Experimental Study of the Microstructural Evolution of Glauberite and Its Weakening Mechanism Under the Effect of Thermal-Hydrological-Chemical Coupling
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processes Article Experimental Study of the Microstructural Evolution of Glauberite and Its Weakening Mechanism under the Effect of Thermal-Hydrological-Chemical Coupling Shuzhao Chen 1,*, Donghua Zhang 2,*, Tao Shang 1 and Tao Meng 3 1 College of Mining Engineering, China University of Mining and Technology, Xuzhou 221116, China; [email protected] 2 College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, China 3 College of Chemical and Biological Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China; [email protected] * Correspondence: [email protected] (S.C.); [email protected] (D.Z.) Received: 19 June 2018; Accepted: 19 July 2018; Published: 24 July 2018 Abstract: The microstructures of rock gradually evolve with changes in the external environment. This study focused on the microstructure evolution of glauberite and its weakening mechanism under different leaching conditions. The porosity were used as a characteristic index to study the effect of brine temperature and concentration on crack initiation and propagation in glauberite. The research subjects were specimens of f3 × 10 mm cylindrical glauberite core, obtained from a bedded salt deposit buried more than 1000 m underground in the Yunying salt formation, China. The results showed that when the specimens were immersed in solution at low temperature, due to hydration impurities, cracks appeared spontaneously at the centre of the disc and the solution then penetrated the specimens via these cracks and dissolved the minerals around the crack lines. However, with an increase of temperature, the dissolution rate increased greatly, and crack nucleation and dissolved regions appeared simultaneously. When the specimens were immersed in a sodium chloride solution at the same concentration, the porosity s presented gradual upward trends with a rise in temperature, whereas, when the specimens were immersed in the sodium chloride solution at the same temperature, the porosity tended to decrease with the increase of sodium chloride concentration. In the process of leaching, the hydration of illite, montmorillonite, and the residual skeleton of glauberite led to the expansion of the specimen volume, thereby producing the cracks. The diameter expansion rate and the expansion velocity of the specimen increased with temperature increase, whereas, due to the common-ion effect, the porosity of the specimen decreases with the increase of sodium chloride solution concentration. Keywords: glauberite cavern for storing oil & gas; thermal-hydrological-chemical interactions; temperature; brine concentration; microstructure; micro-CT 1. Introduction Thermal-hydrological-chemical (THC) interactions are widely recognized and studied by researchers from different fields, including hydraulic engineering, civil engineering and geological environmental engineering [1–5]. The existing research on THC coupling has benefited these different engineering disciplines, with each of them having different key interacting factors influencing various projects. In the area of underground mineral resource development using the method of leach mining, pore structure of geo-materials, solution temperature, and chemical reaction kinetics are the key factors Processes 2018, 6, 99; doi:10.3390/pr6080099 www.mdpi.com/journal/processes Processes 2018, 6, x FOR PEER REVIEW 2 of 19 Processes 2018, 6, 99 2 of 19 reaction kinetics are the key factors to solution migration, solute diffusion and recovery efficiency to[6– solution11]. Leach migration, mining is solute the extraction diffusion of and useful recovery minerals efficiency (elements [6–11 or]. compounds) Leach mining such is the as extractioninorganic ofsalt, useful copper minerals, and uranium (elements which or compounds) are naturally such dissolved as inorganic in a leaching salt, copper, solution and (water, uranium chemical which aresolvents naturally, or microorganisms) dissolved in a leaching [6–8]. The solution method (water, was chemicalfirst used solvents, for sodium or microorganisms) chloride and copper [6–8]. Themining, method and wasin the first late used 1970s, for sodium it was chloride widely andused copper for glauberite mining, and mining in the and late refining. 1970s, it wasGlauberite widely ( ) used( Na2 Ca(SO for glauberite 4 ) 2 ) is an mining important and sulphate refining. deposit Glauberite with ( Napotential2Ca SO industrial4 2) is an importantvalue because, sulphate not only deposit can withit be used potential as a industrialsubstitute valueafter the because, depletion not onlyof mirabilite can it be deposits, used as abut substitute it is also after recognized the depletion as the ofideal mirabilite medium deposits, for the storage but it is of also oil, recognized natural gas as, and the idealcarbon medium dioxide fordue the to storageits low ofpermeability, oil, natural gas,simple and hydrogeology carbon dioxide, and due wide to its geographical low permeability, distribution simple (see hydrogeology, Figure 1) [12 and–14] wide. Atgeographical present, the distributiontraditional mining (see Figure method1)[ used12–14 in]. glauberite At present, (blast the traditional-cave-leach miningmining) method has many used disadvantages in glauberite, (blast-cave-leachsuch as a high labour mining) intensity, has many low disadvantages, recovery rate such, and as creation a high labour of environmental intensity, low pollution. recovery rate,Therefore, and creation some scholars of environmental proposed the pollution. adoption Therefore, of in situ some leaching scholars to mine proposed glauberite the adoption deposits of[13] in. situIn the leaching glauberite to mine mining glauberite (or leaching) deposits process [13]. In theusing glauberite the method mining of (orin leaching)situ leach process mining, using the theglauberite method ore of is in immersed situ leach mining,in different the glauberiteconcentrations ore isof immersedsodium chloride in different solution concentrations (or brine) ofat sodiumdifferent chloridetemperatures solution for a (or few brine) days. at Under different the effect temperatures of hot brine, for athe few microstructure days. Under of the glauberite effect of hotminerals brine, is the changed microstructure due to the of glauberitedissolution mineralsof sodium is changedsulphate dueand tothe the modification dissolution of calcium sodium sulphate andstructure the modification (i.e., the crystallization of calcium sulphate of calcium structure sulphate (i.e., dihydrate) the crystallization [13]. It is of interesting calcium sulphate to note dihydrate)that the leaching [13]. It isprocess interesting is a to typical note that THC the leachingmulti-field process coupling is a typical problem THC which multi-field involved coupling the probleminteraction which and involvedmulti-influence the interaction of fluidand migration, multi-influence mineral ofdissolution, fluid migration, chemical mineral reaction, dissolution, solute chemicaldiffusion, reaction,and heat soluteexchange diffusion,. In this andcompl heatex process, exchange. these In influence this complex factors process, may greatly these influenceaffect the factorspore microstructure, may greatly affecteventually the pore resulting microstructure, in a series eventually of changes resulting in macroscopic in a series properties of changes (i.e., in macroscopicmechanical strength properties and (i.e., hydraulic mechanical conductivity) strength and, as hydraulichappens conductivity),with other rocky as happens materials, with such other as rockycement materials,-based ones such [15 as– cement-based17]. Notably, onesthe pore [15– 17microstructure]. Notably, the is pore closely microstructure related with is closely the leaching related withefficiency the leaching and cavity efficiency stability and of the cavity glauberite stability deposit. of the Given glauberite all these, deposit. it raises Given questions all these, about it raises how questionsthe pore microstructure about how the of pore glauberite microstructure changes of under glauberite the effect changes of underTHC coupling. the effect Will of THC the coupling.solution Willtemperature the solution and temperature concentration and greatly concentration affect the greatly pore affect microstructure the pore microstructure and eventually and eventuallyaffect the affectmechanical the mechanical strength strengthand permeability? and permeability? To address To address the questions, the questions, it is it necessary is necessary to to study study the evolution of the pore microstructuremicrostructure ofof glauberiteglauberite duringduring thethe leachingleaching process.process. Figure 1. (a) In situ leach mining with a cavern for oil, natural gas and carbon dioxide storage; (b) Figure 1. (a) In situ leach mining with a cavern for oil, natural gas and carbon dioxide storage; Glauberite cores. (b) Glauberite cores. Several researchers previously concentrated on the effect of external environment on rock microstructuresSeveral researchers under the previously THC coup concentratedling effect. Meer on et the al. effect [18] have of external discussed environment the microstructure on rock microstructuresevolution of gypsum under in the the THC present coupling of saline effect. pore Meer fluid et and al. [18NaCl] have-free discussed pore fluid. the They microstructure concluded evolutionthat pressure of gypsumsolution inin thegypsum present is controlled of saline poreby the fluid precipitation and NaCl-free reaction,