Search Documents

By J. E. Zeilinger, John Nagy, Irving Hartman

"INTRODUCTION Experience and research have shown that an effective way to reduce structural damage from explosions is to provide adequate pressure-relief vents in the walls of equipment and buildings within which explosions might occur. In general, free or unrestricted openings represent the most efficient type of vent. However, in many manufacturing operations it is impractical to use free vents, and it becomes necessary to cover the openings with diaphragms or blow-out disks, hinged panels, or other devices, in order to give protection against weather changes, to prevent the efflux of dusts or vapors from equipment during normal operations, or for other reasons.A few data on the pressure-relieving capacities of hinged panels and a paper diaphragm were published in Bureau of Mines Report of Investigations 3924,4/ which covered the first phase of the dust-explosion venting study made by the Bureau. The present report describes recent experiments that were made to determine the relative effectiveness of an unrestricted vent and 22 types of closures, including various paper, cloth, and metal-foil diaphragms, hinged doors or panels, and several kinds of scored and unscored glass panes.The tests showed that many of the vent closures were effective in releasing the explosions rapidly during their incipient stage, so that the development of destructive pressures was prevented. Vents of this type should prove useful in safeguarding industrial equipment from severe explosion damage. The experiments disclosed that the venting capacities of the diaphragms are approximately inversely proportional to their static bursting strengths."

Jan 1, 1950

By Shuren Wang, Wenbing Guo, Paul C. Hagan, Yanhai Zhao

"The overlying strata is often destroyed in large scale during shallow coal seam mining, and the sliding instability of the caved roof seriously threatens the safety of the mining field. The typical shallow coal mining in Shendong Mining Area in China was used as engineering background; the load distribution characteristics of the overlying strata were analyzed based on the monitoring data of the support resistance. Through theoretical analysis and numerical simulation, the formation and changing process of the pressure-arch in the overlying strata were studied, and the instability mechanism of the overlying strata was revealed by focusing on the accumulation and release law of the elastic energy in the pressure-arch. The results show that the continuous pressure-arch was formed when the horizontal stress exceeded the primary vertical stress of the mining field, and the elastic energy of the roof was released by the mining unloading effect. The elastic energy was accumulated in the pressure-arch, and the energy arrived the highest at the front arch foot. The accumulated energy at the arch foot was released by coal mining, and the shear zone could be formed. Therefore, the sliding of the caved zone along the shear zone induced strong roof weighting. The concentrated stress and the released energy during each mining increased with the panel advancing, and the height of the shear zone also increased. The conclusions obtained in the study are of important theoretical value to direct similar engineering practice.INTRODUCTIONThe instability of overlying strata during shallow coal mining, such as the large-scale roof falling and step-like ground subsidence, is a key problem that can restrict the safety mining in the mines (Ju and Xu, 2015). The self-bearing structure of the pressure-arch can form in the overlying strata after coal mining, and this structure can support the load of the upper strata and soil layer, so the weighting intensity of the panel is determined by the caved rock in the unloading zone under the inner boundary of the pressure-arch. A large amount of elastic energy is accumulated in the pressure-arch under the concentrated stress, and the released energy for mining is the internal cause of rock failure (Wang et al., 2017). It is an important problem to reveal the distribution characteristics of the stress and energy fields in the mine and to analyze the stability of the overlying strata during shallow coal mining based on the evolution characteristics of the pressure-arch."

Jan 1, 2018

A prototype EM Location System was fabricated and tested at the Geneva Mine near Dragerton, Utah. This system consisted of six manpack transmitters and six manpack receivers covering the frequency band from 900 Hz to 2900 Hz as well as a six channel receiver designed for use, in a helicopter. Tests showed that EM signals from lightweight (3/4 lb) transmitters could be detected both on the surface and from a helicopter through as much as 4450 ft of relatively high conductivity (2 X 10- 2 mhos/m) overburden. Furthermore, the resulting field strength profiles could be used to determine source location to within 150 ft of the actual surveyed location in the mine. Modifications were made to a Monitoring, Locating and Communications System being tested at the Bruceton Mine, Pittsburgh, Pennsylvania. These modifications have improved the performance and quality of the paging and voice telephony mode of the system. Subsurface receiver modifications have improved reception of voice downlink signals.

Jan 1, 1973

By V. V. Zubkov

The systems of geophysical monitoring have been already put into operation at coal mines and ore mines. Since at present it is rather difficult to conduct mine experiments; it is necessary to start a new step in joining of geomechanics and seismic. The linking geomechanical and geophysical monitoring at coal mines and ore mines today became possible due to progress in development of numerical methods of modeling of stress state and technology for processing of seismic information. This paper describes one potential step in linking geomechanical and seismic monitoring at coal mines and ore mines.

Aug 1, 2013

By M. Nejati, A. Paluszny, R. W. Zimmerman

A numerical method is proposed to propagate multiple discrete fractures leading to primary fragmentation in a mine that is being worked using block caving. Deformation is computed using the finite element method, fractures are represented explicitly, and the mine domain is discretized by an unstructured mesh. Bedding planes are represented by systematically varying the elastic modulus of the rock and by defining horizontal weakness planes. Fractures and matrix are represented using parametric surfaces, and tips are defined by their boundary curves. Tip advance is controlled by a failure criterion, and a criterion for propagation direction and magnitude, based on the evaluation of the modal stress intensity factors. A novel domain integral approach is applied to accurately compute stress intensity factors (K) ahead of fracture tips in three dimensions. The method does not require a structured volumetric mesh structure around the crack tip, as integration is performed over a series of virtual surface domains along the crack front. The method is efficient, as it makes direct use of automatically generated, arbitrary tetrahedral meshes, and approximates stress intensity factors (KI, KII, KIII) along each crack front using Interaction-integrals. As opposed to the JIntegral, the method does not decompose K a-posteriori, but instead uses an auxiliary field to directly compute modal K. When using this method, numerical approximations of K do not exhibit dependence on the mesh layout, and require meshes that can generally be ten times coarser than are required by displacement- and stress-based methods. Volumetric meshing requires only, on average, 17% of each computation step. Thus, cracks do not follow any pre-existing mesh structure, and the method is well suited for high-density fracture datasets. The method is demonstrated for primary fragmentation of a mine area covering 110 initial draw points, immediately beneath a 2m high undercut. Displacement is constrained at all boundaries except the surface of the undercut. The growth of the fractures is investigated at the onset of the undercut. Ninety initial disc-shaped fractures are taken into account, each having an initial radius of 10 m. Fractures grow around the undercut, intersect the bedding plane boundaries, and the domain is fragmented as a result.

Jan 1, 2015

By Pengfei Wang, Guorui Feng

With increase of cover depth, dynamic incidents, such as large gateroad convergences and coal bursts, are frequently encountered when mining deep longwall panels. It would be more adverse due to large inclination of the coal bed. Because of the shearer and armored face conveyor (AFC) sliding downhill, toppling of shields, disconnection of AFC to the stage loader are related to it. Tangshan coal mine is a typical deep, inclined coal mine situated in the east of China. The split-level longwall panel layout is being adopted for mitigating problems above, where tailgate entries of longwall panels are driven below gob edge and along the floor, while headgate entries are driven along the roof. Wire mesh and steel sets have been used empirically for support for tailgates for about a decade, which reduces the tunneling speed. In view of this fact, the paper serves to investigate the proper adjustment of the tailgate locations and to analyze the resulting stress and yield conditions according to support strategy. First, field measurements and observations were carried out. Stress data distributed within surrounding rock mass were obtained, especially within the roof since the roof consists of only a coal sheet with a thickness of 2–4 m. Roof pressure of the tailgate represents the pressure at the gob edge since it is below the gob edge. The convergence data were also collected. Numerical modelling using FLAC3D™ was conducted and validated by side abutment stress data. The double-yield constitutive model was used to simulate the gob. The finalized tailgate layout and design were determined through comparison of three scenarios. The research in this paper can help engineers choose the proper tailgate layout and support strategies for coal mines that are going to adopt a split-level longwall panel layout for mining deep, inclined longwall panels.

Jan 1, 2018

By I Canbulat, G Si, Y Duan, X Luo

The presence of unfavourable geological structures such as faults and dykes often can contribute to coal burst risk. Seismic events associated with rock fractures tend to cluster around these structures and it is important to understand the characteristics of seismicity for coal burst risk management. In an Australian underground longwall coalmine, two clusters of seismic activities close to the longwall face and within a section of the maingate roadway which was intersected by a major dyke were respectively identified. The induced seismicity in the dyke-roadway area was closely related to the longwall mining. These seismic events were located and the distribution of seismic events shows that more fractures were distributed on the inbye side of the dyke. The spatial-temporal changes of the seismic event locations delineate the fracture propagation as the longwall mined through the dyke. The frequency-magnitude relationship of seismicity was analysed and it was found that it changed significantly when the longwall was mining towards, through and past the dyke. The monitoring results demonstrate that because of the stiff dyke, the seismicity was intensified in the roadway section. It proves that the dyke promoted stress re-distribution and seismicity in its vicinity. More fractures were induced in the hanging wall of the dyke, whereas less strain energy was accumulated and released in the footwall of the dyke.

Nov 29, 2022

By Hans Jürgen Maier, Hans-Bernward Besserer, Gregory Gerstein, Florian Nürnberger

The formation and development of voids in both the single-phase ferritic steel DC04 and the dual-phase ferritic-martensitic steel DP600 are discussed. Various mechanisms and kinetics of void formation are considered regarding the influence of dislocation glide, precipitates and other parameters. Different characteristic locations in the microstructure are investigated at which strain concentrations during plastic deformation can occur and cause the formation of voids and microcracks. For the specimen preparation ion slope cutting is applied. Voids and cracks revealing ductile damage are analyzed by Scanning Electron Microscopy (SEM). Stress concentrations in consequence of plastic deformation could be shown by extinction contours on images recorded by Transmission Electron Microscopy (TEM).

Jan 1, 2015

By R. J. Mirville

Flocculation is an important step in the treatment of coal preparation waste waters. The effects of polymer charge and molecular weight and solution pH and ionic strength on the adsorption and flocculation characteristics of a series of polymeric flocculants on some typical waste water constituents have been studied experimentally. The results indicate that the nature and extent of flocculation are strongly system-dependent. For the case of coal, both adsorption and flocculation behavior can be correlated with the polymer conformation in solution. For the mineral constituents, on the other hand, adsorption and flocculation depend also on the surface properties of the solid.

Jan 1, 1979

By M. A. Perras, H. A. Kasani, G. Walton

The development of an underground repository for the storage of spent nuclear fuel relies on a multi-barrier system to isolate the used fuel from the environment – ultimately, the biosphere. These barriers include engineered copper and steal canisters, dense bentonite, and the natural rock as well. In order to reduce the potential flow pathway along the excavation induced damage zone, various slots perpendicular to the excavation axis have been proposed. These slots are called cut-off seals and generally are filled with bentonite as it has high swelling potential and very low permeability. Many different geometries have been proposed, but clear guidelines on the selection of the optimum geometry have yet to be developed. In order to assess the optimum geometry, three shapes have been selected for this numerical study, including a trapezoid, a triangle, and a thin rectangular slot. The cut-offs were modelled for a conceptual shaft constructed in the Queenston Formation from Ontario over a stress to strength ratio that has been found at many underground research laboratories. It was found that the excavation of a triangular and thin slot cut-off had the smallest increase in the damage as quantified based on the extensile volumetric strain within the plastic yield zone. Both shapes showed an increase in the damaged zone by 13 cm at a stress to strength ratio of 2.75. The trapezoidal cut-off damage zone increased by 20 cm at the same stress to strength ratio. Further study is required to better define the optimum cut-off shape for wider variety of conditions and other geological formations.

Jan 1, 2020

By Kathy S. Kalenchuk

Damages have been observed in shafts, backfill infrastructure and the slick line at the Leeville Mine. Shaft damage (shearing and spalling of the concrete liner) is attributed to mine-induced rock mass deformations concentrated where regional faults intersect the shafts. Aggregate bin, mixer station and conveyor drive damage (shotcrete shearing, heave of concrete floor slabs and bending of a vertical steel beams) is attributed to rock mass squeezing induced by elevated magnitudes of the vertically orientated major principal stresses. Separation of the slick line has occurred do to extension of the stressshadowed rock mass immediately above the mined out areas. Three-dimensional numerical models have been developed and calibrated to simulated rock mass deformations with magnitudes similar to those believed to have produced the observed damages. By stepping through the mine sequence, models reproduce deformations at the same mining stage as when damage has occurred. Calibrated models have been used to forward project further damage over the remaining mine life.

Jun 1, 2012

By Alan Bugden

Goedehoop colliery in South Africa has experienced a number of substantial roof falls in roadways and intersections. Many of these have been associated with geological slips and/or horizontal compressive stresses. This paper describes a numerical modelling study conducted after one such fall with the objective of clarifying the roof failure mechanism associated with these falls. The fall was located at an intersection and occurred as the last cut was being made to complete the intersection. One bounding surface of the fall was formed by the geological slip; the others were formed by fresh failure surfaces. The fall took place despite the slip being identified as a hazard and additional bolts being placed by the workforce. In conjunction with the modelling work tests were conducted on samples from the roof in which the fall occurred, these gave a U.C.S. of 80 -100MPa. Several in-situ stress tests had already been conducted at the mine; a further test was conducted close to the fall site. The tests show that the largest stresses are horizontal. However, the maximum stresses of 6-12MPa are small compared to the compressive strength of the root. Computer modelling highlighted a mechanism arising from the conjunction of a geological slip and horizontal compressive stresses that could result in tensile failure of the roof leading to the fall. Sensitivity studies were conducted to identify risk factors associated with this mechanism and the modelling results used to assist in formulating a support strategy to combat falls of this nature.

Jan 1, 2003

By Xicai Gao

With the intensification and expansion of deep coal mining, the surrounding rock stress environment becomes further deteriorated because of the complicated geological environment and powerful mining effect, so the major deformation of roadway appeared frequently. The proportions of floor heave are increasing and , the most serious problem is the floor heave of preparatory workings. The coal seam of Binchang coal area in Shaanxi province is at a depth of 621.6 ~ 817.0 m, the average thickness of the coal seam is 25 m, and the overburden and coal measure strata are the main regional aquifers and are highly permeable. The properties of water-weakening are obvious in the floor strata. The main preparatory workings or chambers were driven in the coal seam. The severe floor heave of the water chamber appeared during early mining and the amount was 180 ~ 910mm, and seriously affected the normal installation and use of the water chamber. Theoretical study and numerical simulation as well as the in-situ measurements were carried out to investigate the deformation and failure characteristic of the surrounding rock with non-support in the floor and the main reasons which caused the floor heave were determined. A new combined support technology was proposed which consists of overcutting, grouting, backfilling and invertedarch and bolt-grouting. The field monitoring showed that the cumulative convergence of the water chamber was 20mm, the maximum convergence speed is 1.2 ~ 2mm/d, the accumulative convergence of two ribs was 36mm and the maximum convergence speed is 2mm/d. The combined support technology will enhance the support intensity between the roof and floor, the two sides of the entry and improve the loading capacity of the floor effectively and the floor heave can be controlled effectively.

Jan 1, 2013

By L. St-Georges

In aerospace and transportation industries, severe problems are related to fatigue failure. Aluminium is frequently used in these industries to optimize the solidity and minimise the weight of the structures. Nevertheless, the fatigue life of aluminium alloys is limited and specific efforts have to be made to reduce the vibratory stresses in the mechanical parts. When riveted, the life of the sheet metal structures can be increased compared to welded structures, due to the energy dissipated by the friction between the rivets and the sheet metal and the sheets themselves. In the past decade, a new tool was developed to assemble aluminium structures; the friction stir welding. With this new technique, fatigue life of the aluminium welded joints can be increased by a factor from two to ten. However, less energy can be damped by the friction between the components and increased vibratory stresses may occur. In this investigation, the potential of the friction stir welding to create damped aluminium sheet metal structures is evaluated.

Jan 1, 2006

By D Reid

The engineering properties of soils and tailings often exhibit significant anisotropy. One area where this has been well documents is undrained shear strength. Materials sheared in directions close to the principal stress direction in situ, i.e. triaxial compression, typically exhibit higher undrained strengths than those sheared in other directions such as simple shear. Importantly, along the most common critical failure surfaces relevant to tailings stability, whether primarily through tailings or a critical foundation layer, saturated materials are generally much closer to the simple shear loading direction than triaxial compression. Therefore, testing undrained shear strength of tailings or foundation soils in triaxial compression is likely to result in overestimation of the relevant strengths for design. Although these issues have repeatedly documented in the geotechnical literature for over three decades, they are not appreciated sufficiently across our industry. In an attempt to address and highlight these issues, a review is carried out of historical reference to the importance of simple shear loading. Attention is then drawn to the techniques applied in three recent post-failure investigations, where simple shear testing was applied. Finally, some relevant laboratory test results from a recent project in which the author was involved are outlined.CITATION:Reid, D, 2018. Importance of the simple shear loading direction to the stability of tailings storage facilities, in Proceedings Mine Waste and Tailings Stewardship Conference 2018, pp 274–284 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Jul 23, 2018

By Bo-Hyun Kim, Mark K. Larson

"Researchers at the National Institute for Occupational Safety and Health (NIOSH) have advocated the use of instrumentation in underground mine ground control studies to aid in understanding failure mechanisms and to calibrate numerical models. The purpose of these efforts is to reduce accidents and fatalities in underground mines resulting from inadequate design of mining layout or supports. A recent examination of some results of stress change measurement raised some questions about the validity of elastic solutions in ground, such as coal. Particularly under deep cover as is prevalent in the western United States, the coal around a borehole would likely have yielded so that post-yield behavior would govern the response of the instrument.A numerical experiment was conducted by NIOSH researchers using the FLAC software (Itasca Consulting Group, 2016) to investigate the response of the borehole pressure cell (BPC) and the biaxial stressmeter to varying degrees of plastic behavior. The pre-encapsulated BPC cell pressure exhibits increasing departure from the elastic case with decreasing post-peak-strength slope of the stress-strain curve. On the other hand, because the BSM usually provides much more confinement to the surrounding rock than does the BPC, the measurements obtained from the BSM do not depart significantly from the changes in secondary principal stresses and directions as calculated by the elastic solution.INTRODUCTIONSeveral years ago, Larson et al. (2000) described several instruments that NIOSH had used in underground field studies and how to monitor them with dataloggers. This information is still relevant because of the need to understand how excavation affects the stability of mines and thus, the safety of mining personnel. One class of instruments described in that paper includes those instruments that measure change in rock stress. Although several such instruments might be used to measure stress change, the focus of this paper is on two instruments with which NIOSH has had more recent experience—the borehole pressure cell (BPC) (Geokon, 2018a; RST Instruments, 2018) and the biaxial stressmeter (BSM) (Geokon, 2018b). Other instruments, such as the CSIRO hollow inclusion stress cell (HICell) (Earth Sciences, 2019; Worotnicki and Walton, 1976) may be included in future studies.Early studies in the development of the BPC were those of Panek and Stock (USBM, 1964), Sellers (1970), Lu (1984), and Babcock (1986). Babcock’s work is particularly useful, but his setting and testing range was far below what we experienced in the NIOSH study, and it was Babcock’s data reduction scheme from his laboratory experiments that was used to calculate stress change in the NIOSH study (NIOSH, 2019a)."

Jan 1, 2019

By O. Hungr, V. Gischig, G. Preisig

Deep-seated landslides are characterized by slow time-dependent movements induced by constant gravitational creep, and periods of acceleration associated with stress changes within the rock and the preexisting fracture network. Natural stress changes are typically caused by seasonal variations in surface temperatures and groundwater pressures, but also by rare and more violent events such as earthquakes. These processes are responsible for cyclic loading and unloading that leads to rock mass damage, fatigue and progressive failure with time, and may explain sudden, unexpected rock slope acceleration phases. In this work, we present a set of distinct-element models to investigate and compare the role of hydromechanical fatigue and seismic fatigue in the history of a deep-seated landslide. Hydromechanical fatigue arises from the fluctuation of pore pressure in fractures accompanying periods of significant groundwater recharge. In alpine areas, there are usually one or two cycles per year associated with snow melt and intense rainfall periods. In the presence of a deep confined aquifer within the rock slope, the amplitude of pressure fluctuations may reach values of 0.25-0.5 MPa. Hydromechanical models considering such pressure changes are able to reproduce observed slope accelerations-decelerations phases. Substantial accelerations may occur even in the absence of a cycle of exceptional amplitude, depending on the current state of fatigue. At a given time, one or more fractures may be critically-stressed, and a small change in applied stress is enough to cause local failure and induce a crisis of enhanced slope acceleration. This is augmented if the rock slope contains planes of weakness associated with its tectonic history, glacial debuttressing, etc. The corresponding behaviour is analogous to laboratory experiments on rock and metal fatigue. Similar fatigue effects have also been proposed for repeated earthquake events in seismically active regions. Unlike hydromechanical stress changes that occur in a more localized portion of the rock slope, those induced by passing seismic waves involve the entire rock mass. Moreover, the frequency and amplitude of stress perturbations due to earthquakes are different than those induced by pore pressure variations, although both likely contribute in combination to the degradation and progressive failure of large deep-seated landslides when both are present. The results presented illustrate that: (1) assessment of slope stability near critical infrastructure (e.g., hydroelectric reservoirs) should not only consider static stability, but also failure mechanisms related to fatigue that evolve over time and may result in an apparently stable slope progressing towards a critical state due to relatively small but repetitive stress changes; and that (2) characterization of fatigue processes affecting a deep-seated landslide requires multiparametric continuous monitoring at different observation points.

Jan 1, 2015

By N. S. Tanious

This paper summarizes the results of an investigation into possibly the largest coal pillar failure in the USA. The newly opened drift mine is located in Southeastern Kentucky and worked an 11 feet thick coal seam, all other mines in the District worked seams 4-5 feet thick. The initial mine development consisted of 14 main entries (Fig. 1) which were driven in two sets of 7, under increasing depth of overburden. As the left seven entries came under deeper overburden the pillar ribs began sloughing which caused many of the concrete-block ventilation stoppings to crack. The mine management stopped the advance in the left seven and continued with the right seven entries. During the spring of the following year development resumed in the left seven, rib sloughage occurred again and this time severely cracked many stoppings and many were totally disintegrated from the excessive deformation. Within few weeks the sloughage was so heavy that many of these entries behind the active face were filled with piles of sloughed coal slabs. When the face reached a depth approximately 1150 feet, the sloughage became a slow, massive squeeze. The entry convergence (closure) reduced their height from 11 to 7-8 feet and the squeeze spread back to shallower depth covering an area roughly 300 pillars.

Jan 1, 1989

By Matej Petružálek, Tomáš Svitek, Tomáš Lokajícek

"Migmatite from the Skalka region (Czech Republic) was chosen as the experimental rock material. It has macroscopically visible plane parallel structure (foliation). The foliation was caused mainly by biotite grains arrangement. The cylindrical samples of migmatite with subhorizontal, subvertical and oblique foliation were uniaxially loaded up to the failure. Network of 8 broad band sensors was employed for acoustic emission monitoring and ultrasonic sounding. A grid search method with anisotropic velocity model was used for AE hypocenters localization. The source types of successfully localized events were determined from the average first arrival amplitude. Structural anisotropy of tested rock material caused the anisotropy of its mechanical properties (peak strength, Young’s modulus) as well as a different way of fracturing process in dependence on the angle between axial stress and foliation plane. The combination of splitting, shearing and sliding was found to control the fracturing of sample with subhorizontal foliation. The shearing and sliding was dominant in fracturing of samples with oblique foliation. With greater dip of foliation, the role of sliding increased at the expense of shearing. Due to the favorably oriented system of microcracks already present, the shearing and splitting was at the same level during fracturing of the sample with subvertical foliation before nucleation began.INTRODUCTIONThe process of failure of low porosity rocks depends on their mechanical properties and actual stress and temperature condition. At the low pressure and low temperature, the most common way is brittle failure, which is a progressive process requiring the initiation, growth and coalescence of cracks (Lockner, 1993). Stress strain behavior of low porosity crystalline rocks, during the laboratory compression experiments, is divided into four characteristic stages: crack closure, elastic region, stable crack growth and unstable crack growth which leads to the brittle failure, Brace et al. (1966) , Bieniawski (1967), Lajtai (1974), Cai et al. (2004), Nicksiar and Martin, 2012 and 2013.The fracturing process of stressed rock begins with the crack initiation (sci), which for low porosity rocks occurs approximately at 40-50% of peak strength (sp) (Cai et al., 2004; Nicksiar and Martin, 2013). After the sci, the dilatancy begins and stable crack growth follows up to the crack damage threshold (scd), which is approximately at 80% of sp (Cai et al., 2004). After crossing the crack damage level, there is an unstable crack growth accompanied with nucleation of fault plane (sn) at 97-100% of sp. (Rao et al., 2011). The stress drop accompanied with formation of macro scale shear failure plane follows after peak stress is crossed."

Jan 1, 2015

By W Jahn Archer

El rendimiento de extracción de mineral de cobre en Mufulira aumentó durante el periodo de 1971 - 1973. Por ello, los experimentos iniciales se concentraron en tratar de comprobar las potenciales ventajas de un tiempo de flotación prolongado, de forma que se incremente el rendimiento en cobre. En ese sentido, el texto señala las mejoras en los circuitos de flotación a través de la instalación de una celda de flotación DAVCRA en el circuito triturador. Dicha celda se intercaló con el circuito central y en el circuito de residuos.

Jul 1, 1976