Experimental Study of the Microstructural Evolution of Glauberite and Its Weakening Mechanism Under the Effect of Thermal-Hydrological-Chemical Coupling

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 φ3 × 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 chemical first used solvents, for sodium or microorganisms) chloride and copper [6–8]. Themining, method and was in the first late used 1970s, for sodium it was chloride widely and used copper for glauberite mining, and mining in the and late refining. 1970s, it was Glauberite widely ( ) used( N a C242 fora( S O glauberite ) ) 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]. glauberiteAt present, (blast the traditional-cave-leach mining mining) 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 creationa 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 the using glauberite the method mining of (or in leaching)situ leach process mining, using the theglauberite method ore of isin immersed situ leach mining,in different the glauberiteconcentrations ore is of immersed sodium 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 minerals of sodium is changedsulphate dueand to the the modification dissolution of calciumsodium 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 is process interesting is a to typical note that THC the leaching multi-field process coupling is a typical problem THC which multi-field involved coupling the probleminteraction which and involved multi-influence the interaction of fluidand migration, multi-influence mineral of dissolution, 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 affect eventually 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 hydraulic happens conductivity), with other rocky as happens materials, with such other as rockycement materials,-based ones such [15 as– cement-based17]. Notably, ones the pore [15– 17 microstructure]. Notably, the ispore 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 thegreatly pore affect microstructure the pore microstructure and eventually and eventually affect the affectmechanical the mechanical strength strength and permeability? and permeability? To address To address the questions, the questions, it is it necessaryis 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, pore and fluid. diffusion They concludedis unlikely thatto be pressure the rate solution-controlling in gypsum mechanism is controlled of pressure by the solution. precipitation Zhao reaction,et al. [19] and investigated diffusion is evolution unlikely tocharacteristics be the rate-controlling of pores and mechanism the residual of pressure porous skeleton solution. of Zhao glauberite, et al. [ 19 and] investigated concluded that evolution three Processes 2018, 6, 99 3 of 19 characteristics of pores and the residual porous skeleton of glauberite, and concluded that three zones are present from dissolution interface to the outside in the process of glauberite dissolution. Zhang et al. [14] studied the cementation state and the grain arrangement mode of the halite and salt interlayers with scanning electron microscope analyses. They concluded that halite and argillaceous anhydrite are mixed together, and the pores of the halite are filled with argillaceous minerals. Liang et al. [20] have studied the mechanical properties of gypsum soaked in hot brine. They concluded that the weakening coefficient of gypsum increases with the increase of brine temperature, whereas it decreases with the increase of brine concentration. Yu et al. [21] experimentally presented the meso-structure of gypsum rock after treatment with pure water, half saturated brine, and saturated brine. They asserted that the weakening effect of half-saturated brine on gypsum is the most severe. Meng et al. [4] studied the weakening mechanism of gypsum under brine saturation. They concluded that the foremost reason for the weakening of gypsum is that the heat or water can easily separate the hydrogen bonds, which will lead to molecular chain and layer structure separation and dislocation. In summary, the external environment may greatly affect the physical properties of rock, and the porosity is an important parameters to characterize the microstructure of rock [22–25]. However, all the research discussed above showed the microscopic damaging behaviours of rock salt, granite, and gypsum under different conditions and, to the authors’ best knowledge, there is little research on microstructural evolution and the weakening mechanism of glauberite under the effect of THC coupling. It is well known that minerals variably respond in the presence of chemicals depending on their crystal structure and chemical composition [4]. Thus, in this study, the micro-computed tomography (MCT) scanning was conducted to obtain the porosity of specimens under different conditions and to discuss the corresponding weakening mechanisms. This study is of great significance to the development of micromechanics, in situ leach mining, and technological progress.

2. Methods and Materials

2.1. Test Pieces and Test Methods

Glauberite ores were selected with the main component (75%) being Na2Ca(SO4)2, and the remaining components illite (10%), chlorite (5%), quartz (4%), mica (4%), and montmorillonite (2%). The core containing the glauberite was retrieved by the China National Petroleum Corporation in the Yingcheng Mine. We were not involved in the drilling work. However, we performed the work of core sampling and plug preparation. Note that we did not use the conventional method (a core-drilling machine) because it would cause unavoidable damage to the plug during the sample preparation. Therefore, we used a line cutting machine, which produces less damage, as shown in the figure below. The diameter of steel wire that was used in this paper is 0.3 mm. Glauberite cores were obtained from a bedded salt deposit buried more than 400 m underground in the Yunying salt formation, China, which belongs to a marine precipitate sequence. The Yunying Salt Mine is located in the middle of the Yunying Basin, in the northeast of the Jianghan Basin, Hubei Province. The Yunying Basin is an interior terrestrial faulted saline lake basin formed in Cretaceous and Tertiary systems. Well ZK-1053 was drilled by the China National Petroleum Corporation in the Yingcheng Mine to provide the formation data for the experimental analysis (see Figure2). In order to observe the evolution of the meso-structure clearly, the glauberite ores were processed into many small (φ3 × 10 mm) cylindrical specimens (see Figure2b,c). The lithology of glauberite is chiefly middle-fine salt with good sorting and low texture maturity. The shape of the crystals is mainly subangular and sub-rounded, and the content of matrix is low; the particle is coarser, and the fractures are very clear; the phenocryst is mainly composed of sodium sulphate, calcium sulphate, and illite, and the matrix is quartz; and the rock is characterized by low porosity and low permeability. Processes 2018, 6, 99 4 of 19

Processes 2018, 6, x FOR PEER REVIEW 4 of 19

(a)

(b) (c)

Figure 2. (a) Regional geological map of Yunying mine [14]; (b) glauberite ores ( 3 × 10 mm); (c) the Figure 2. (a) Regional geological map of Yunying mine [14]; (b) glauberite ores (φ3 × 10 mm); (c) the thin sectionthin section of glauberite of glauberite (200 (200×)×). .

2.2. Test Equipment 2.2. Test Equipment The main equipment used in this study was the μCT225kVFCB high-accuracy (μm grade) CT Thetest analysismain equipment system (the used machine in this is study madewas in Taiyuan the µCT225kVFCB University of high-accuracy Technology, Shanxi (µm grade) province, CT test analysisChina system) (see (theFigure machine 3a). It isconsists made inof Taiyuan a digital University flat detector, of Technology, a micro-focus Shanxi X-ray province, machine, China) a (see Figurehigh-precision3a). It consists work turntable of a digital and flat fixture, detector, an acquisition a micro-focus and analysis X-ray machine, system and a high-precision other structural work turntablecomponents, and fixture, as shown an acquisition in Figure and2. The analysis micro-focus system X-ray and machine other structural in the micro components, CT test system as shown in Figure2. The micro-focus X-ray machine in the micro CT test system allowed weak currents (only 0.01–3.00 mA) which issued a small cone beam of X-ray scanning and then projected the scanned image to the digital flat-panel detector for display. The minimum focus size of the micro-focus X-ray machine Processes 2018, 6, x FOR PEER REVIEW 5 of 19

allowed weak currents (only 0.01–3.00 mA) which issued a small cone beam of X-ray scanning and then projected the scanned image to the digital flat-panel detector for display. The minimum focus size of the micro-focus X-ray machine was 3 μm, the focal length was 4.5 mm, and the maximum power was 320 W. In high-magnification tests for small specimens, a low power of 20–30 W ensured sharp focus. Due to all of these features, the system could realize three-dimensional CT scanning Processes 2018, 6, 99 5 of 19 analysis of various metal and non-metallic materials. The magnification is 1–400 times, the rock sample size is φ1–50 mm, and the scanning unit resolution is 0.194 mm per magnification. For a wasspecimen 3 µm, the 400 focal times length magnified, was 4.5 the mm, size andof the the scan maximum cell was power0.5 μm, was which 320 could W. In distinguish high-magnification 0.5 μm testspores for and small cracks. specimens, a low power of 20–30 W ensured sharp focus. Due to all of these features, The basic principle of CT scanning relies on the penetrating power of the X-ray. In the scanned the system could realize three-dimensional CT scanning analysis of various metal and non-metallic image, the dark areas relate to regions of low density, and the bright colours indicate the materials. The magnification is 1–400 times, the rock sample size is ϕ1–50 mm, and the scanning unit high-density regions—in grayscale, the whiter areas represent the higher density. Therefore, by resolution is 0.194 mm per magnification. For a specimen 400 times magnified, the size of the scan cell scanning slices and unit analysis in all directions, the morphological distribution of the rock particles was 0.5 µm, which could distinguish 0.5 µm pores and cracks. and the distribution and connectivity of the pore fissures would be clear.

(a) (b)

Figure 3. (a) μ-CT225kvFCB MCT test system. (b) Self-heated system with constant temperature Figure 3. (a) µ-CT225kvFCB MCT test system. (b) Self-heated system with constant temperature (SHSWCT). (SHSWCT). 2.3. Test Method The basic principle of CT scanning relies on the penetrating power of the X-ray. In the scanned Then, the specimens were immersed in three different solutions and at three different image, the dark areas relate to regions of low density, and the bright colours indicate the high-density temperatures; thus, there were nine different sets of experimental conditions. The solutions were regions—in grayscale, the whiter areas represent the higher density. Therefore, by scanning slices and pure water, half-saturated brine and saturated brine. The temperatures were 20, 50, and 80 °C. Six unitspecimens analysis were in all tested directions, under the each morphological set of conditions; distribution thus, 54 tests of the were rock performed particles andin total. the distribution and connectivityFirstly in this of the experiment, pore fissures MCT would scanning be clear.was conducted for the specimens in a dry state, and then each specimen was immersed in pure water, semi-saturated, and saturated brine, respectively, 2.3. Test Method at three different temperatures of 20, 50, and 80 °C. An intelligent water bath thermostatic device wasThen, used the to accurately specimens control were the immersed temperature in three throughout different the solutions process. Due and to at the three changes different of temperatures;microstructure thus, of therespecimens were immersed nine different in different sets of experimentalconditions, it was conditions. important The to solutionsconduct the were MCT pure water,scanning half-saturated of the specimens brine and at regular saturated intervals. brine. TheThe temperaturessoaking duration were for 20, the 50, specimens and 80 ◦C. was Six 30 specimens h, and werethe tested MCT scans under of each the setcolumn of conditions; specimens thus, were 54conducted tests were while performed they were in total.in the dry state and after theyFirstly had inbeen this immersed experiment, forMCT 10, 20 scanning, and 30 wash. In conducted order to ensure for the the specimens comparability in a dry of state, the results and then eachbefore specimen and after was the immersed scanning, in the pure same water, part semi-saturated, of the specimen and was saturated selected brine,for comparison, respectively, and at the three differentcorresponding temperatures microstructural of 20, 50, evolutions and 80 ◦C. were An intelligentanalysed. The water detailed bath thermostaticexperimental deviceprocedures was are used shown below. to accurately control the temperature throughout the process. Due to the changes of microstructure Experimental process: (1) Put 2000 mL of the pre-prepared liquid (pure water, half-saturated of specimens immersed in different conditions, it was important to conduct the MCT scanning of brine, or saturated brine) into dry and clean reagent bottles and mark them with numbers; (2) Heat the specimens at regular intervals. The soaking duration for the specimens was 30 h, and the MCT scans of the column specimens were conducted while they were in the dry state and after they had been immersed for 10, 20, and 30 h. In order to ensure the comparability of the results before and after the scanning, the same part of the specimen was selected for comparison, and the corresponding microstructural evolutions were analysed. The detailed experimental procedures are shown below. Experimental process: (1) Put 2000 mL of the pre-prepared liquid (pure water, half-saturated brine, or saturated brine) into dry and clean reagent bottles and mark them with numbers; (2) Heat each bottle using the SHSWCT (see Figure3b) for approximately 20 min to ensure that the liquid Processes 2018, 6, 99 6 of 19 temperature in each bottle rises to the predetermined temperature; (3) Gently place each specimen into the correct bottle filled with liquid and seal each bottle to ensure that the concentration of the solution in the bottles does not change via heating. Maintain a stable temperature for the predetermined duration; (4) Tweeze the specimens out of the bottles lightly to make sure that the specimens are not damaged by the tweezers, and dry the surface of each specimen with filter paper; (5) Dry the specimens using a drying oven for approximately 10 min at 30 ◦C to ensure that the specimens are desiccated; (6) Scan the specimens, including the original specimens using MCT; (7) After performing the MCT tests, put the same specimens into the bottle again for the next soaking saturation, and repeat the steps 4–7. After the MCT tests, the MCT dataset needs to be constructed by binarization, in which a distinct set of classes is created and every single datapoint is assigned to a class. That is, the binarization of an image is to set the gray value of the pixel on the image to 0 (black) or 255 (white). However, the value of the final segmentation result is highly dependent on the thresholding techniques; hence, the porosity partially reflects the choices made by the operator. Note that conventional thresholding techniques (e.g., global thresholding) usually fail to extract narrow cracks and low porosity. In this paper we, therefore, chose to apply 3D voxel-based segmentation using the multiscale Hessian filter (MSHFF) of [26]. This approach, which involves low computational requirements, cannot only precisely extract the fractures and apertures from the MCT database but also better estimate the micro-porosity for rock types with smaller ranges of pore sizes. The MSHFF method has been implemented as macrocode in MATLAB. Second, the areas of the extracted fractures and apertures are calculated by using MATLAB. Then, the porosity may be obtained by dividing the fracture and aperture areas by the total area of the specimen.

3. Test Results

3.1. MCT Analysis of Microstructural Evolution Figures4–6 show the microstructure evolution of glauberite in pure water at different temperatures. Obviously, the microstructure evolution of glauberite varied greatly according to the pure water temperature. As can been seen in Figure4, the structure of the specimen was compact with no obvious cracks after 10 h of immersion in pure water. However, when the specimens were immersed for 20 h, arbitrary branched and intersecting micro-cracks appeared inside the specimens. After 30 h, the aperture and length of the cracks increase considerably. In Figure5, the water temperature was 50 ◦C. Micro-cracks emerged after 10 h of immersion, and the micro-crack density increased with the soaking time. In addition, after 30 h, banded dissolved areas were formed around the micro-cracks, and secondary branching cracks were nucleated in the main crack. Figure6 indicates that the crack number greatly increased over time. After 30 h of immersion, the specimen broke up completely, and notches appeared at the edges. However, vastly distinct from Figures4 and5, large-scale dissolved areas (i.e., dark areas) were observed in Figure5 when the specimens were only immersed for 10 h at 80 ◦C, whereas the dissolved areas in Figures4 and5 were negligible when immersed even for 30 h after 30 h of immersion. Overall, from the comparison of these MCT images, it could be determined that temperature may have accelerated the breakdown and dissolution rate of the glauberite. Figures6–8 display the microstructure evolution of the specimen in a 80 ◦C solution with various concentrations. It is clear in Figure7 that there were a great number of internal cracks and point-like dissolved areas inside the specimen after immersion in half saturated brine for 10 h. Additionally, the crack density and dissolution areas increased over time until, ﬁnally, the specimen broke up thoroughly. However, as seen in Figure7, the dissolution areas and the number of cracks were much smaller than those shown in Figure6, and no notches on the edges occurred. Figure8 shows that the dissolved areas were only concentrated around the crack lines. The location of dissolved areas was totally different from that of Figures6 and7. Furthermore, the dissolved areas appeared after Processes 2018, 6, 99 7 of 19

Processes 2018, 6, x FOR PEER REVIEW 7 of 19 immersionProcesses 2018 in saturated, 6, x FOR PEER brine REVIEW for 30 h. Overall, by comparing the variation trend of dissolved7 of 19 areas, appeared after immersion in saturated brine for 30 h. Overall, by comparing the variation trend of it was concluded that the chloride ion inhibited the dissolution of glauberite. appeareddissolved afterareas, immersio it was concludedn in saturated that the brine chloride for 30 ion h. inhibitedOverall, by the comparing dissolution the of variationglauberite. trend of dissolved areas, it was concluded that the chloride ion inhibited the dissolution of glauberite.

(a) (b)

Figure 4. Microstructure evolutions(c) of glauberite specimens immersed(d) in pure water at 20 °C : (a) 20 Figure 4. Microstructure evolutions of glauberite specimens immersed in pure water at 20 ◦C: (a) 20 ◦C °C pure water immersion, 0 h; (b) 20 °C pure water immersion, 10 h; (c) 20 °C pure water immersion, pureFigure water immersion,4. Microstructure 0 h; (bevolutions) 20 ◦C pure of glauberite water immersion, specimens 10immersed h; (c) 20 in◦ Cpure pure water water at 20 immersion, °C : (a) 20 20 h; °C20 hpure; and water (d) 20 immersion, °C pure water 0 h; immersion,(b) 20 °C pure 30 waterh. immersion, 10 h; (c) 20 °C pure water immersion, and (d) 20 ◦C pure water immersion, 30 h. 20 h; and (d) 20 °C pure water immersion, 30 h.

(a) (b)

Figure 5. Microstructure evolutions of glauberite specimens immersed in pure water at 50 ◦C: (a) 50 ◦C pure water immersion, 0 h; (b) 50 ◦C pure water immersion, 10 h; (c) 50 ◦C pure water immersion, 20 h; and (d) 50 ◦C pure water immersion, 30 h. Processes 2018, 6, x FOR PEER REVIEW 8 of 19

Processes 2018, 6, x FOR PEER REVIEW 8 of 19 Figure 5. Microstructure evolutions of glauberite specimens immersed in pure water at 50 °C : (a) 50 Processes 2018, 6, 99 8 of 19 Figure°C pure 5 water. Microstructure immersion, evolutions 0 h; (b) 50 of°C glau pureberite water specimens immersion, immersed 10 h; (c) in50 pure°C pure water water at 50 immersion, °C : (a) 50 °C20 hpure; and water (d) 50 immersion, °C pure water 0 h; (immersion,b) 50 °C pure 30 waterh. immersion, 10 h; (c) 50 °C pure water immersion, 20 h; and (d) 50 °C pure water immersion, 30 h.

(a) (b)

Figure 6. Microstructure evolutions(c) of glauberite specimens immersed(d) in pure water at 80 °C: (a) 80 °C Figure 6. Microstructure evolutions of glauberite specimens immersed in pure water at 80 ◦C: (a) 80 ◦C pure water immersion, 0 h; (b) 80 °C pure water immersion, 10 h; (c) 80 °C pure water immersion, pureFigure water immersion,6. Microstructure 0 h; (evb)olutions 80 ◦C pureof glauberite water immersion, specimens immersed 10 h; (c) 80in pure◦C pure water water at 80 immersion,°C: (a) 80 °C 20 h; pure20 h; andwater (d immersion,) 80 °C pure 0water h; (b )immersion, 80 °C pure 30 water h. immersion, 10 h; (c) 80 °C pure water immersion, and (d) 80 ◦C pure water immersion, 30 h. 20 h; and (d) 80 °C pure water immersion, 30 h.

(a) (b)

Figure 7. Microstructure evolutions of glauberite specimens immersed in half saturated brine at 80 ◦C: (a) 80 ◦C half saturated brine immersion, 0 h; (b) 80 ◦C half saturated brine immersion,10 h; (c) 80 ◦C half saturated brine immersion, 20 h; and (d) 80 ◦C half saturated brine immersion, 30 h. Processes 2018, 6, x FOR PEER REVIEW 9 of 19

Figure 7. Microstructure evolutions of glauberite specimens immersed in half saturated brine at 80 °C: Processes(a) 201880 °C, 6 ,half 99 saturated brine immersion, 0 h; (b) 80 °C half saturated brine immersion,10 h; (c) 80 °C9 of 19 half saturated brine immersion, 20 h; and (d) 80 °C half saturated brine immersion, 30 h.

(a) (b)

Figure 8. Microstructure evolutions of glauberite specimens immersed in saturated brine at 80 °C : (a) Figure 8. Microstructure evolutions of glauberite specimens immersed in saturated brine at 80 ◦C: 80 °C saturated brine immersion, 0 h; (b) 80 °C saturated brine immersion, 10 h; (c) 80 °C saturated (a) 80 ◦C saturated brine immersion, 0 h; (b) 80 ◦C saturated brine immersion, 10 h; (c) 80 ◦C saturated brine immersion, 20 h; and (d) 80 °C saturated brine immersion, 30 h. brine immersion, 20 h; and (d) 80 ◦C saturated brine immersion, 30 h. 3.2. The Porosity of Glauberite under Varying Conditions 3.2. The Porosity of Glauberite under Varying Conditions Figure 9 and Table 1 shows the average porosity evolution of glauberite with different immersionFigure 9times and Tableunder1 showsdifferent the condition average porositys. The porosity evolution refers of glauberite to the ratio with of differentthe total immersionarea of the minutetimes under voids different in the cross conditions. section Theof the porosity specimen refers to tothe the total ratio area of theof the total cross area section. of the minute In this voids paper, in thethe self cross-developed section of MCT the specimen image analysis to the total software area of was the applied cross section. so as Into thisstatistically paper, the analyse self-developed the grey valuesMCT image of the analysis glauberite software MCT wasimages, applied and sodetermine as to statistically the porosity analyse under the different grey values conditions. of the glauberite As can beMCT seen images, in Table and 1, determinethe porosity the of porosityglauberite under specimens different varied conditions. greatly depending As can be seenon the in solution Table1, temperaturethe porosity and of glauberite concentration. specimens When immersed varied greatly in pu dependingre water at room on the temperature, solution temperature the porosity and of glauberiteconcentration. was When 7.29% immersed after immersing in pure for water 30 h. at roomWhen temperature, the temperature the porosity was raised of glauberite to 50 °C, wasthe 7.29% after immersing for 30 h. When the temperature was raised to 50 ◦C, the porosity of glauberite porosity of glauberite was 7.52% after immersing for 20 h and the porosity of glauberite after◦ immersingwas 7.52% after for 10 immersing h at 80 °C for was 20 h 7.84%. and the Obviously, porosity of temperature glauberite afterplayed immersing an important for 10 role h at in 80 theC dissolutionwas 7.84%. of Obviously, the glauberite temperature and the evolution played an of important pore microstructure. role in the dissolutionFurthermore of, increasing the glauberite the temperatureand the evolution shortened of pore the dissolving microstructure. time. Furthermore, increasing the temperature shortened the dissolving time. When comparing the porosity of specimens immersed in solution at 80 °C◦ with different concentrations,When comparing it could thebe found porosity that of the specimens porosity reached immersed 19.48% in solution after 30 ath of 80 immersionC with different in pure waterconcentrations, which was it could 16.23 be times found that that of the the porosity initial reached porosity 19.48% (dry condition), after 30 h of while immersion the porosity in pure water was, respectivelywhich was 16.23, 7.52% times and that 6.71% of the after initial immersion porosity (dry in half condition),-saturated while and the saturated porosity sodiumwas, respectively, chloride solution,7.52% and which 6.71% wasafter 8.2immersion6 times in and half-saturated 3.92 times higherand saturated than the sodium initial chloride porosity. solution, The differences which was between8.26 times the and porosity 3.92 times growth higher further than theproved initial that porosity. the concentration The differences of a between solution the had porosity an inhibiting growth effectfurther on proved the evolution that the concentrationof glauberite pores. of a solution The higher had an the inhibiting concentration effect onthe the stronger evolution the ofinhibition. glauberite pores. The higher the concentration the stronger the inhibition. Processes 2018, 6, 99 10 of 19

In summary, by analysing the micrographs and porosity of glauberite after immersion in pure water at different temperatures, it could be concluded that the dissolution was very slow at low temperature; the cracks appeared to initiate in the centre of the discs, and then acted as seepage channels for further dissolution. The dissolution rate increased greatly as the temperature rose, and the dissolved areas and kinked cracks were generated simultaneously in the specimens. Additionally, the crack density and the dissolution area increased with the temperature. By analysing the micrographs and porosity of glauberite after immersion in brine with varying concentrations, it could be concluded that the crack density and dissolution rate decreased with the increase in brine concentration.

Table 1. The average porosity of glauberite specimens in different immersion conditions.

Porosity (%) Regression Equations of Temperature Immersing Condition 0 h 10 h 20 h 30 h Glauberite Porosity with Time 20 ◦C Pure water 1.57 1.63 4.08 7.29 y = 1.2898e0.0552x Half-saturated brine 1.88 1.91 1.97 2.12 y = 1.8557e0.0039x Saturated brine 1.42 1.44 1.47 1.52 y = 1.4135e0.0022x 50 ◦C Pure water 1.73 3.29 7.52 10.97 y = 1.7910e0.0637x Half-saturated brine 2.21 2.41 2.75 3.43 y = 2.1417e0.0145x Saturated brine 1.86 1.91 1.99 2.14 y = 1.8402e0.0046x 80 ◦C Pure water 1.82 7.84 13.13 19.48 y = 2.4756e0.0763x Half-saturated brine 0.91 2.39 4.86 7.52 y = 1.0378e0.0705x Saturated brine 1.71 2.87 4.16 6.71 y = 1.7491e0.0447x

7 y = 1.2898e0.0552x R² = 0.9117 6 20℃ Saturated brine ) 5 ％ 20℃ Half- 4 saturated brine

0.0039x 3 y = 1.8557e 20℃ Water Porosity( R² = 0.8985 2

1 y = 1.4135e0.0022x R² = 0.9632 0 0 10 20 30 40 Soaking time (h)

(a)

Figure9. Cont.

Processes 2018, 6, 99 11 of 19

y = 1.791e0.0637x 12 R² = 0.9818 50℃ Saturated 10

brine )

％ 8 50℃ Half- saturated brine 6

y = 2.1417e0.0145x 50℃ Water Porosity( 4 R² = 0.9581

2 y = 1.8402e0.0046x 0 R² = 0.9512 0 10 20 30 40 Soaking time (h)

(b) 30

25 y = 2.4756e0.0763x R² = 0.9015 80℃ Saturated

20 brine )

％ 80℃ Half- 15 saturated brine y = 1.0378e0.0705x 80℃ Water Porosity( 10 R² = 0.9726

5 y = 1.7491e0.0447x R² = 0.9964 0 0 10 20 30 40 Soaking time (h)

(c)

Figure 9. Variation in the average porosity of glauberite specimens with time under different immersion conditions. (a) 20 ◦C; (b) 50 ◦C; (c) 80 ◦C.

4. Discussion

4.1. Weakening Mechanism of Glauberite

Glauberite (Na2Ca(SO4)2) is a special rock, with the main components of sodium sulphate (water-soluble) and calcium sulphate (slightly water-soluble) [12–14]. The microstructure evolution (or leaching) of glauberite in solution is a complicated process of dissolution-crystallization interaction [13]. Namely, the dissolution of sodium sulphate in glauberite is almost simultaneous with the crystallization of calcium sulphate dehydrate. When the specimen dissolution was totally complete, the surface of the specimen residuals were covered with a layer of white needle-like gypsum crystals. A detailed leaching process of glauberite is as follows: There are two different kinds of evolution processes for the specimens soaked in liquid. The two evolutionary processes are shown below.

2 Processes 2018, 6, x FOR PEER REVIEW 12 of 19

There are two different kinds of evolution processes for the specimens soaked in liquid. The two evolutionary processes are shown below. 1) For the intact specimens with no cracks, the microstructural evolution process was very regular Processes 2018, 6, 99 12 of 19 and orderly. As can be seen in Figures 10–12, the specimens were gradually dissolved from the outside to the inside, and the pores and three zones were formed during the leaching process. (1) TheFor the zones intact were specimens the dissolved with no area cracks, (or theresidual microstructural skeleton, porous evolution structure), process wasthe veryundissolved regular areaand orderly. (i.e., dense As can and be intact seen in area) Figures, and 10 the–12 ,circular the specimens solid-liquid were gradually interface (i.e., dissolved circular from line), the

respoutsideectively. to the As inside, we all and known, the pores glauberite and three ( Na zones2 Ca(SO were 4 ) 2 formed) is a special during rock, the leaching with the process. main The zones were the dissolved area (or residual skeleton, porous structure), the undissolved area components of sodium sulphate ( NaSO ) (water-soluble) and calcium sulphate ( CaSO ) (i.e., dense and intact area), and the circular4 solid-liquid interface (i.e., circular line), respectively.4 (slightly water-soluble). The sodium sulphate is water-soluble, whereas the calcium sulphate is As we all known, glauberite (Na2Ca(SO4)2) is a special rock, with the main components of poorly soluble in water. Hence, when the glauberite was in contact with water, the sodium ions sodium sulphate (NaSO4) (water-soluble) and calcium sulphate (CaSO4) (slightly water-soluble). + 2- andThe sodium calcium sulphate ions ( Na is water-soluble,, SO4 ) in the whereas crystal the lattice calcium were sulphate electrostatically is poorly attracted soluble in by water. the waterHence, molecules when the with glauberite an opposite was in charge, contact withand if water, the attractive the sodium force ions of andthe calciumwater molecule ions (Na to+, 2− theSO 4ions) in was the crystalsufficient lattice to wereovercome electrostatically the attractive attracted force bybetween the water the molecules ions in the with crystal an opposite lattice, thecharge, glauberite and if thecrystal attractive lattice force was ofdestroyed the water and molecule the soluble to the minerals ions was sufﬁcientwere dissolved to overcome by water. the Theattractive chemical force formula between is theas follows: ions in the crystal lattice, the glauberite crystal lattice was destroyed

and the soluble minerals were dissolved by water.+ The chemical formula is as follows: Na2 Ca(SO 4 ) 2 2Na  SO 4  CaSO 4 (1) + Na2Ca(SO4)2 → 2Na + SO4 + CaSO4 (1) 2+ 2- CaSO Ca +SO (2) 442+ 2− CaSO4 → Ca +SO4 (2) After dissolution, the remaining insoluble matter (i.e., CaSO4 or CaSO42 2H O ) in · glauberiteAfter became dissolution, the residual the remaining skeleton. insoluble On one matterhand, this (i.e., increasedCaSO4 or theCaSO porosity4 2H2 Oof) the in glauberitespecimen becameand, on thethe residualother hand, skeleton. the penetration On one hand, resistance this increased of the solut the porosityion was ofgreatly the specimen reduced and,due onto the otherseepage hand, paths the (i.e., penetration residual resistanceskeleton) of(see the Figure solutions 10 was–12).greatly Then, the reduced solution due easily to the flowed seepage via paths the (i.e.,preferential residual skeleton) seepage (see paths Figures through 10–12 ). the Then, dissolved the solution zones easily and ﬂowed came via into the preferential contact with seepage the pathsnewly through-exposed the minerals dissolved to zones form aand new came solid into-liquid contact interface. with the Eventually, newly-exposed the sodium minerals sulphate to form aw newould solid-liquidbe completely interface. dissolved Eventually, and removed, the sodium whereas sulphate the calcium would besulphate completely would dissolved completely and removed,become the whereas residual the skeleton. calcium sulphate would completely become the residual skeleton.

Figure 10. The dissolved areas (residual porous skeleton area) and undissolved areas. Figure 10. The dissolved areas (residual porous skeleton area) and undissolved areas. Processes 2018, 6, 99 13 of 19

Processes 2018, 6, x FOR PEER REVIEW 13 of 19

Figure 11. The spherical ore specimen (i.e., intact specimen with no cracks) dissolving process and Figure 11. ideal modelThe sphericaldiagram. (a ore) liquid specimen immersion, (i.e., 0 intact h; (b) liquid specimen immersion, with no 10 cracks)h; (c) liquid dissolving immersion, process 20 h; and Figure 11. The spherical ore specimen (i.e., intact specimen with no cracks) dissolving process and idealand model (d) liquid diagram. immersion, (a) liquid 30 h. immersion, 0 h; (b) liquid immersion, 10 h; (c) liquid immersion, 20 h; ideal model diagram. (a) liquid immersion, 0 h; (b) liquid immersion, 10 h; (c) liquid immersion, 20 h; and (d) liquid immersion, 30 h. and (d) liquid immersion, 30 h.

Figure 12. Dissolution of a small salt-gypsum specimen by pure water after different immersion times [19]. Images were made using MCT (250×). Figure 12. Dissolution of a small salt-gypsum specimen by pure water after different immersion Figure 12. Dissolution of a small salt-gypsum specimen by pure water after different immersion times [19]. Images were made using MCT (250×). times [19]. Images were made using MCT (250×). Processes 2018, 6, 99 14 of 19

(2)ProcessesFor 2018 the, 6, specimenx FOR PEER REVIEW with cracks, the microstructural evolution process was very irregular14 of 19 and complex. By comparison with the microstructures evolution process at different soaking duration, 2) Forthere the were specimen four different with cracks, zones the formed microstructural in the MCT evolution images. process The zones was were very dissolved irregular and area (or complex.residual skeleton), By comparison cracks area, with undissolvedthe microstructures area, and evolution circular solid-liquid process at interface, different soakingrespectively. Theduration, reason there why were the four evolution different processeszones formed is complexin the MCT is images. that the The cracks zones formed were dissolved during the area (or residual skeleton), cracks area, undissolved area, and circular solid-liquid interface, leaching process will greatly affect the flow field and path line of liquid. That is, the liquid respectively. The reason why the evolution processes is complex is that the cracks formed will concurrently flow via the preferential seepage paths through the dissolved zones (or residual during the leaching process will greatly affect the flow field and path line of liquid. That is, the skeleton) and cracks zones, and come into contact with the newly-exposed minerals to form liquid will concurrently flow via the preferential seepage paths through the dissolved zones several new solid-liquid interfaces (i.e., the cracks will become the new solid-liquid interfaces). (or residual skeleton) and cracks zones, and come into contact with the newly-exposed Then,minerals the to dissolution form several trajectories new solid initiated-liquid at interfaces the previous (i.e., circularthe cracks interface will become and the the crack new lines, solidand spread-liquid tointerfaces). the surrounding Then, the undissolved dissolution areas trajector (seeies in initiated Figures 6 at–8 theand previous13). By comparison, circular interfacethere was and only the one crack interface lines, and during spread the wholeto the surrounding leaching process undissolved for the specimens areas (see within Figure no cracks.s 6Hence,–8 and theFigure evolution 13). By process comparison, was very there irregular was only and one complex. interface It isduring worth the to notewhole that, leaching primarily processdue to thefor the clay specimens (i.e., mainly with illite no cracks. and montmorillonite) Hence, the evolution hydration, process was swelling very irregular and the and thermal complex.cracking (more It is details worth see to in note the Section that, primarily 4.1), the cracking due to can the easily clay occur (i.e., to mainly the specimens illite and when montmorillonite)soaked in liquid. hydration, swelling and the thermal cracking (more details see in the Section 4.1), the cracking can easily occur to the specimens when soaked in liquid.

Figure 13. The spherical ore specimen (i.e., fractured specimen with cracks) dissolving process and Figure 13. The spherical ore specimen (i.e., fractured specimen with cracks) dissolving process and ideal model diagram. ( a) liquid immersion, 0 h; (b) liquid immersion, 10 h; (c) liquid immersion, 20 ideal model diagram. (a) liquid immersion, 0 h; (b) liquid immersion, 10 h; (c) liquid immersion, 20 h; h; and (d) liquid immersion, 30 h. and (d) liquid immersion, 30 h. As shown in Figure 14, due to the action of dissolution–penetration, sodium sulphate in the specimenAs shown was dissolved in Figure into14, due the to solution the action or water, of dissolution–penetration, and the insoluble calcium sodium sulphate sulphate in the in the specimenspecimen was formed dissolved the resid intoual the skeleton solution (the or water, black and parts the represent insoluble the calcium seepage sulphate pathways in the which specimen formedresulted the from residual the dissolution skeleton (theof sodium black partssulphate represent, whereas the the seepage white pathwaysparts are the which calcium resulted sulphate from the dissolutionor calcium ofsulphate sodium dihydrate). sulphate, whereasFinally, the the impermeable white parts areglauberite the calcium salt rock sulphate gradually or calcium became sulphate a dihydrate).permeable porous Finally, medium the impermeable with calcium glauberite sulphate salt(or rock calcium gradually sulphate became dihydrate) a permeable as the solid porous mediumskeleton, withmechanically calcium supporting sulphate (or and calcium sustaining sulphate the network dihydrate) of seepage as the pathways solid skeleton, and eventually mechanically supportingchanging and and affecting sustaining the physical the network and ofmechanical seepage pathwaysproperties andof the eventually glauberite. changing Additionally and affecting, the processes of crack initiation, propagation, and coalescence can also be observed on the petrographic the physical and mechanical properties of the glauberite. Additionally, the processes of crack initiation, thin section. For example, the cracks is not obvious for the dry specimen slice. However, after 5 min propagation, and coalescence can also be observed on the petrographic thin section. For example, of dissolution, several cracks initiated at the centre of the petrographic thin section due to the the cracks is not obvious for the dry specimen slice. However, after 5 min of dissolution, several cracks impurity hydration and swelling. After 10 min of dissolution, the width of the cracks became initiated at the centre of the petrographic thin section due to the impurity hydration and swelling. obviously larger. This is because the pore liquid which infiltrated into the crack zones takes the crack After 10 min of dissolution, the width of the cracks became obviously larger. This is because the pore lines as the new solid-liquid interfaces, and then preferentially dissolved the undissolved areas around the cracks. After 15 min of dissolution, the morphology of specimen underwent a Processes 2018, 6, 99 15 of 19 liquid which inﬁltrated into the crack zones takes the crack lines as the new solid-liquid interfaces, and then preferentially dissolved the undissolved areas around the cracks. After 15 min of dissolution, Processes 2018, 6, x FOR PEER REVIEW 15 of 19 the morphology of specimen underwent a considerable change; cracks completely disappeared; a varietyconsiderableof punctate change; pores cracks were completely randomly disappe distributedared; a atvari theetycentre of punctate of microscope pores were images; randomly and a visibledistributed deformation at the occurredcentre of microscope at the residual images; skeleton. and a This visible is because deformation the calcium occurred sulphate at the residual may have reactedskeleton. with This water is because to form the calcium calcium sulphate sulphate dihydrate may have inreacted the process with water of leaching to form (i.e.,calcium crystallization, sulphate seedihydrate in the Equation in the process (3)), which of leaching will create (i.e., the crystallization, swelling force. see Furthermore,in the Equation the (3 specimens)), which will contained create a largethe percentageswelling force. of impurities Furthermore (mainly, the specimens illite and montmorillonite) contained a large inpercentage addition toof theimpurities glauberite. (mainly When theillite impurities and montmorillonite) were immersed in in addition water, to they the absorbed glauberite. moisture, When the leading impurities to an were increase immersed in volume. in Déwater,kány etthey al. absorbed [27] asserted moisture, that leading the free to swell an increase ratio of in illitevolume. and Dékány montmorillonite et al. [27] asserted could reachthat the 40% andfree 56%, swell respectively. ratio of illite Hence, and montmorillonite the porous structures could reach underwent 40% and an 56%, obvious respectively. deformation, Hence, and the the cracksporous disappeared. structures underwent an obvious deformation, and the cracks disappeared. Crystallization Ca SO +2H+ O−−−−−−−−−→ Ca SO· 2H O (3) Ca22SO4 42H 22O←−−−−−−−−−Dissollution Ca 22SO 44 2H2 2O (3) Dissollution

Figure 14. Microscope photos of glauberite sections dissolved by pure water at different times (250×) Figure 14. Microscope photos of glauberite sections dissolved by pure water at different times [13]. (a) Original dry slice; (b) after 5 min of dissolution; (c) after 10 min of dissolution; (d) after 15 (250×)[13]. (a) Original dry slice; (b) after 5 min of dissolution; (c) after 10 min of dissolution; min of dissolution. (d) after 15 min of dissolution. Undoubtedly, all of these behaviours (i.e., chemical reactions and physical reactions) would greatlyUndoubtedly, improve the all of porosity these behaviours of the specimen (i.e., chemical and further reactions accelerate andd thephysical dissolution reactions) rate. would For greatlyexample, improve as shown the porosity in Table of 1, the compared specimen with and the further porosity accelerated of the dry the specimen, dissolution the rate.porosity For example,of the asspecimen shown in soaked Table1 ,in compared pure water with at 20 the °C porosity increased of by the 1.03 dry times specimen, after 10 the h of porosity immersion, of the 2.59 specimen times soakedafter in20 pure h of waterimmersion at 20, ◦andC increased 4.64 times by after 1.03 30times h of after immersion. 10 h of immersion, Obviously, 2.59 this times may have after fully 20 h of immersion,reflected the and special 4.64 times permeability after 30 hcharacteristics of immersion. of Obviously,glauberite during this may the have leaching fully process. reﬂected the special permeability characteristics of glauberite during the leaching process. 4.2. The Effect of Temperature on the Microstructure of Glauberite 4.2. TheThe Effect MCT of Temperature scan images on of the glauber Microstructureite after immersing of Glauberite in pure water at 20 °C showed that the dissolutionThe MCT rate scan of glauberite images of was glauberite very slow. after After immersing 10 h of immersion, in pure water the porosity at 20 ◦ increasedC showed by that only the dissolution3.8%; after rate 20 h of of glauberite immersion was, the veryporosity slow. increased After 10 by h 159.8 of immersion,%, and the thecracks porosity appeared increased to initiate by onlyat the centre of the discs, crisscrossed together. The reason for this is that the specimen contained a 3.8%; after 20 h of immersion, the porosity increased by 159.8%, and the cracks appeared to initiate large percentage of impurities (mainly illite and montmorillonite, 12%) in addition to glauberite [13]. at the centre of the discs, crisscrossed together. The reason for this is that the specimen contained a Dékány et al. [27] asserted that part of the impurities could be characterized by the large percentage of impurities (mainly illite and montmorillonite, 12%) in addition to glauberite [13]. super-hydrophilic performance and large expansibility factor. Based on their experimental results, Dékány et al. [27] asserted that part of the impurities could be characterized by the super-hydrophilic the free swell ratio of a specimen could reach 40%. Therefore, when immersed in pure water, the specimen volume expanded, thus, generating cracks inside the specimen. Subsequently, the solution or water accumulated in the crack regions, and as a result, the crack faces (or crack lines) became the Processes 2018, 6, 99 16 of 19 performance and large expansibility factor. Based on their experimental results, the free swell ratio of a specimenProcesses 2018 could, 6, x FOR reach PEER 40%. REVIEW Therefore, when immersed in pure water, the specimen volume expanded,16 of 19 thus, generating cracks inside the specimen. Subsequently, the solution or water accumulated in the cracknew solid regions,-liquid and contact as a result, surfaces. the crack Thus, faces after (or being crack immersed lines) became for 30 the h, newnew solid-liquid dissolving regionscontact surfaces.appeared Thus, around after the being crack immersed lines (see forFigure 30 h, 4) new. When dissolving the specimens regions were appeared immersed around inthe pure crack water lines at (see50 °C Figure and 804). °C When for only the specimens10 h, complex were cracks immersed appeared in pure at the water centre at 50 of ◦theC and discs, 80 and◦C for the only porosity 10 h, complexgrowth rate cracks of theappeared pore number at the centre increased of the with discs, the and temperature. the porosity For growth example, rateof the the exponent pore number (i.e., increasedregression with equations the temperature. of glauberite For porosity example, with the time) exponent in 80 °C (i.e., pure regression water (immersing equations for of 30 glauberite h) were porosity1.19 times with and time) 1.20 times in 80 that◦C pure of 50 water °C, and (immersing 1.38 times for and 30 1.43 h) were times 1.19 that times of 20 and°C. The 1.20 mai timesn reasons that of 50are◦ asC, follows: and 1.38 times and 1.43 times that of 20 ◦C. The main reasons are as follows: 1) The dissolution rate of the soluble minerals in the specimen increased with the temperature, (1) The dissolution rate of the soluble minerals in the specimen increased with the temperature, which eventually resulted in the increase of porosity. As shown in Figure 6, when immersed in which eventually resulted in the increase of porosity. As shown in Figure6, when immersed in pure water at 80 °C for 10 h, it dissolved along the edges from the surface inwardly on a large pure water at 80 ◦C for 10 h, it dissolved along the edges from the surface inwardly on a large scale, instead of forming dissolving areas around the crack lines in the specimen; while in scale, instead of forming dissolving areas around the crack lines in the specimen; while in 20 ◦C 20 °C pure water, the evolution process began with cracks, and then dissolved with the new pure water, the evolution process began with cracks, and then dissolved with the new solid-liquid solid-liquid contact surfaces created by the cracks, inwardly. By comparing the dissolution contact surfaces created by the cracks, inwardly. By comparing the dissolution regions and areas, regions and areas, it could be concluded that the dissolution rate of glauberite increased with it could be concluded that the dissolution rate of glauberite increased with the temperature. the temperature. (2)2) AsAs the the temperature increased, increased, the the activity activity of of the water and the impuritiesimpurities (illite and montmorillonite)montmorillonite) increased, increased, which accelerated the impurity/hydration impurity/hydration reaction, reaction, and thereby exacerbatedexacerbated the the expansion expansion of of the the specimen [28] [28].. (3)3) AfterAfter the the temperature temperature rose, rose, the chemicalthe chemical reaction reaction rate of ratecalcium of sulphate calcium andsulphate water accelerated.and water accelerated.Lewis et al. [Lew29] maintainedis et al. [29] thatmaintained the specimen that the volume specimen could volume increase could by 30%increase when by the 30% calcium when thesulphate calcium completely sulphate turned completely into dihydrate turned into calcium dihydrate sulphate, calcium and the sulphate swelling, and force the could swelling reach force584–840 could kPa. reach It was 584– the840 volume kPa. It was expansion the volume that enlargedexpansion the that lattice enlarged spacing the lattice of the spac specimen,ing of theand specimen, thereby producing and thereby cracks. producing cracks.

For quantitative explanation on the expansion characteristics of the specimen at different temperatures, the diameter of the specimens after soaking in pure water at differentdifferent temperaturestemperatures werewere calculated. calculated. As As shown shown in Figure in Figure 15, the 1 diameter5, the diameter grew gradually grew gradually with the increasing with the temperature, increasing whichtemperature, was consistent which with was the consistent variational with trend the of vartheiational porosity trendin the paper. of theAdditionally, porosity in this the strongly paper. supportsAdditionally the, analysis this strongly mentioned supports above. the analysis mentioned above.

3.12 3.11 3.1 3.09 3.08 3.07 20℃ water 3.06 50℃ water 3.05 80℃ water

3.04 The The diameter(mm) 3.03 3.02 0 5 10 15 20 25 30 35 Soaking time(h)

Figure 15. The relationship between the diameter of the glauberite specimen and the immersing time.

4.3. The Effect of Brine Concentration on Microstructure of Glauberite By comparing the MCT scanning images of glauberite immersed in 80 °C solution with varying concentrations, it can be determined that, after 10 h of immersion in pure water, a wide range of dissolving areas appeared; after 30 h of immersion, the specimen broke completely, and the Processes 2018, 6, 99 17 of 19

4.3. The Effect of Brine Concentration on Microstructure of Glauberite By comparing the MCT scanning images of glauberite immersed in 80 ◦C solution with varying Processes 2018, 6, x FOR PEER REVIEW 17 of 19 concentrations, it can be determined that, after 10 h of immersion in pure water, a wide range of dissolvingdissolution areas notch appeared; was observed. after 30 However, h of immersion, this phenomenon the specimen w brokeas not completely, observed when and the immersed dissolution in notchsemi-saturated was observed. brine However,and saturated this phenomenonbrine. By analysing was not Figure observed 9 and when the data immersed of Table in 1 semi-saturated, it is revealed brinethat the and growth saturated rate brine.of glauberite By analysing pore varied Figure greatly9 and the—the data difference of Table1 between, it is revealed them thatis large, the growtheven by ratean order of glauberite of magnit poreude. variedFor example, greatly—the when immersed difference in between pure water, them the is large,exponent even in bythe an regression order of magnitude.equations of For glauberite example, porosity when immersed with time in purewas 0.water,0552; the in exponent semi-saturated in the regressionsolution, the equations exponent of glauberitedecreased porosityto 0.0039 with; in saturated time was 0.0552;salt solution, in semi-saturated it was as low solution, as 0.0022 the exponent. From Tab decreasedle 1, the toporosity 0.0039; indecreased saturated gradually salt solution, with itthe was increasing as low as brine 0.0022. concentration. From Table1 ,For the example, porosity decreasedwhen immersed gradually for with30 h, the porosity increasing in brinepure concentration.water at 80 °C were For example, 2.59 times when higher immersed than that for of 30 half h,- thesaturated porosity brine, in pure and water were ◦ at2.90 80 timesC were higher 2.59 than times that higher of satur thanated that brine. of half-saturated The comparison brine, strongly and were confirmed 2.90 times that higher the thanchloride that ofion saturated had an brine. inhibitory The comparison effect on strongly sodium conﬁrmedsulphate dissolution, that the chloride and ion the had phenomenon an inhibitory could effect onbe sodiumexplained sulphate by the dissolution, Debye-Huckel and theory the phenomenon, namely, sodium could be chloride explained was by a the symmetrical Debye-Huckel electrolyte theory, namely,[30–33]. Wh sodiumen the chloride concentration was a symmetricalof sodium chloride electrolyte solution [30– 33was]. Whenincreased, the concentrationthe activity coefficient of sodium of chloridecalcium solutionsulphate wasdecreased increased, and the calcium activity coefﬁcientsulphate formed of calcium ion sulphate pairs. Thus,decreased the and short calcium-range sulphateelectrostatic formed interaction ion pairs. was Thus, enhanced, the short-range lowering electrostatic the solubility interaction of the calci wasum enhanced, sulphate, lowering thereby theaccelerating solubility the of dissolution the calcium rate sulphate, of sodium thereby sulphate accelerating. In summary, the dissolution the solubility rate and of sodium dissolution sulphate. rate Inof sodium summary, sulphate the solubility decreased and with dissolution the increase rate of of sodium sodium ch sulphateloride concentration decreased with (see the Figure increase 16), as of sodiumdid the corresponding chloride concentration porosity (seeof glauberite. Figure 16), as did the corresponding porosity of glauberite.

Figure 16. Na SO -NaCl-H O phase diagram of three element system (20◦ °C). Figure 16. Na2SO4− 4NaCl − H2 O phase diagram of three element system (20 C).

5.5. Conclusion Conclusionss Glauberite is a a special special kind kind of of salt salt rock. rock. Its Its microstructure microstructure evolution evolution in in the the process process of ofleaching leaching is isnot not only only relat relateded to tomining mining efficiency efﬁciency,, but but also also to to the the deformation deformation and and stability stability of of its its mineral mineral layer. layer. This paper paper focused focused on onthe microstructural the microstructural evolution evolution of glauberite of glauberite under different under leaching different conditions. leaching Theconditions. main conclusions The main conclusions are as follows: are as follows: (1) When the specimens were immersed in a low-temperature solution, the dissolution rate was (1) When the specimens were immersed in a low-temperature solution, the dissolution rate was very very slow. With the increase of temperature, the dissolution rate greatly improved, and the slow. With the increase of temperature, the dissolution rate greatly improved, and the initiation initiation of micro-cracks and the dissolution regions were produced spontaneously. of micro-cracks and the dissolution regions were produced spontaneously. (2) In the process of leaching, the effects of temperature and concentration of solution on the (2) In the process of leaching, the effects of temperature and concentration of solution on the porosity of glauberite was significant. When immersed in brine with the same concentration, porosity of glauberite was signiﬁcant. When immersed in brine with the same concentration, the porosity of the specimens increased considerably as the temperature increased; when at the the porosity of the specimens increased considerably as the temperature increased; when at the same temperature, the porosity of the specimen decreased with the increase in brine same temperature, the porosity of the specimen decreased with the increase in brine concentration. concentration. (3) When the specimens were immersed in pure water or brine, the sodium sulphate in glauberite dissolved and the remaining solid insoluble substances became the residual skeleton. The specimens are gradually dissolved from the outside to the inside, and the pores and three zones are formed during the leaching process. The zones are the dissolved area (or residual skeleton, porous structure), the undissolved area (i.e., dense and intact area), and the circular solid-liquid interface (i.e., circular line), respectively. Processes 2018, 6, 99 18 of 19

(3) When the specimens were immersed in pure water or brine, the sodium sulphate in glauberite dissolved and the remaining solid insoluble substances became the residual skeleton. The specimens are gradually dissolved from the outside to the inside, and the pores and three zones are formed during the leaching process. The zones are the dissolved area (or residual skeleton, porous structure), the undissolved area (i.e., dense and intact area), and the circular solid-liquid interface (i.e., circular line), respectively. (4) With the increase of temperature, the hydration of illite and montmorillonite in glauberite and the hydration of calcium sulphate were improved, resulting in a gradual increase of the expansion rate of glauberite and fracture density increased as well. With the increase of sodium chloride concentration, the porosity of the specimen decreased.

Author Contributions: S.Z.C. contributed to the experimental design and sourcing of the laboratory equipment and raw materials; D.H.Z. contributed to the experimental operation, data collection, and writing of the manuscript; T.S. carried out the CT analysis and the correction and editing of the manuscript; T.M. also contributed in the experimental design and correction of the manuscript. Funding: This paper was support by the Natural Science Funds for Young Scholar (2016YFC0501103), Major Research and Development Plans of Shanxi Province (grant no. 201603D121031), the School Fund of Tai Yuan University of Technology and Science (no. 20172018, 20182008), Science and technology innovation project of colleges and universities in Shanxi (Grant No. 2017158), Shanxi Scholarship Council of China (Grant No. 2017-086), State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology(Grant No. mkx201702). Acknowledgments: The authors gratefully acknowledge Tao Meng, who has given us many useful ideas and suggestions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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