Study On Microstructure Evolution Of Glauberite Under The Coupling Action

Glauberite salt rock(Na₂SO₄·Ca SO₄)is an important sulphate ore deposit,which is the main storage form of glauberite.In Sichuan,Qinghai,Jiangsu,Xinjiang and other places in China,glauberite reserves are extremely rich,and co mmonly used in chemical raw materials,pharmaceutical industry,printing and dyeing industry,food industry,etc.,with great industrial value.Glauberite salt is one of the principal minerals for industrial extraction of sodium sulfate.Some scholars have presented the method of in-situ solution mining to mine it.The in-situ solution mining of glauberite salt is a physicochemical process affected by hydro,thermo,concentration and mechanical.Each factor will have an effect on the evolution of its internal microstructure,the development of cracks,and then on its mechanical properties.The macroscopic mechanical properties of rock materials are closely linked to the evolution of their internal microstructure.The effects of temperature,concentration and axial stress on the solution mining of glauberite were fully considered,and the mechanism of increasing efficiency of solution mining in-situ was studied.The study on the microstructure and dissolution mechanism of glauberite under multi-field coupling is of huge significance to the efficient exploitation of salt mines and the construction of oil and gas reservoirs in China.In this paper,in order to study the mesostructure evolution of internal pore cracks under the coupling of THMC,different temperatures(20,65,95?)and different concentrations(fresh water,semi-saturated salt solution,saturated Salt solution)and different axial pressures(0,4,8,9.6 MPa),the uniaxial loading was carried out on the thenardite after leaching,and the micro-CT scan was used to observe the different axial directions Evolution characteristics of cracks inside glauberite under stress.Control experiments were designed to study the structural characteristics of glauberite under natural conditions.The micro-CT images were processed and statistically analyzed to explore and co MPare the changes in pore parameters of glauberite,and analyze the effect mechanism of temperature,concentration and axial stress on them.This study mainly draws the following conclusions:(1)Changes in cracks of glauberite under different conditions were observed by micro CT.When the axial stress is not applicable,the internal structure is co MPact and no corrosion cracks appear.When the axial stress is 4MPa,fine cracks begin to appear in the inner part of the specimen,and the difference of crack morphology is small in each condition;when the axial stress is 8 MPa,the development of cracks in glauberite under different conditions begins to appear difference;when the axial stress is 9.6 MPa,the development of cracks in glauberite salt under different conditions appears difference obviously.Under the action of axial stress of 9.6 MPa,the number of cracks of glauberite in fresh water is the most,the length of cracks is short,the opening is the largest,the connectivity between cracks is fine,the maximum length of cracks is between 1.8-2.5 mm,the maximum opening is between 0.15-0.3 mm;the number of cracks produced by half saturated salt is large,the opening is small,the maximum length of cracks is about 2 mm,the maximum opening is about 2 mm The crack number of glauberite is the least,the length is the longest,the opening is larger,the maximum length is more than 3 mm,and the maximum opening is between 0.1-0.28 mm.(2)Different solutions have different degrees of leaching and erosion effect on glauberite.The leaching and softening effect of glauberite specimens in fresh water is better than that in salt solution.Comparing the effect of temperature on the cracks of glauberite,the number of cracks generally increases with increasing temperature,but the effect is smaller.As the concentration of leaching increases,the number of cracks decreases and the length of the cracks increases.(3)With the increase of the concentration of the leaching liquid,the crystals of glauberite salt leached in the saturated salt solution have larger particles of crystals precipitate and adhere to the surface of the test piece,and the crystal particles become larger as the temperature of the solution increases.On the one hand,after the concentration of the solution increases,the dissolution rate of the soluble matter of the surface of the calcium glauberite sample decreases.On the other hand,the crystals adhere to the surface of the specimen,blocking the contact between the specimen and the liquid,thereby further inhibiting the dissolution of the glauberite specimen.Reducing the weakening effect of the liquid in the test piece.(4)Using image analysis software to carry out binary processing on the micro CT images,statistically calculate the changes of pore parameters of glauberite pore.Regardless of whether the test piece is subject to the leaching test,the change law is universal.With the increase of axial stress,the number of primary pores with the equivalent diameter less than 0.05 mm decreases,and the number of pores with the equivalent diameter between 0.05-0.5 mm increases gradually.When the stress is 8 MPa,cracks with the equivalent diameter more than 0.5 mm appear,and when the stress is 9.6 MPa,the crack opening becomes larger,and the porosity continues to increase.However,the pore parameters of glauberite also have certain differences in quantity.Regardless of whether there is an axial stress or not,the total number of pores of calcium glaze after leaching in fresh water is the largest,and as the concentration of the leaching liquid increases,the total number of pores gradually decreases.In the natural state,the glauberite sample has the lowest porosity.(5)Under the coupling effect of THMC,glauberite salt mainly affects the changes in the internal structure of the test piece,including the temperature,concentration,axial load of the solution,and the length of the immersion time of the test piece.Liquid leaching has a softening effect on the mechanical properties of glauberite,reducing its mechanical strength.The axial load will further aggravate the occurrence and development of cracks in the sample.The occurrence of cracks provides a channel for the solution to break into,which greatly aggravates the dissolution effect of the salt.The change of calcium thenardite after compression is also related to the length of the leaching time of the test piece.Under the condition of the same leaching time,the size of the applied load plays a decisive role in the occurrence of internal cracks in the test piece,and the level of the solution concentration is the fundamental effect.Compared with the control experiment,under the coupling effect of THMC in the solution,it can better increase the porosity of glauberite salt and improve the efficiency of leaching mining.