Abstract: In order to analyze the form of the isolated extraction zone (IEZ) in side drawing and get optimal independent advance with large structural parameters, drawing models with the mesomechanical properties of ore granular media were constructed based on the particle flow theory and PFC3D code. The reliability of the drawing models was validated by comparative analysis between existing research conclusions and simulated results. On this basis, a study on independent advance of side drawing at different end wall angles in the structural parameters of 18 m×20 m was performed. The results show that the IEZ form is incomplete and is not a regular ellipsoid at different end wall angles. When the drawn mass is equal, the IEZ height increases with decreasing end wall angle and the IEZ form becomes more and more slender. The trend of the IEZ height at both the infinite boundary condition and the boundary condition of different end wall angles can be divided into two stages:in the first stage, the IEZ height rapidly increases in the exponential form at the initiation of side drawing and its growth rate decreases with the increasing of drawn mass, while in the second stage the IEZ height linearly increases with the increasing of drawn mass. It is suggested that the end wall angle of 85°-90° and the independent advance of 4. 8 m should be used in the structural parameters of 18 m×20 m.
Abstract: To simulate the sandstone ore leaching process, some specimens with different grades were made with river sand, cement and salt as the raw materials. On the basis of the monitoring of leaching solution concentration in a water-solution mining experiment, the time evolution of leaching solution concentration and leaching rate was examined. Through phase space reconstruction from the time series of leaching solution concentration, the distance evolution of points of specimens with different grades in the phase space was revealed by the chaotic theory. A prediction model of the distance evolution of phase points was established by using the grey theory, and a reasonable leaching period was ascertained. The results show that the leaching solution concentration holds chaotic characteristics. After the phase space reconstruction, the delicate changes of leaching solution concentration are enlarged, and the internal rules are fully demonstrated. The ores of different grades have different nonlinear dynamic behaviors. Based on the phase space reconstruction, the leaching period has been predicted by means of the residual modification grey model, which provides a new research method and theory to solution mining.
Abstract: The mineral composition variation of Indonesian seaside titanomagnetite at different roasting temperatures was studied in the direct reduction roasting process with coal slime as a reductant. The roasted products were analyzed by X-ray diffraction and scanning electron microscopy. It is found that as the roasting temperature rises the titanomagnetite is gradually reduced. Iron minerals in the titanomagnetite are first reduced to FeO and finally into metallic iron. Titanium minerals contained in the titanomagnetite are first reduced to titanium spinel and finally into ilmenite with a small amount of pseudobrookite. In the direct reduction roasting process, metallic iron particles generate at about 1100℃ and continue to grow. At 1250℃, molten metallic iron particles obviously increase and metallic iron particles continue to grow in the later thermal process. The separation of metallic iron and ilmenite is relized in this process.
Abstract: The basic characteristics of ironsand were systematically studied by chemical analysis, scanning electron microscopy and X-ray diffraction. A series of experiments were performed to investigate reduction-magnetic separation for Fe and Ti from iron-sand. The effects of reduction temperature, reduction time, carbon ratio, magnetic density, and grinding fineness on the reduction-magnetic separation process were investigated. It could be found that the ironsand mainly consisted of titanomagnetite and titanohematite. The optimal process parameters were obtained as follows:reduction temperature, 1300℃; reduction time, 30 min; C/O molar ratio, 1.1; magnetic density, 50 mT; and the grinding fineness of -0. 074 μm, 86. 34%. Under such conditions, the metallization rate, the iron grade of magnetic substance and the iron recovery reached to 94.23%, 97.19% and 90. 28%, respectively, and the titanium grade of non-magnetic substance and the titanium recovery were 57. 94% and 87.22%, respectively. The effective separation of iron and titanium could be achieved in this reduction-magnetic separation process.
Abstract: In this work, microwave energy was used for roasting treatment of diaspora bauxite mixed with saturated NaOH solution as an additive. The influences of microwave heating temperature and NaOH addition on the phase transformation of the diaspora ore-sodium hydroxide system were investigated. The phase constitution of the roasted ore in microwave heating and conventional heating processes was compared under the same experimental conditions. The phase composition and microstructure of the roasted ore were comparatively analyzed by X-ray diffi'action analysis and scanning electron microscopy. The results show that microwave roasting can facilitate the rapid and complete reaction between NaOH and the diaspore ore, leading to more NaAI02 phase forming at lower temperature as compared to conventional heating. After microwave heating, the roasted ore is porous, good for subsequent digestion processes.
Abstract: In order to investigate the effect of particle size difference and density difference on the mixing mechanism of binary granular materials within a rotating kiln, a kinematic model of particles was established using the discrete element method, and the transverse mixing process of granular materials in the rolling regime within the rotating kiln was simulated. The mixing index was defined by contact-number between particles, and the effects of particle volume ratio and density ratio on the mixing index were analyzed using Hong's theory:competition between percolation and condensation. The results show that the percolation mechanism improves with increasing volume ratio σ between particles, but the condensation mechanism improves with increasing density ratio η. Segregation will occur in the radial direction of the rotating kiln no matter percolation or condensation plays the leading role resulting in the decrease of mixing degree. Percolation and condensation can balance each other for some reasonable particle volume and density combinations, where the binary granular materials could get uniformly mixed. There is a power function relationship between particle volume ratio and density ratio when percolation and condensation balance each other.
Abstract: The load from the upper sinter bed is one of the important factors which deteriorate the permeability of the combustion zone in the sintering process, which restricts the further development of thick bed sintering. To reduce the load of the sinter bed and improve the permeability of the combustion zone, the influences of different loads which were coordinated by the height of support stands on the combustion zone and sinter bed were researched. Sintering pot test results indicated that the permeability of the combustion zone and sinter bed was obviously improved after the support stand being installed in the sinter bed and the tumble index (TI) of sinter ores were more than 65%. The velocity of sintering dramatically increased and achieved the maximum value of 28.4 mm. rain-i The yield of sinter ores did not significantly change compared to ordinary sintering process. The sinter productivity increased from 1.89 t·m-2·h-1 to 2. 31 t·m-2·h-1. The fuel consumption of the sintering process decreased by 1.32%. Theoretical analysis and experiments showed that load reduction sintering process had a positive impact on the permeability of the sinter bed, resulting in the decrease of fuel consumption and the improvement of sinter quality.
Abstract: High basicity (CaO/SiO2 mass ratio) glass-ceramics were prepared to improve the usage of high calcium metallurgical slags in raw materials. Glass-ceramics with a high basicity of 0. 9 were prepared using steel slag as the main raw material through one- step sintering process. The influences of heat treatment on the microstructure, linear shrinkage, bulk density and bending strength of glass-ceramics were investigated by X-ray diffraction, scanning electron microscopy and performance testing. The results show that one-step sintering process is suitable for preparing high basicity glass-ceramics. The sintering process of the glass-ceramics is basically completed after heat treatment at 1100℃ for 120 min, with the largest bulk density of 2. 4 g·cm-3 and the optimal bending strength of 56.4 MPa. The main crystalline phase of the glass-ceramics is gehlenite and the secondary crystalline phase is augite. The nucleation and crystallization process is completed in the heating process, while in the heat preservation process the sintering densification and crystal growth are dominant. After heating to 1100℃ and holding for 30 min, the bending strength of the glass-ceramics exceeds 45 MPa. Columnar crystals are intertwined together to form the crystal skeleton which constitutes the microstructure with residual glassphase inside the glass-ceramics. Besides, the linage shrinkage, bulk density and bending strength of the glass-ceramics increase with the increasing of holding time. The crystal morphology transforms from spherical particles and short columns into long rod-like columns with the increasing of holding time, while the crystalline phase content stays constant. The crystal morphology of the glass-ceramics and the integrated dense microstructure formed by the crystals and the residual glass phase are two primary factors of the improvement in mechanical properties of the glass-ceramics.
Abstract: The Hopkinson experiment system was used to do the dynamic tensile experiment of 800 MPa grade cold rolled dual phase steel (DP800). The strain rate was determined as 500, 1000 and 2250 s-1 By comparing the experimental results, both the yield strength (Rp0.2) and the tensile strength (Rm) of the dual phase steel increase with strain rate in the exponential form. The plastic deformation at high strain rate leads to adiabatic temperature rise effect. The adiabatic temperature rise is 89℃ at the 2250 s-1 strain rate. Based on the J-C (Johnson-Cook) model and Z-A (Zerilli-Armstrong) model, the constitutive model of the dual phase steel was researched. The quadratic polynomial of strain rate effect of the J-C model was modified. The average coefficient of determination increases from 0. 9228 to 0. 9886 by modifying the J-C model.
Abstract: The effects of pretreated microstructure on the multiphase microstructure features at room temperature and the mechanical properties of low-carbon steel were studied by the treatment of water cooling after hot-rolling in the γ phase region or in the γ + α phase region followed by the treatment of intercritical reheating-quenching-partitioning (IQ&P). Two different pretreated microstructures, which are martensite and martensite + ferrite, are obtained by the two different hot-rolling and quenching processes. After IQ&P, the steel with martensite as pretreated microstructure, in which the volume fraction of retained austenite is 8. 2%, is turned into a multi-phase structure composed of lath intercritical ferrite, block tempered martensite, needle-like untempered martensite, and retained austenite. While the steel with martensite + ferrite as pretreated microstructure, in which the volume fraction of retained austenite is 14. 3%, is turned into a multi-phase structure composed of lath intercritical ferrite, block or needle-like untempered martensite, and short needle-like or block retained austenite. With the same IQ&P process parameters, the tensile strength of the steel with martensite as pretreated microstructure is 770 MPa, the elongation is 28%, and the product of strength and elongation is 21560 MPa·%. While the tensile strength of the steel with martensite + ferrite as pretreated microstructure is 834 MPa, the elongation increases to 36. 2%, and the product of strength and elongation reaches to 30190 MPa·%. In other words, the steel has obtained a good combination of strength and ductility.
Abstract: An austenite grain growth model was established by analyzing the quantificational relationship between precipitates and austenite grain size. The austenite grain size of Ti microalloyed steel with the interaction of TiN and TiC panicles was calculated by using the austenite grain growth model. The calculated resuhs of the precipitation particle theory indicate that the volume fraction of precipitates gradually decreases and the panicle radius increases with increasing temperature. The austenite grain growth is strongly impeded by TiC panicles, while the pinning force of TiN on austenite grains is weak. The austenite grain size of the tested steels was measured by a test method at different heating temperatures within 30 min. The results are in good agreement with theoretical calculations.
Abstract: To improve the wear resistance of maraging stainless steel Custom 465, the 580℃ aged steel was modified by salt bath nitriding at 440, 480 and 520℃ for 2 h, respectively. The surface phases and hardness, the microstructure of the nitrided layer, and the corrosion and wear resistance of the steel were characterized by microhardness testing, X-ray diffractometry, electrochemical corrosion workstation, ball-on-disc tribometry, surface profilometry, and scanning electron microscopy. With increasing nitriding temperature, the surface hardness increases but the corrosion resistance decreases. When the nitriding temperature is 520℃, the surface hardness is 1240 HV, much greater than 400 HV of the unnitrided specimen, and the case depth is 22 μm. The surface phase is nitrogen supersaturated solid solution in the martensite substrate at 440℃ and the pitting potential decreases by about 60 mV. A small quantity of CrN precipitates at 480℃, which decreases the pitting potential by about 180 mV and reduces the wear volume by 43%. The CrN content significantly increases at 520℃, the corrosion potential decreases by about 70 mV, and no passive region appears in the polarization region. The wear volume gets a significant reduction, about 82% less than that of the unnitrided sample. The 520℃ nitrided sample has a better wear resistance.
Abstract: The microstructure and texture evolution of Hi-B steel produced by thin slab casting and rolling (TSCR) process in the laboratory was studied by metalloscopy and scanning electron microscopy. It was found that the microstructure and texture of the hot rolling slab of Hi-B steel, which was manufactured by simulated TSCR process in the laboratory, was inhomogeneous along the thickness direction. Ferrite grains in the surface decarbonization layer of the normalizing slab obviously coarsen; moreover, the texture of the normalizing slab inherited the texture type of the hot rolling slab, with only a difference in texture density. After cold roiling with big reduction rate, the coarse grains and grain boundaries were stretched to be fibrous bands in the rolling direction, and the α fiber texture and the γ fiber texture were the main texture style. With the deearburizing annealing carrying out, the recovery and reerystallization of the cold rolling slab occurred, primary recrystallization grains formed, and the texture was more centrally distributed. When the temperature increased to 1000℃, secondary recrystallization appeared; at 1010℃ the Goss grains grew up, and the Goss texture intensity reached to 61. 779. By testing, the products had a magnetic induction of 1. 915 T and an iron loss of 1. 0671 W.kg-1.
Abstract: The fatigue crack growth rates of powder metallurgy (P/M) superalloy FGH97 with different grain sizes, γ' phase sizes and Hf contents were measured at a 650℃ high temperature condition and compared with those of FGH95 and FGH96 alloys. The fatigue fracture behavior characteristic of FGH97 alloy in each stage was analyzed by the quantitative analysis method. It is found that FGH97 alloy with coarser grains has a lower fatigue crack growth rate. Reasonable match of secondary and tertiary γ' phases and Hf addition can get a higher fatigue life. Compared with FGH95 and FGH96 alloys, FGH97 alloy has the highest fatigue cack intiation resistance and exihibits the lowest fatigue crack propagation rate at high temperature.
Abstract: The effects of pouring temperature, squeeze pressure, squeeze speed and cooling type on Si segregation in 6066 alloy were studied in indirect squeeze casting. Taking the difference between the silicon content of the hot spot zone after solidification and the initial silicon content of the alloy as the segregation degree, the Si segregation was analyzed according to orthogonal design which considers two levels, four factors and partial first-class interactions. The results show that pouring temperature, squeeze pressure, squeeze speed and cooling type all have effect on the Si segregation, but pouring temperature is the most significant factor. With the increase of pouring temperature, the segregation degree of Si decreases. Squeeze pressure and squeeze speed have less influence on the Si segregation than pouring temperature, but their influence trends are opposite. The influence degree of mold cooling is similar to that of the partial first class interaction of squeeze pressure and speed. The Si segregation in a copper sleeve (high cooling speed) is lighter than that in a steel sleeve (low cooling speed). Negative Si segregation was found in the hot spot zone of indirect squeeze cast work-pieces.
Abstract: The semi-solid slurry of A380 aluminum alloy was prepared by using a graphite serpentine channel and the microstructure evolution during solidification was investigated. The result showed that, initial solidification occurred in the serpentine channel, and a large number of primary free grains generated in the alloy melt under the influence of chilling and heterogeneous nucleation caused by the serpentine channel inner wall. Secondary non-dendrites formed during secondary solidification which occurred in the remaining alloy melt of the semi-solid slurry in the collective crucible. The drifting model of primary grains displayed that a part of primary free grains directly grew to spherical grains, and others developed to dendrites. The "self-stirring" of the alloy melt made dendrites neck and fuse, and they got initial spheroidization and ripening through the " self-rotating" in the serpentine channel. Secondary non-dendrites formed during secondary solidification got initial spheroidization and ripening in the collective crucible. Mean-while, the primary grains got further spheroidization, ripening and uniform distribution in the collective crucible.
Abstract: A mesoporous SBA-15/silica composited aerogel was prepared by a two-step acid-alkali method with SBA-15 as an additive. The effects of the amount of SBA-15 on the formation process and the performance of the eomposited aerogel were investigated. The microstructure of the composited aerogel was characterized by scanning electron microscopy and N2 adsorption-desorption isotherms, and the mechanical properties and thermal conductivity were tested. It is found that as a small amount of SBA-15 is added, the gelation time of silica greatly shortens, the mechanical properties improves, the thernlal conductivity still keeps small, while the specific surface area slightly decreases.
Abstract: The temperature distribution of billets in a heating furnace is simulated by Deform software and is verified by the "black box". When the heating time is 70 min the temperature difference between surface and center is about 66℃, but it reduces to 15℃ after 80 min. The core and surface temperatures of wire rods in roiling and cooling are calculated and testified by thermometric indicator, and afterwards, the accurate friction heat, plastic deformation heat and heat transfer coefficient of water cooling are obtained. The wind field of fans is computed by Fluent software and is confirmed by anemometer. A temperature model of the lapping point is built and used to calculate the forced convection, natural and radiation heat transfer coefficients and the latent heat of phase transformation on the air-cooling line, which are validated by thermal imager. All the numerical simulation results agree well with the test data.
Abstract: The virtual flux space vector of a pulse width modulation (PWM) rectifier usually can be calculated by integrating the grid voltage vector. In practice, the pure integral part is replaced with a first-order low-pass filter (FOLP) to counteract the DC offset error and high frequency harmonic interference. However, the FOLP will result in the amplitude variation and phase shift of the grid voltage vector, which can lead to inaccurate observations of the virtual flux. In order to eliminate the effect of the FOLP on the grid voltage, this paper proposes an improved virtual flux orientation strategy based on the vector reconstruction principle. According to the amplitude-frequency characteristics and phase-frequency characteristics of the FLOP, the amplitude and phase of the voltage vector were separately reconstructed to improve the accuracy of the amplitude and phase values in virtual flux estimation. Furthermore, the proposed method was applied to a virtual flux oriented PWM rectifier direct power control system. Simulations and experimental results show that, compared with the traditional first-order low-pass filter, this method improves the accuracy of virtual flux estimation, effectively reduces the DC bus vohage fluctuation in dynamic response, and is more conducive to filter out grid current harmonics control.
Abstract: The stress distribution characteristics of adjacent horizontal parallel tunnels are revealed by comprehensively utilizing the complex function theory, analytic continuation method and Schwarz alternating method. Then a mechanical model of adjacent horizontal parallel tunnels is constructed in combination with the D-P yield criterion which considers the effect of intermediate principal stress. The concept of connected radius of the tunnel plastic zone is proposed, the solving equation is established and its correctness is verified by numerical simulation. The critical distance of the plastic zone connected between tunnels is considered as a reasonable spacing, which has a better agreement with the spacing of tunnels when the displacement and settlement of surrounding rock do not basically change with the spacing of tunnels calculated by numerical simulation software FLAC3D, thus demonstrating its feasibility as the reasonable spacing of adjacent horizontal parallel tunnels.
Monthly, started in 1955 Supervising institution：Ministry of Education Sponsoring Institution：University of Science and Technology Beijing Editorial office：Editorial Department of Chinese Journal of Engineering Publisher：Science Press Chairperson：Ren-shu Yang Editor-in-Chief：Ai-xiang Wu ISSN 2095-9389CN 2095-9389
