Scielo RSS <![CDATA[Journal of the Southern African Institute of Mining and Metallurgy]]> vol. 118 num. 5 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>The story of the decline of the South African mining industry</b>]]> <![CDATA[<b>Where will our future metallurgists come from?</b>]]> <![CDATA[<b>The future of Africa is not so dark</b>]]> <![CDATA[<b>Optimization of the load-and-haul operation at an opencast colliery</b>]]> The current coal mining climate is characterized by coal price volatility, political instability, high labour costs, and increasing operational costs. This is exacerbated by a steady decline in the growth of global coal demand due to the increased use of alternative and renewable fuels in the energy industry. Locally, the overall mining cost inflation indices shows a yearly increase of 2% over the national consumer inflation. In order for coal mines to survive and mine profitably, they need to capitalize on the opportunity to improve their productivity and focus on one factor they can control: operational efficiency. Increasing productivity is one of the key drivers to counter diminishing profit margins. Increasing production effectively reduces operating costs. However, the emphasis should not only be on increasing output with the same input, but increasing the output while decreasing the input, and ultimately adding optimum value to current resources. Research shows that an increase in production will ultimately decrease the operation's unit cost, especially fixed costs. In this study a load-and-haul fleet optimization approach has been used to identify the opportunities for operational improvement at an opencast colliery. The study combines the results of a literature review, on-site time studies, and statistical data analysis in order to determine the best loader-truck fleet combinations for increased production. Several relevant key performance indicators (KPIs) for the evaluation and identification of productivity improvement opportunities were defined during this study. These KPIs are bucket fill factor, loading conditions, loading cycle time, utilization, and deviations from schedule. The priority delays determined by on-site time studies compared to the time book for each delay showed that idle or waiting time by the loaders, face preparation and relocation, and process delays had significant deviations. However, the results showed that this operation is under-trucked, hence optimizing the loader-related inputs proved less effective than optimizing truck-related inputs. The results indicated that a homogeneous truck fleet consisting of five Caterpillar 789C trucks, combined with a Caterpillar 994K loader, is the most efficient fleet option and will produce 1455 t/h. The combined optimized effect of each identified KPI of production led to a tonnage improvement opportunity of 5421 t per shift. <![CDATA[<b>Test work to examine the potential for improving gold leaching performance at Navachab Gold Mine, Namibia</b>]]> The Navachab Gold Mine is an open-pit gold operation located in Namibia. The metallurgical flow sheet consists of crushing, milling, leaching by CIP (carbon in pulp), and electrowinning. Extraction in the leach section is approximately 67%, increases to approximately 86% by the end of carbon adsorption, indicating that extraction in the leach section is far from complete. The purpose of this study was to examine the potential for improving the performance of the leach section. Bottle roll leach tests were conducted using different parameters to observe their impact on gold extraction and cyanide consumption. The factors varied included the cyanide concentration and pH. The effect of introducing lead nitrate on the leach performance was also investigated. Furthermore, it is suspected that constituents in the ore that adsorb gold from solution (known as preg-borrowing constituents) suppress the overall extraction of gold, and thus the benefit of CIL (carbon in leach) was investigated. Finally, bottle roll tests as well as plant tests were conducted to determine the influence of residence time, as it was thought that this may be a limiting factor. An increase in extraction was observed when the cyanide levels were increased and when lead nitrate was introduced. The CIL test also showed an improvement in extraction compared to the normal leach tests. An increment in the pH did not show the same benefit, however. Plant tests showed a decline in the solid residue grade with an increase in residence time. The results indicate possible areas for further research in order to improve leach performance. <![CDATA[<b>Numerical analysis and case study on the mitigation of mining damage to the floor of no. 5 coal seam of Taiyuan Group by grouting</b>]]> Based on the geological condition of panel 22507 of the no. 5 coal seam at Dongjiahe coal mine, numerical analysis of the stability and water permeability of the floor aquiclude before and after grouting was conducted using Rock Failure Process Analysis (F-RFPA2D). The dynamic development, extension, and distribution of the cracks in the aquiclude are discussed. It is shown that grouting can increase the effective thickness of the aquiclude, reduce the floor damage depth, and control the floor water inrush path, thus effectively improving the overall strength and water blocking capability of the aquiclude. These numerical study results were applied to facilitate relevant engineering work at Dongjiahe coal mine. The mitigation measures involved grouting the floor of the no. 5 coal seam, allowing panel 22507 to be mined safely above confined water. This exercise provided invaluable experience in preventing water inrush hazards. <![CDATA[<b>Surrounding rock control mechanism in the gob-side retaining entry in thin coal seams, and its application</b>]]> The stability of the roadside filling body is the key to successful gob-side entry retention in a thin coal seam. However, the existing failure mechanism of the roadside filling body is unclear, and no proactive measures for the high-stress regulation of the roadway-surrounding rock have been proposed. Taking the 2704N working face of Xinyang mine, China, as an example, this study aimed to elucidate the failure mechanism and characterize the mechanical environment of the roadside filling body in order to implement proactive stress adjustment in gob-side entry retention and measures for the structural control of the surrounding rock. The mode of failure of the roadside filling body was analysed, and numerical simulations and field experiments were subsequently performed to characterize the stress environment of the roadside filling body under caving and gob-filling conditions. Finally, regulatory measures for regional stress were proposed. The results showed that adjusting the three-dimensional stress fields in the surrounding rock causes the majority of the principal deviatoric stress to concentrate on the filling body, thus causing the surrounding rock to fail. Therefore, decreasing the subsidence space of the roof strata and preventing the accumulation of excessive principal deviatoric stress on the roadside filling body are crucial for regulating the regional field stress in gob-side entry retention and structurally controlling the roadway-surrounding rock. After gob filling, neighbouring units in the 'coal-roadside filling body-immediate roof-filling body in the gob' support system jointly form a skeleton structure that supports the overlying rock through contact force. A powerful force chain network is stored in the skeleton structure. The strong force chains in the gob-filling body bear the majority of the stress imparted by the overlying rock. Relative movement between key blocks is restricted because the key blocks compress each other. Thus, the principal deviatoric stress on the roadside filling body decreases and stability is significantly improved. A new technology that combines a mechanized gob-side entry in a thin coal seam and proactive roof guarding in the gob filling behind the hydraulic supports was designed in accordance with the 'green mining' concept of underground separation of coal from gangue and the requirements for pumped filling materials. The results of this study contribute to the theory and technology of gob-side entry retention and provide a basis for an effective stress adjustment and structural control method in the gob-side entry retention area. <![CDATA[<b>Experimental study on the stability of surrounding soft rocks of gob-side entry retaining in fully mechanized caving</b>]]> Theoretical and field investigations were combined to analyse and determine the design parameters for a field roadway and roadside support. A gob-side entry retaining model was created with similar materials, and physical tests and numerical simulation were carried out to study the stability of surrounding soft rocks in fully mechanized caving using specific roadway and roadside supports with different stress states. The study shows that mining the upper and lower panels causes a superposition effect to the gob-side entry retaining roadway, and the deformation of the surrounding rock is further exacerbated during mining of the lower panel. Moreover, the lateral residual abutment stress in the gob area could easily cause the entire roadside support to topple. A support method coupling 'long/short anchor cable plus bolts plus net' to a composite suspension beam provided excellent support for roadways with fractured roofs. As long as the roadside support strength is maintained, the secondary support provided by roof-contact yielding material will facilitate benign deformation of the roadway roof to release energy and stabilize the roadside support. <![CDATA[<b>Determination of stable spans in UG2 excavations</b>]]> The hangingwall of the UG2 Reef is characterized by stratification in the form of thin and weakly cohesive chromitite stringers that can vary in number and height above a stope. These stringers, in conjunction with shallow-dipping thrust faulting, endemic across the platinum-bearing reefs of the Bushveld Complex, affect the maximum span that can be safely mined. The paper describes the research that was carried out to determine the interaction of support with the rock mass in both conventional and mechanized workings, and provides insights into stable span determination where excavations are intersected by a shallow-dipping thrust structure. Four important issues are highlighted. 1. Mine pole and pack support has a greater influence on stability than span in the context of the studied database for conventional mines. 2. Shallow-dipping discontinuities are a dominant feature in stability analyses. 3. The height of the vertical tensile zone does not restrict the fallout height if persistent shallow-dipping structures are present. 4. In the context of the numerical modelling shown in the paper, a span of 6 m is safe when 1.8 m long, full-column resin bolts are used at a support resistance of 48 kN/m², regardless of the k-ratio or height of the triplets (intersections must be dealt with separately). Several leading practices have been developed in recent years to identify and timeously support hazardous structures. <![CDATA[<b>Effect of mould geometry, coating, and plate thickness on the thermal profile of continuous casting moulds</b>]]> A three-dimensional model for heat transfer analysis of continuous casting mould plate has been developed to predict the hot face temperature. Actual geometry has been considered by incorporating the bolt holes used for fixing bolts to tighten the plate with the backup water box, and the effect of these bolt holes on the hot face temperature of the mould plate is analysed. The high thermal conductivity of copper makes it suitable for use as mould plate material where high heat transfer rates are required. However, copper has poor resistance to abrasion and this disadvantage becomes apparent in the lower part of the mould, where severe wear of the copper surface takes place. To eliminate this problem, high-hardness materials like nickel are coated over the copper surface. This does not affect the heat transfer significantly. The purpose of this study is to investigate the effect of the bolt holes, as well as the nickel coating, on the hot face temperature. <![CDATA[<b>Automatic generation of feasible mining pushbacks for open pit strategic planning</b>]]> The design of pushbacks in an open pit mine has a significant impact on the mine's profitability. Automatic generation of practical pushbacks is a highly desirable feature, but current automatic solutions fail to sufficiently account for complex geometric requirements of pushbacks, including slopes, phase bench and bottom width, smoothness, and continuity. In this paper, we present a tool to fill this gap. Our proposed algorithm is based on modification of sets of blocks obtained by parametric optimization of the pit using a maximum flow method. A set of geometric operators is developed to modify the sets to present a feasible geometry for mining. The geometric operators are essentially derived from mathematical morphological tools. Case studies show that the proposed method generates practical pushback designs that meet all geometric constraints. The algorithm can be used to create a solution for medium-size pits in minutes. This significantly improves the efficiency of designing pushbacks for open pit mines. <![CDATA[<b>A new approach for predicting bench blasting-induced ground vibrations: A case study</b>]]> The paper describes efforts to find an effective and reasonable method for predicting ground vibrations induced by bench blasting. In order to reflect the effect of actual topography and geological conditions, two concepts - the equivalent path and equivalent distance - are introduced to take into account the effects of topographic features and properties of the rock and rock mass. An equivalent-path-based equation, the EPB equation, is thus proposed, which takes into account the impacts of maximum charge quantity and explosion heat of the explosive, acoustic impedance of the rock, integrity coefficient of the rock mass, as well as the equivalent distance. A total of 48 field seismic monitoring tests were carried out and the constants of the EPB equation were determined. Comparison of the predicted peak particle velocity values with those measured shows that the average error of the prediction is much lower, demonstrating the applicability of the EPB equation in the prediction of bench blasting-induced ground vibrations. <![CDATA[<b>Effect of cooling conditions on the leachability of chromium in Cr<sub>2</sub>O<sub>3</sub>-containing steelmaking slag</b>]]> The effects of temperature and atmosphere during cooling on the leachability of chromium from stainless steel slag were investigated. Experiments were performed under an oxidizing atmosphere in a muffle furnace and in a reducing atmosphere in an induction furnace. The slags were subjected to leaching tests using the standard procedure prEN12457-2. Results indicated that an oxidizing atmosphere enhances the leachability of chromium and that the extent of leaching increases with temperature. A reducing atmosphere tends to suppress the leaching of chromium. Differences between the chromium leached from the samples cooled under different atmospheres revealed that it is difficult for CO to completely reduce chromium Cr6+ to Cr³+ once hexavalent chromium has been generated. Therefore, it is necessary to utilize a neutral or reducing atmosphere for cooling the slag so as to prevent the formation of hexavalent chromium. <![CDATA[<b>Mathematical simulation and water modelling of liquid steel interaction with an argon bubble curtain in a one-strand continuous casting tundish</b>]]> The injection of argon into a continuous casting tundish stimulates the zones of active flow (plug and ideal mixing flow) in the liquid steel and reduces the stagnant volume flow. In this research, numerical simulations and laboratory experiments were conducted on the use of a gas-permeable barrier (GPB) in the one-strand tundish. The investigations were aimed at increasing active liquid steel flow in the tundish, without using standard flow-control devices, under variable thermal conditions. The software program ANSYS-Fluent was used to perform numerical simulations. A water model was used to validate the numerical model. The water motion vector fields confirm the hydrodynamic patterns obtained from the computer simulations. The argon flow rate, location of the GPB, and the thermal conditions influence the liquid steel motion in the tundish. The best GPB positions were identified based on the lowest value of stagnant flow at argon injection flow rates of 15 and 5 Nl/min. <![CDATA[<b>Particle size distribution and energy consumption during impact crushing of single granite particles</b>]]> To study the effects of crushing parameters on particle size distribution and energy consumption of rock impact crushing, a custom-made experimental rig was used for impact crushing tests on single particles of granite. The particle size distribution of the crushed product was analysed, as well as the sand production ratio and energy consumption per unit sand product under different impact velocities and with different primary particle sizes. The results show that the particle size distribution of the crushed granite under different experimental parameters is in accordance with the Weibull distribution. With a decrease in the primary particle size, or a specified impact velocity, the discreteness of the particle size distribution of the fragments becomes greater. An optimal impact velocity exists at which the energy consumption per unit sand product is minimized. Also, it was found that the smaller the primary particle size, the lower the energy consumption per unit sand produced.