An empirical formula for calculating the non-dimensional distribution of high-pressure mobile water jet velocity along the path was proposed.Based on the modified erosion theory,the mechanism of destruction of viscous soil under the action of mobile high-pressure submerged water jets was further revealed,and the effects of nozzle diameter,jet pressure,and nozzle movement speed on the morphology and depth of the erosion surface of the soil were studied.The distribution of shear stress and pressure along the erosion surface was calculated based on the attenuation formula of the jet flow velocity,which provided theoretical support for the construction design of the artificial grouting bottom plate in new efficient liquid collection technology.(5)In practical production,a rare earth mine that is about to be mined was chosen as the target mine.Considering the spatial variability of soil strength,the strength of soil at different depths was measured by in-situ testing,and the basic erosion parameters were determined through erosion tests,which served as the basis for optimizing the design of jet construction parameters for increased permeability holes and grouting trenches.Based on the research conclusions of the mechanism of fixed high-pressure submerged water jet excavation,a design method for the jet parameters of increased permeability holes for the target rare earth mine was proposed,and the specific implementation process was introduced.Based on the research conclusions of the mechanism of mobile high-pressure submerged water jet excavation,a design method for the jet parameters of grouting trenches for the target rare earth mine was proposed. |
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