Characterization of meso-scale mechanical properties of Longmaxi shale using grid microindentation experiments

Mechanical properties,such as the hardness H,Young’s modulus E,creep modulus C,and fracture toughness Kc,are essential parameters in the design of hydraulic fracturing systems for prospective shale gas formations.In this study,a practical methodology is presented for obtaining these properties through microindentation experiments combined with quantitative observations of the mineralogical phases using X-ray diffraction(XRD),scanning electron microscopy(SEM)with backscattered electron(BSE)imaging,and energy-dispersive X-ray spectroscopy(EDS)analyses.We apply this method in the case of three types of Longmaxi shales with different mineralogies(i.e.carbonate-,clay-,and quartz-rich,respectively),which allows us to determine the characteristic indentation depth,hc?8e10 mm,beyond which the mechanical response of the carbonate-rich shale is homogeneous and independent of its complex heterogeneous microstructure.Moreover,exploiting the results of a large number of indentation tests,we demonstrate that the indentation modulus M of the shale increases as a power-law of hardness H,and its creep modulus C increases linearly with H.We also compute the fracture toughness Kc from the indentation data by assuming a perfectly plastic behavior of the sample.Our results are in good agreement with independent measurements of Kc determined by microscratch tests.Finally,further tests on quartz-and clay-rich samples of the Longmaxi shale suggest further variations in the samples’mechanical properties depending on their burial conditions and the mechanical properties of their dominant mineral phases.

Process mineralogy of Dalucao rare earth ore and design of beneficiation process based on AMICS

The test results of the automated mineral identification and characterization system(AMICS),including the mineral composition,particle size distribution,dissemination state and degree of liberation of the target minerals,could be used to improve the beneficiation process.Taking the Dalucao rare earth ore located in Dechang,Sichuan Province,China(with an average content of 2.40 wt%)as the research object in this paper,the chemical composition,phase composition and dissemination state of the minerals were tested by AMICS,and the minerals of different fineness were ground.The concentrate yield,grade and recovery rate of the minerals of different fineness were compared through flotation tests.When the grinding lasted for 5 min and 82.60%of mineral grains passed through the-74-μm sieve,the yield,grade and recovery rate could reach 20.19%,8.75%and 73.64%,respectively(as the best grinding fineness),under the same flotation conditions.

Behavior of alkali minerals in oxyfuel co-combustion of biomass and coal at elevated pressure

Combustion of biomass or coal is known to yield aerosols and condensed alkali minerals that affect boiler heat transfer performance.In this work,alkali behavior in the pressurized oxyfuel co-combustion of coal and biomass is predicted by thermodynamic and chemical kinetic calculations.Existence of solid minerals is evaluated by X-ray diffraction(XRD)analysis of ashes from pressure thermogravimetric combustion.Results indicate that a rise in pressure affects solid alkali minerals negligibly,but increases their contents in the liquid phase and decreases them in the gas phase,especially below 900℃.Thus,less KCl will condense on the boiler heat transfer surfaces leading to reduced corrosion.Increasing the blend ratio of biomass to coal will raise the content of potassium-based minerals but reduce the sodium-based ones.The alkali-associated slagging in the boiler can be minimized by the synergistic effect of co-combustion of sulphur-rich coal and potassium-rich biomass,forming stable solid K2SO4 at typical fluidized bed combustion temperatures.Kinetics modelling based on reaction mechanisms shows that oxidation of SO2 to SO3 plays a major role in K2SO4 formation but that the contribution of this oxidation decreases with increase in pressure.