Jo Nov 28, 2023
Mining Method Selection (MMS) is the first and the most critical problem in mine design, and it depends on some parameters such as geometric and geological features and economic factors.
The ultimate goals of mining method selection are maximizing profit, enhancing mining recovery rate and providing a safe mining environment.
Selection of an appropriate mining method is a complex task that requires consideration of many technical, economic, social, and historical factors.
Pak Myong Chun, a section head at the Faculty of Mining Engineering, determined the factors affecting MMS with the help of some mining experts, and selected the most suitable mining method using the hesitant fuzzy group decision-making (HFGDM) and technique for order performance by similarity to the ideal solution (TOPSIS). These factors included type of deposit, slope of deposit, thickness of orebody, depth below the surface, grade distribution, hanging wall Rock Mass Rating (RMR), footwall RMR, ore body RMR, recovery, capital cost, mining cost, annual productivity, and environmental impact.
Firstly, he proposed a group decision-making (GDM) method to determine the weights of several attributes based on the score function with decision-makers’ weights, in which the n-dimensional hesitant fuzzy environment takes the form of hesitant fuzzy sets (HFS). Then, he calculated the weights of these factors using the HFGDM method. He compared seven mining methods for an apatite mine to select the optimal mining method using the TOPSIS method.
The results showed that the sub-level stoping method with priority of 0.811 3 was the best for the studied mine.
For more information, please refer to his paper “Suitable Mining Method Selection using HFGDM-TOPSIS Method: a Case Study of an Apatite Mine” in “Journal of Mining and Environment” (EI).
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Jo Nov 25, 2023
Generally, least squares (LS) method treats only random errors of observation vector in adjustment function models. However, both observation vector and elements of coefficient matrix of an adjustment function model contain random errors. Therefore, the adjustment result of least square method does not guarantee a global-optimal solution.
Since total least square (TLS) method takes into account both random errors of observation vector and coefficient matrix based on an errors-in-variables (EIV) model, it is possible to improve the accuracy more than the result by LS method. TLS method has been further systematically developed and widely applied to many science and engineering problems, namely some practical problems, such as those in signal processing, statistical calculation and regression analysis.
Kim Jung Hyang, a researcher at the Faculty of Earth Science and Technology, has described a parameter adjustment method based on Weighted Total Least Square (WTLS) method and verified effectiveness of this method through application in simulated network. He has shown its advantage in comparison with classical LS and TLS methods.
The results show that the WTLS method based on an EIV model can further improve the accuracy of adjustment results as it handles simultaneously all kinds of random errors involved in the observation system.
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Jo Nov 24, 2023
In the environmental protection and petroleum industries, advanced technologies for separating water and oil are essential. Selective separation of oil and water is an important means of oil and water separation, and superoleophilic and superoleophobic properties of microstructure surface in water have attracted attentions of many researchers. Contact angle is greatly affected by interactions between the liquid and the solid and by chemical characteristics and roughness of a solid surface. For a certain solid surface, surface roughness is a key factor in controlling contact angle.
Contact angle is usually used for study of wetting phenomenon while wetting equations connect experimental measurement variables and wetting phenomenon. Wettability phenomenon can be theoretically explained by wettability equation.
Previous numerical simulations on static contact angle and dynamic contact angle of droplets on surfaces were mainly directed to water droplets in the air. Two-dimensional numerical simulations were used to study static and dynamic properties of water droplets in the air. Numerical simulations on static contact angles and dynamic contact angles of oil droplets in water were rarely found.
Choe Hyon Chol, a researcher at the Faculty of Mechanical Science and Technology, has conducted a 3-dimensional numerical simulation on the underwater apparent contact angle of oil droplets on the microstructure-textured surface by VOF method.
The results are as follows.
The height of a microstructure layer on the surface is an important variable that affects the wettability of a solid surface. When the surface of material is oleophobic in water, an increased microstructure height increases roughness factor, apparent contact angle of an oil droplet and oleophobic nature of surface. When the surface of material is oleophilic in water, an increased microstructure height reduces apparent contact angle and makes the surface more oleophilic.
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Jo Nov 22, 2023
Incomplete data are a common problem arising in data analysis. There are many methods to treat incomplete data including deletion and imputation.
However, they may be unsuitable in case of relatively large amount of missing values.
Yang Won Chol, a researcher at the Faculty of Materials Science and Technology, has proposed a method to develop a multiple regression model with block missed incomplete data.
The outline of the method is as follows: (a) separate submatrices from block missed incomplete data, (b) develop multiple regression submodels from the submatrices, and (c) develop a final multiple regression model by linear or nonlinear combination of the submodels.
By applying the proposed method, he conducted a simulation experiment on three datasets and developed a prediction model of casting density for A380 according to die casting process parameters.
The results demonstrated that the performance and data usage rate of the proposed method is far superior to the previous methods.
First, the proposed method to develop a multiple regression model with block missed incomplete data ensures a high performance, statistical stability and reliability of the final multiple regression model, and ensures data usage rate of 100%.
Second, the final multiple quadratic regression model by quadratic combination of regression submodels has better performance than the final multiple linear regression model by linear combination of regression submodels.
Third, when some features of a new object are not observed, it is possible to predict the target value by using final regression submodels.
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Jo Nov 20, 2023
Recently, rare earth elements (REEs) are widely used in special material science, and therefore, preparation and application of REEs have been rapidly developed.
La, Sm, Pr and Nd among REEs are common additives. It is important to determine amounts of them in materials since they change the mechanical and electronic properties of materials.
Kim Pyong Hun, a section head at the General Assay Office, has studied atomization process of La, Sm, Pr and Nd by GFAAS. He also investigated the possibility of preconcentration of REEs by selective evaporation of matrix, and found proper atomization and pyrolization temperatures. Then, he made sure the use of chemical modifier and mini-flow of internal gas at atomization step resulted in relative enhancement in sensitivity.
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Jo Nov 18, 2023
Organic and inorganic compounds have been used as a modifier for enhancing bonding force and strength of sodium silicate sand at room and high temperatures. Sodium silicate is generally used as an inorganic binder for producing molds and cores which are hardened by CO2 or ester.
The additive amount of sodium silicate should be decreased by raising its binding capacity to improve the strength and collapsibility. Therefore, it is necessary to refine colloidal particles and change the structure of sodium silicate by modifying it.
Many studies on modification of sodium silicate have been concentrated on application of organic compounds with functional group.
O Kwang Hi, an institute head at the Faculty of Materials Science and Technology, has modified the size of colloidal particles of sodium silicate by sodium phosphate. Then, he has measured and compared the sizes of colloidal particles of modified and unmodified sodium silicate by using SEM. After that, he has observed the change of crystal of modified and unmodified sodium silicate by XRD.
For modification, he set the pressure at 0.4~0.5MPa, time at four to five hours and additive amount of modifier at 4%.
The main conclusions of his study are as follows:
Firstly, the heating temperature of sodium silicate core modified by sodium phosphate is 760~1 400℃ for cast iron and steel castings with wall thickness of 8 to 60mm, and after being heated at the temperature of 800~1 200℃ the retained compressive strength and the knockout energy of the modified sodium silicate sand are about 1/3 and about 1/2 lower than those of unmodified one.
Secondly, when the modified sodium silicate is heated and cooled, collapsibility of the mold is improved due to the difference in the shrinkage stresses between silicate glass and phosphate glass.
Thirdly, when sodium phosphate is added to sodium silicate as a modifier, Na2O-SiO2 binary system is changed into Na2O-SiO2-P2O5 ternary system, and net colloidal structure is formed to enhance the binding strength as a result of interaction between phosphate tetrahedroid and silicate tetrahedroid.
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