Jo Sep 25, 2024
Graphite has chemical inertness and corrosion resistance as well as good thermal and electrical conductivity. At present, flake graphite, the most common graphite form, has a high world market share. Crystalline flake graphite is a plate-like particle of hexagonal structure with irregular fragment edges. Graphite has a layered structure, in which carbon atoms in the monoatomic layers share a strong covalent bond, but the relatively weak π-π bonds connect the layers.
In the graphite structure, gangue minerals have the characteristics of accumulation between layers, stacks or clusters. These impurities can coexist with the graphite flake, either adhering to the surface of the graphite flake or being trapped by adjacent graphite flakes. Therefore, it is essential to study the release of impurities from graphite.
Ri Ho Chol, a section head at the Faculty of Mining Engineering, has succeeded in converting graphite flotation concentrate GFC into high purity graphite from laboratory scale to pilot scale through flotation, alkali roasting and acid leaching processes.
In GFC, carbon content was 85.6%, and quartz and hematite were the main gangue minerals. The high-grade flotation concentrate with 95% carbon content was produced by low pulp concentration flotation. And the silicate mineral was converted to the soluble salt at 500℃ by alkali roasting. The soluble roasting products and other impurities such as hematite were dissolved by acid, and the carbon content of the produced high purity graphite was higher than 99.8%.
You can find the details in his paper “Effective purification of graphite via low pulp concentration flotation-low temperature alkali roasting-acid leaching route: From laboratory-scale to pilot-scale” in “Minerals Engineering” (SCI).
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Jo Sep 23, 2024
An important problem arising in underground excavation is stabilizing the country rock that surrounds openings at depth. An increase in the in situ rock stresses is the essential difference between rocks at depth and near the surface. As a consequence of such an increase in the ground stress, rock burst may be of common occurrence in hard rocks, or large squeezing deformations may appear in soft and weak rocks. It has been detected in lots of mines that such phenomena begin to occur at the depth of about 600–800m below the surface level and become more significant below 1 000m. At these depths, conventional support devices may not be adapted for severe rock conditions.
Over the years, many researchers have tried to develop various ground support techniques and products for support and retention of the newly exposed faces and internal reinforcement of the soil and rock masses surrounding the excavations. One of them is a D bolt.
A D bolt is a smooth steel bar with a number of anchors along its length. The anchors, which can be spaced evenly or unevenly along its length, are firmly fixed within a borehole using either cement grout or resin, while the smooth sections of the bolt between the anchors may freely deform in response to rock dilation.
Pyon Kwang Nam, a section head at the Faculty of Mining Engineering, has carried out a numerical simulation of D bolt, a type of energy-absorbing rock bolt which is not fully encapsulated but multipoint anchored in a fully grouted borehole. Then, he considered the influence of the spacing arrangement of its anchors on its axial stress distribution and determined reasonable structural parameters.
The numerical simulation results show that for the D bolt with its whole length of 2.4m, the length of its exposed section of 0.1m, 4 anchors with the length of 0.1m and the ratio of the spacing between anchors (RSA) of 30:40:50:70, the maximum tensile stress of 3.25GPa is generated, which is about 1.13–1.31 times lower than other D bolts with different ratio of spacing, and the changing range of stress is also the smallest. Here, the ratio of 30:40:50:70 indicates a ratio of lengths of deformable sections which is determined by turns from the innermost section of rock mass around roadway to the outermost section of roadway space.
For more information, please refer to his paper “Numerical Simulation Study on Influence of a Structural Parameter of D Bolt, an Energy-Absorbing Rock Bolt, on its Stress Distribution” Advances in Civil Engineering” in “Advances in Civil Engineering” (SCI).
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Jo Sep 22, 2024
The mechanical parameters and initial stress of coal have predominant influences on the stability of the surrounding coal of an entry in the coal seam. In practice, however, it is very difficult to determine the values of time-dependent soft coal mass parameters. To obtain these values of the time-dependent soft coal mass parameters based on very limited amounts of measured data, back-analysis is the most commonly used approach worldwide. Field measurement data may include displacements, strains, and stresses.
Because the displacements of a rock mass can be measured easily and reliably, back-analysis based on displacements has long been an active topic of research. The basic methods used in most of the studies have been numerical simulation methods and optimization algorithms. In these methods, numerical simulations were used to compute the displacements of the surrounding rock based on certain assumed values of the mechanical parameters and the initial stress. Then, by optimizing the error between the computed displacements and the measured displacements via an optimization algorithm, the initially assumed values were corrected into real values.
However, there are few papers that deal with back-analysis method based on timber load and displacement measured in the entry when the rock pressure and displacement of the entry increase continuously owing to the coal softening with time in very soft coal such as anthracite coal.
In order to ensure the accuracy in determining the mechanical parameters of time-dependent anthracite coal and to ease consideration and calculation in a set of algorithm, Sin Myong Nam, a researcher at the Faculty of Mining Engineering, has conducted a FLAC3D numerical simulation based on measured load and displacement, established a three-layer BPNN from it and found a methodology for predicting the mechanical parameters by SSA.
The proposed method can provide a useful basis for predicting time-dependent behavior of media surrounding the drift or chamber driven in such soft rock as the anthracite coal.
You can find the details in his paper “Determination of Mechanical Parameters of Anthracite Coal using Flying Squirrel Search Algorithm with Timber Load and Displacement Data” in “Journal of Mining and Environment” (EI).
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Jo Sep 20, 2024
GFRP, the polymer matrix composite, has been widely used in many industrial domains including the development of natural energy by wind, construction, ship-building, automobile, power and chemical industries, and it occupies the majority of the total production of composite materials all around the world.
VARTM, one of the Liquid Composite Molding (LCM) processes for producing large composite parts such as wind turbine blades or hulls at room temperature, is a useful manufacturing method for high fiber content, low cost of equipment, high material quality and low porosity. VARTM process usually uses room temperature cured resins such as unsaturated polyesters and room temperature-curable VARTM epoxy resin.
There are various kinds of methods of manufacturing GFRP pipes, for example, hand lay-up, filament winding, pultrusion, pre-deposited fabric winding, centrifugal casting, RTM, etc., whereby long or short pipes with different dimensions and properties for various fields are produced.
However, there are very few studies on the process for GFRP pipes by VARTM.
Sin Song Bom, a researcher at the Faculty of Materials Science and Technology, has investigated the effect of several parameters on the resin filling behavior when high temperature cured epoxy GFRP pipes undergo VARTM in the heating conditions, in association with ANSYS simulations.
He found that resin flow gets faster when it is at 60℃ for infusion and semi-curing, and room temperature VARTM materials such as PE vacuum films and other auxiliary material can be used.
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Jo Sep 19, 2024
Aluminum and its alloys are light and cheap with good thermal and electrical properties and excellent mechanical properties and machinability, so they are widely used in various fields of the national economy such as mechanical industry, aerospace industry, daily necessities industry, electronic industry, building materials industry, etc.
Aluminum and its alloys are easily oxidized in the atmosphere because of their strong chemical activity, resulting in a thin oxide film on the surface, which is protective because of its good corrosion resistance in dry air, but is easily damaged in contaminated atmosphere, water, seawater, soil and various corrosive media. Therefore, surface treatment is usually applied to enhance the surface performance of aluminum and its alloys such as corrosion resistance, wear resistance and decorativeness.
Surface treatment methods of aluminum and its alloys usually include mechanical surface treatment (mechanical grinding), chemical surface treatment (chemical coating), electrochemical surface treatment (anode oxidation, electroplating), and physical surface treatment (coating, deposition), etc.
Anode oxidation is the most commonly used electrochemical surface treatment method, which allows aluminum and its alloys to form protective coatings with a major component of amorphous aluminum oxide and a thickness of several tens of micrometers under the action of an external electric field in an acidic electrolyte (e.g. sulfuric acid, oxalic acid, phosphoric acid, etc.). The increase in the temperature of electrolyte due to the electrical resistance heat during anode oxidation promotes the dissolution of the surface oxide film and decreases its thickness, which leads to the reduction of its mechanical properties and corrosion resistance. Therefore, in the anode oxidation process of aluminum and its alloys, it is necessary to cool the electrolyte with cooling equipment to prevent excessive rise in the temperature, which means consumption of a large amount of energy.
Kim Chang Sok, a researcher at the Science Engineering Institute, has proposed a surface treatment method for excellent surface properties and low energy consumption by applying wide temperature anode oxidation technique to the surface treatment of aluminum alloy 6061 commonly used as dry bulb.
Addition of polycarboxylic acid and inorganic salts to the sulphuric acid electrolyte of aluminum alloys can inhibit or retard the electrochemical and chemical dissolution of aluminum, resulting in an anodic oxide film with excellent surface properties even at wide temperatures, which can significantly reduce energy consumption and cost during anode oxidation. Considering it, he selected inexpensive oxalic acid as polycarboxylic acid and nickel sulfate which is widely used in the coloration process of aluminum alloy as inorganic salt, and conducted a study to increase the allowable electrolytic temperature of anodic oxidation.
The results showed that the surface characteristics of the samples anodized at 30℃ are almost the same as those of the samples anodized by the existing method at 20℃, so that the acceptable temperature range of anode oxidation can be increased by 10℃ and the energy consumed for cooling of electrolyte during anodization can be significantly reduced.
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Jo Sep 18, 2024
The real-world optimization problems which we encounter nowadays are becoming more and more complicated and difficult to solve by traditional heuristic methods. For those problems, metaheuristic algorithms have proven to be efficient and reliable techniques for finding near optimal solutions with a reasonable computational cost. Most of the today’s successful metaheuristic methods were inspired by swarm behavior of animals, biological systems and natural phenomena.
Recently, gravitational search algorithm (GSA) has been newly introduced, imitating the law of gravity in physics. The GSA is distinguished from other metaheuristic algorithms for its unique concepts and operators and it is regarded as one of the powerful metaheuristic algorithms. Actually, it has been widely utilized to solve various kinds of optimization problems arising in many fields, for example, image processing, effective solving of aircraft landing planning (ALP) problems in the air traffic control, predicting the structure of protein and estimating the minimum ignition energy.
Hybrid algorithms of GSA with other optimization methods have been presented in several works. Still, a large number of open problems exist for GSA despite these exertions.
Choe Thae Ryong, a researcher at the Faculty of Applied Mathematics, has investigated a method for improvement of GSA by mixing it with the invasive weed optimization, thus proposing a new hybrid algorithm called IWO-GSA. In IWO-GSA, the agents generate new seeds and scatter them at each iteration of GSA. Then, elite agents are selected according to their fitness.
He applied IWO-GSA to 23 benchmark functions and compared it with GSA and IWO. From the results of numerical experiments, he found that the proposed algorithm is superior to the competitors for most benchmark functions.
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