The Infiltration Model And Soil-water Characteristics For Unsaturated Ion-absorbed Rare Earth Ore

Ion-absorbed type rare earths are important strategic mineral resource in the world.At present,in-situ leaching is a commonly used exploitation method.In the process of minging,the seepage law and mechanism of ion-absorbed rare earth have not been clearly understood in in-situ leaching.And in general,leaching solution injection well net parameters and injection velocity are determined by experience,which leads to a great waste of resources in ion-absorbed rare earth mining.Therefore,the theory researches of infiltration law and mechanism of ion-absorbed rare earth in-situ leaching need to be carried out,and the effective theoretical model need to be established.In order to describe the unsaturated flow phenomenon,two constitutive relations,the permeability coefficient model and the soil-water characteristic curve,must be studied.Based on the above analysis,the research of the infiltration model and soil-water characteristics for unsaturated ion-absorbed rare earth ore was proposed.Based on the related research and engineering practice,through combination of laboratory test and theoretical analysis,and by using multi-disciplinary knowledge such as leaching mining,unsaturated soil mechanics,seepage mechanics,groundwater dynamics,fractal theory and so on,the seepage law and influencing factors of ion-absorbed rare earth leaching process and the soil-water characteristic curve model were studied.The infiltration characteristic parameters calculation models with different liquid-injection heads and different particle sizes were established,the influence mechanism of particle size and leaching on the soil-water characteristics of ion-absorbed rare earth was revealed,and the theoretical method,which used fractal theory to predict soil-water characteristic curve,was explored.The main research work completed in this paper is as follows.?1?Based on the self-made seepage device,the infiltration experiments under different constant head conditions were carried out,and the two-dimensional seepage law of ionic rare earth and its infiltration model were studied.The results show that the shape of the wetting body during the injection is approximately a semi-ellipsoid.In the initial stage of the injection,the distance of the horizontal wetting front is greater than the distance of the vertical wetting front.As the infiltration progresses,the distance of the vertical wetting front gradually approaches the distance of the horizontal wetting front,and the shape of the wetting body develops from a semi-ellipsoid to a hemisphere.The horizontal infiltration distance and vertical infiltration distance have a good power function relationship with time:y?t?=at^b.There is a significant linear relationship between the volume of wetting body and the cumulative infiltration volume,indicating that the increment of average moisture content in wetting body of the soil is basically kept at a fixed value.?2?The relationship between wetting front migration velocity and time accords with the Philip model:v?t?=ct^-0.5+d.At the beginning of liquid injection,the movement velocity of the wetting front reaches the maximum rapidly,and then decreases sharply.With the passage of infiltration time,the movement velocity of the wetting front decreases slowly,and then tends to a stable level.The larger the head height,the greater the movement speed of the wetting front.Based on the Green-Ampt model and the principle of water balance,a model for calculating the characteristic parameters of wetting body in the infiltration process is established.The model results are in good agreement with the test results and have good application value.?3?Based on the self-made seepage device and digital image technology,a two-dimensional infiltration test of ionic rare earths with different particle sizes and particle gradations was carried out.The shape of the wetting body was characterized by visualized results,and the evolution process of the wetting body was intuitively described.Under the same injection infiltration duration,the shape of the wetting body increases with the increase of the coarse particle size content.In the same injection infiltration duration,the more fine particles in the soil,the smaller the infiltration distance,and the smoother the infiltration curve.The directional velocity ratio?is defined.In the process of liquid infiltration,the directional velocity ratio gradually decreases from greater than 1 to close to 1.It can be seen that the initial horizontal infiltration rate is fast,and then the vertical infiltration rate gradually approaches or even exceeds the horizontal infiltration rate.?4?The theoretical derivation shows that in the process of ionic rare earth in-situ leaching,when the leaching solution is fixed,the permeability coefficient is highly related to the shape factor and calculation particle size of soil particles.An empirical calculation model of characteristic parameters of particle gradation,such as d₆₀?d₅₀?d₃₀?d₁₀?C_u?C_c,and stable infiltration rate i_s are established.The linear correlation between the limited particle size d₆₀ and the stable infiltration rate is the highest.The results provide theoretical guidance for calculating the stable infiltration rate of ionic rare earth ores.?5?The soil-water characteristic curve test was performed on soil samples with different particle sizes and before and after leaching by using a pressure plate instrument.Under the same matrix suction,the smaller the particle size,that is,the more the fine particle content of the soil sample,the greater the water content of the soil and the greater the water holding capacity of the soil.With the same volume water content,the matrix suction is inversely proportional to the particle size.When the soil is in low moisture content and high suction value,the suction is mainly formed by short-range interparticle reactions controlled by the surface properties of soil particles.We had used MLA650 scanning electron microscope to analyze the microstructure of soil samples with different particle sizes.The results show that the smaller the size of the soil particles,the larger the opening pores in the particle,and the wider the space provided for free water flow and storage,so the smaller the size of soil particles is,the more water can be absorbed.After high water content and low suction value,pore water is mainly influenced by capillary action.In the theory of capillary water,the larger the particle size of the soil,the mean meniscus curvature radius will increase,and the matrix suction will decrease accordingly.In the process of drying and wetting,as the particle size increases and the content of coarse particles increases,the range of change in water content gradually decreases.?6?According to the soil-water characteristic curve of rare erath before and after leaching,it can be concluded that under the same matrix suction,compared with the soil samples before leaching,the water content of the soil samples after leaching is smaller,indicating that leaching reduces the water-holding capacity of the soil.In the process of drying and wetting,the range of water content change after leaching is smaller than that of water content before leaching.According to the analysis,the leaching effect increases the thickness of the electric double layer,increases the pore water pressure,and reduces the matrix suction,which leads to a decrease in the water holding capacity of the soil after leaching.?7?The particle size distribution of single-grained ionic rare earth has good fractal characteristics.Soil samples in different particle size ranges have different fractal dimensions.The fractal dimension can characterize the particle size range of the soil.The smaller the particle size,the larger the fractal dimension,and the larger the grain size of the soil,the smaller the fractal dimension.In the process of dehumidification and moisture absorption of ionic rare earth,as the particle size increases,the matrix suction decreases,the water holding capacity weakens,the saturated moisture content gradually decreases,the intake air value gradually decreases,and the larger the fractal dimension,the higher the saturated water content and intake air value.The prediction of SWCC model by using the fractal dimension of PSD has good applicability,and the predicted results are in good agreement with the measured results.It provides a feasible method for predicting the characteristic curve of ionic rare earth soil-water,which effectively reduces the test time of matrix suction measurement,and lays a theoretical foundation for studying the soil-water characteristics of ionic rare earth from the fractal point of view.The results are helpful to predict and timely regulate the in-situ leaching process,which can improve the leaching rate of rare earth resources.It provides a theoretical basis for engineering liquid injection well pattern design and determination of liquid injection intensity,and has important practical significance for improving in-situ leaching technology and promoting scientific exploitation of ion-absorbed rare earth.