Water Retention Characteristics And Strength Evolution Of Ion-absorbed Rare Earth Under Different Leaching

As an important strategic mineral resource in China,ionic rare earths are of significant significance to national defense and industrial development,and how to achieve safe mining of ionic rare earth resources is an important issue.In-situ leaching method has become the recommended development method for ionic rare earths due to the advantages of simple production process,low environmental pollution and low production cost.In the process of in situ leaching of ionic rare earths,the water content of the ore body increases continuously due to the injection of leaching solution,coupled with the climatic characteristics of the rain-abundant area in Ganan,which makes the ore body in a highly saturated environment for a long time and its shear strength weakens continuously,thus inducing geological disasters such as landslides and threatening the safety of people’s lives and properties in the mining area.In order to further realize safe and efficient mining of rare earths,it is urgent to study the water-holding characteristics of ionic rare earths under different leaching effects and reveal the mechanism of the weakening of ionic rare earth strength under leaching.In this paper,the ionic rare earths in the Zudong mine of Longnan,Jiangxi Province were studied by combining theoretical and experimental methods to investigate the soil-water characteristics and strength properties of ionic rare earths under different leaching effects.The dissolution leaching test was carried out with a homemade indoor simulated leaching device,and the effects of the type of leaching solution and the concentration of leaching solution on the waterholding characteristics and the "hysteresis effect" of ionic rare earths were analyzed with the soilwater characteristic curve test.Direct shear tests and unconfined compressive strength tests were conducted to investigate the effects of leach solution type,leach solution concentration,leach time and wet and dry cycles on the shear strength and unconfined compressive strength of saturated ionic rare earths.The main findings of this paper are as follows.(1)The soil-water characteristic curves of ionic rare earths were fitted by three typical models(Van Genuchten model,Fredlund & Xing three-parameter model and Fredlund & Xing four-parameter model)with the help of pressure plate apparatus to obtain the soil-water characteristic curves of specimens after the action of different types and concentrations of leaching solutions.The results show that all three models fit the ionic rare earth-water characteristic curves well,among which the Fredlund & Xing four-parameter model has the best fit.The variation patterns of the parameters of the soil-water characteristic curve models were basically the same under the leaching action of different types of leaching solutions(pure water → 3% magnesium sulfate → 3% ammonium sulfate)and different concentrations of leaching solutions(0 → 2%ammonium sulfate → 3% ammonium sulfate → 5% ammonium sulfate),where the parameter a was related to the inlet value,the parameter n was related to the moisture reduction rate,and the parameter m was related to the residual water content.The parameter values obtained from different models varied,with parameters a,n and m gradually increasing in the Van Genuchten model,and parameters a and m gradually decreasing and n gradually increasing in the Fredlund &Xing three-parameter model and the Fredlund & Xing four-parameter model.(2)By analyzing the soil-water characteristic curves of the specimens after the action of different types of leaching solutions,it was found that the water-holding capacity of the three specimens after leaching with pure water,3% magnesium sulfate and 3% ammonium sulfate decreased in order.Compared with pure water,the "hysteresis effect" was significantly reduced in the specimens with 3% magnesium sulfate and 3% ammonium sulfate.By analyzing the soil-water characteristic curves of the specimens with different concentrations of leaching solution,it was found that the water-holding capacity of the specimens decreased gradually with the increase of the leaching solution concentration(0 → 2% ammonium sulfate → 3% ammonium sulfate → 5%ammonium sulfate),and the significant degree of "hysteresis effect" showed an exponential function decreasing,when the leaching solution concentration increased from 0 to 2%,the "hysteresis effect" of the specimens decreased.The significant degree of "hysteresis effect" of the specimens decreased the most obviously when the concentration of lysis solution increased from 0to 2%.(3)The direct shear tests of ionic rare earth specimens after different dissolution leaching effects were carried out,and the damage type of the specimens was found to be plastic damage.The weakening of the shear strength of saturated ionic rare earth specimens after different dissolution leaching effects was mainly achieved by reducing the cohesion and internal friction angle,with the cohesion having a greater effect.The cohesive force and internal friction angle of the three specimens after dissolution leaching with pure water,3% magnesium sulfate and 3%ammonium sulfate showed a linear decreasing pattern,with the cohesive force decreasing more significantly.With the increase of the concentration of the leaching solution,the changes of the cohesion and the angle of internal friction were in accordance with the law of single exponential decay function.For the specimens with different leaching time,the decrease of cohesive force was most significant before 24 h,and the change was relatively flat between 24 h and 48 h.The change of internal friction angle with the leaching time was not significant.The cohesive force and internal friction angle of the specimens after different number of wet and dry cycles were basically the same,both of them decreased significantly after the first wet and dry cycle,and were basically stable after the third wet and dry cycle.(4)The unconfined compressive strength of ionic rare-earth specimens after different dissolution leaching was found that the axial stress of the specimens after different types and concentrations of dissolution leaching solution showed a pattern of rapid increase,then leveled off and finally decreased slowly with the increase of axial strain,and the damage type belonged to plastic damage.The unconfined compressive strength of the three specimens after dissolution leaching with pure water,3% magnesium sulfate and 3% ammonium sulfate showed a linear decreasing pattern,and the relationship between the unconfined compressive strength and the concentration of the leaching solution was more consistent with the Holliday model.The effect of the concentration of the leaching solution on the lateral limit compressive strength was significantly greater than that of the length of the leaching time within 48 h.After the specimens were subjected to wet and dry cycles,the axial stresses reached their peaks significantly faster than those specimens that did not undergo wet and dry cycles,and the peaks were smaller,i.e.,the specimens were more prone to damage.