Macroscopic And Microscopic Mechanical Properties Of The Bedrock Under The Erosion Of Rare Earth Leaching Solution In Magnesium Salt System

Ionic rare earth materials are widely used in cutting-edge technology fields,and they are of great strategic significance for China’s social progress and technological development.To solve the problem of ammonia-nitrogen pollution caused by traditional ammonium salt in-situ leaching of ionic rare earths,the magnesium salt in-situ leaching method has been widely used.However,the problem of the leaching erosion effect of the in-situ leaching method on the bedrock throughout the leaching period and a period of time afterward,which leads to a decrease in the bearing capacity of the bedrock,still exists.In addition,the degradation mechanism of the leaching solution of the magnesium salt system on the bedrock of ionic rare earth ore is not clear,which brings many safety hazards to mining production and post-mining ecological restoration.Therefore,this paper takes the bedrock of ionic rare earth ore under the action of the leaching solution of the magnesium salt system as the research object,and uses indoor experiments and theoretical analysis to study the basic physical properties,mechanical behavior,and micro-damage evolution law of the bedrock under leaching erosion,revealing the mechanical damage mechanism of the bedrock samples under leaching erosion.The main research results and conclusions of this paper are as follows:(1)A 150-day indoor simulation leaching test was designed and conducted,with the leaching solution of the magnesium salt system for rare earths as the experimental group and water as the control group.p H tests were carried out every 30 days for both leaching environments,and longitudinal wave velocity tests and uniaxial compression tests were performed on the bedrock samples.(2)The p H of the leaching solution of the magnesium salt system showed an increasing trend under the simulated leaching effect,increasing from 3.247 to 4.680 before and after leaching,while the p H of water changed from weakly alkaline to weakly acidic.The longitudinal wave velocity of the bedrock samples in both leaching environments showed a significant decreasing trend,and the decrease of the longitudinal wave velocity of the bedrock samples in the magnesium salt system simulation leaching environment was more obvious,indicating that the acidic erosion effect had a deeper impact on the internal structure of the bedrock samples.(3)The uniaxial compressive strength of the rock samples in the magnesia salt leaching environment decreased by 20.89% after 150 days of leaching,and the proportion of the compaction stage in the stress-strain curve was significantly higher than that in the water soaking environment,indicating that the mechanical properties of the rock samples were significantly damaged under the erosion of leaching.With the increase of leaching time,the shear cracks of the rock samples increased gradually under the action of the magnesia salt leaching solution,and different degrees of flaky peeling occurred on the surface of the rock samples in the late stage of leaching.The failure mode of the rock samples under water soaking mainly manifested as tensile failure.(4)The stress concentration areas in the DIC surface strain field cloud map of the rock samples under the action of magnesia salt and rare earth leaching solution were mostly located in the middle of the samples.In the natural state,the stress concentration areas of the rock samples basically ran through the entire sample,while under water soaking,they were mostly concentrated at the ends of the samples.The relative calm period of the acoustic emission ringing count signal of the rock samples under the action of magnesia salt and rare earth leaching solution was more obvious,further indicating that the leaching effect caused changes in the internal structure of the rock samples.(5)Scanning electron microscopy results showed that under the simulation of magnesia salt and rare earth leaching solution,the number,length,and opening degree of microcracks in the rock samples changed significantly with the increase of leaching time,and the internal structure gradually became loose with the increase of leaching time.Obvious new biomaterials were visible on the surface in the late stage of leaching;under water soaking,the surface of the rock samples was smooth,and the layered structure was obvious,exhibiting brittle characteristics.(6)The physical and chemical interactions such as dissolution and precipitation between the leaching solution and rock samples result in the dissolution of primary minerals and the generation of new biomaterials,causing changes in the internal structure of the samples.With the increase of leaching time,the primary structure of the rock samples gradually becomes loose under the alternating effects of dissolution and precipitation adsorption,and transforms into a new compact structure,leading to macroscopic changes in the mechanical properties of the rock samples under different leaching durations.The deterioration of the mechanical properties of the rock samples soaked in water for 150 days is only significant in the initial stage of leaching,and the reason may be the change in the saturation degree and the distribution of fluids at the microscopic scale range.The reaction degree between the rock samples and water is weaker.