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By R. J. Hafsten, E. S. Davenport, R. A. Grange

THE influence of austenite grain size upon the hardenability of steel is now fairly well understood; for a given austenite, increasing the grain size increases the depth of hardening, and, since the latter is related to the fundamental transformation behavior of austenite, it follows that austenite decomposition must, in general, be retarded by increasing grain size. To date, very little quantitative information bearing directly on the effect of austenite grain size upon the progress and result of isothermal transformation has been published. Davenport and Bain,1 and Zimmerman, Aborn and Bain2 have shown that the transformation is definitely retarded in hypereutectoid plain carbon steels at temperatures near and above the upper region of rapid transformation (Ar') when the austenite grains are increased in size. More recently, Griffiths, Pfeil and Allen3 investigated the effect of a series of different heating (austenitizing) temperatures upon the progress of isothermal transformation at 1245°F. (675°C.) and 745°F. (395°C.) in a medium carbon, low-alloy steel; while they reported no measurements of the austenite grain size, their results suggest that increase in grain size retards austenite transformation at 1245°F. (675°C.), but has no appreciable effect at 745°F (395°C.). The present paper deals with the results of a study of the isothermal transformation of austenite in a single commercial alloy steel in which a fine-grained or a coarse-grained state was induced by appropriate selection of austenitizing temperature. The data are presented in the form of isothermal transformation diagrams, which clearly show that coarsening the grains lessens the rate of transformation of austenite to proeutectoid ferrite and pearlite-that is, at temperature levels above about 1000°F. (540°C.)-but is without significant effect at lower temperatures where the transformation product is bainite or martensite. It is to be presumed that the foregoing statement would be valid, at least in principle, for many other grades of steel; it confirms again that the mechanism of transformation to a pearlitic structure is fundamentally different from that to an acicular structure. A number of related observations were made, which seem to throw some light on the transformation process as it takes place in alloy steels, particularly at temperature levels in the vicinity of 1000°F. (540°C.), where the transition from lamellar to acicular structures occurs. These data are discussed briefly in the belief that they may lead to a clearer understanding of this aspect of the subject. MATERIAL AND EXPERIMENTAL PROCEDURE The material investigated was a commercial grade of S.A.E. 4140 steel, the composition of which was as follows: 0.37 per cent C, 0.77 per cent Mn, 0.98 per cent Cr and 0.21 per cent M0. It was hot-rolled to a 1 1/8-in. round bar and normalized by air-cooling from 1800°F. (980°C.).

Jan 1, 1941

By P. -É. Boileau, S. Mandapuram, Shiping Ma, S. Rakheja

The absorbed power (PAbs) has been suggested as a better measure of human responses to whole-body vibration, since it relates to the cumulative energy dissipated by the body exposed over a given duration. Moreover, unlike the other measures, the PAbs can adequately account for the intensity of exposure. Although, the vast majority of off-road vehicles impose considerably severe vibration along the horizontal axes, the vast majority of studies on biodynamic response characterization consider only vertical vibration. Only a few studies have reported PAbs responses of the seated human body exposed to horizontal vibration and the major contributing factors [1]. This study aims to characterize the PAbs responses of seated human subjects to horizontal (uncoupled x- and y-axis) vibration as functions of the vibration intensity, subject mass, seat height and the, type of back support. Methods Experiments were conducted using a rigid seat with an adjustable backrest inclination and seat height. The seat was installed on a horizontal vibration simulator and the forces at the seat base and the backrest were measured by three-axis force plates. Two single-axis accelerometers were installed on the seat back and the platform, oriented along the axis of motion. The experiments were performed using three different seat heights (350, 390 and 410 mm), back support conditions (NB- no back support and sitting erect; Wb0- Upper body supported against a vertical back support; and WbA- back supported against an inclined backrest, while sitting relaxed) and three different magnitudes of broad band excitations in the 0.5-10 Hz frequency range (0.25, 0.5 and 1 m/s2 rms acceleration under x-axis and y-axis, applied independently). A total of 8 healthy adult male volunteers with total body mass ranging from 59.4 kg to 92 kg and aged between 21-51 years took part in the experiments. The subjects were seated with their hands in lap, and feet supported on the moving platform for each posture. Each measurement was repeated 2 times, while the data were analysed using a bandwidth of 50 Hz and frequency resolution of 0.0625 Hz. [ ] The data were analyzed to derive the absorbed power characteristics of the body at both seat pan and backrest interfaces, while the coherence among the measured forces and accelerations wer

Jan 6, 2006

By Morland R

Recent influence of backfill stability on stope production in the Copper Mine at Mount Isa Mines Limited is described. Changes to the mining process to improve fill stability are described, including increased focus on fill performance, modifications to extraction techniques, increased fill quality, changes to drill and blast practice, and research into backfill stability. Several case studies are described.

Jan 1, 1995

By Frederic Dewey

HAVING shown1 the difficulty of assaying so-called cyanide bullion and the extreme variations often found in the results, an investigation was undertaken to discover if possible the causes of these variations by a series of tests upon the specific action of various metals during cupellation. Zinc.-In order to test the effect of zinc, 48 assays were made in sets of 6, 2 rows of 3. The charge for the middle cupels consisted of 500+ mg. Au, 1,125 mg. Ag, 10 mg. Cu. To the end cupels 50, 75, 100 and 125 mg. Zn were added, and a set of each proportion was cupelled with 4 and 8 grants Pb. The cupels were assayed and showed the following gold contents in milligrams, the figures being arranged as the cupels stood in the muffle: TABLE 1 50 Mg. Zn 75 Dig. Zn 100 Mg. Zn 125 Mg. Zn 4 grams, Pb 0.2S 0.25 0.2S 0:29 0.27 0.34 0.33 0.28 0.41 0.31 0:30 0.36 0.30 0.33 0.34 0.31 0.31 0.29 0.35 0.37 0.42 0.41' 0.34 0.38 8 grams, Pb 0.40 0.43 0.42 0.38 0.36 0.43 0.35 0.34 0.41 0.33 0.37 0.37 0.54 0.17 0.59 0.44 0.43 0.43 0.38 0.39 0.41 0.57 0.38 0.46 _ In many instances the zinc does not appear to affect the cupel gold, but various exceptions are apparent, generally in the cupels in the back row. With 100 and 125 mg. Zn, in some cases the Zn burned with its characteristic flame before it could alloy with the other metals. Also, less fusible oxidized lumps were observed during the cupellation and some cupels showed a scum. In order to lessen the premature burning out of the Zn, an alloy of 2 parts Ag to 1 part. Zn was used. A uniform charge of 500,+ mg. Au,

Jan 7, 1917

By Vikram P. Mehrotra

A laboratory investigation was carried out to study the influence of asphalt, bentonite and corn starch binders on the pelletization kinetics of coal fines and on the quality of the resulting pellets. Pellet quality was evaluated in terns of the compression strength, impact resistance (drop test), abrasion resistance and resistance to severe weather conditions. The amount of binder addition did not significantly influence the pellet growth kinetics. However, the pellet quality varied markedly depending upon the type and the amount of binder used. Experimental results are explained on the basis of the mechanism of interaction between the coal particles and the binders. Of the three binders tested, corn starch proved to be the best in all respects but for the resistance to water penetration. Finally, a combination of corn starch (imparting strength) and asphalt emulsion (imparting resistance to water penetration) was found to yield the best quality coal pellets.

Jan 1, 1981

By K. V. S. Sastry, V. P. Mehrotra

A laboratory investigation was carried out to study the influence of asphalt, bentonite, and corn starch binders on the pelletization kinetics of coal fines and on the quality of the resulting pellets. Pellet quality was evaluated in term of the compression strength, impact resistance (drop test), abrasion resistance, and resistance to severe weather conditions. The amount of binder addition did not significantly influence the pellet growth kinetics. However, the pellet quality varied markedly depending upon the type and the amount of binder wed. Experimental results are explained on the basis of the mechanism of interaction between the coal particles and the binders. Of the three binders tested, corn starch proved to be the best in all respects but for the resistance to water penetration. Finally, a combination of corn starch (imparting strength) and asphalt emulsion (imparting resistance to water penetration) was found to yield the best quality coal pellets.

Jan 1, 1983

By M. Mohammadi Zahrani

The morphological modification of primary and eutectic Mg2Si phases by bismuth (Bi) addition in Mg-5wt.% Si in-situ composite was investigated. It was found that with addition of 0.5wt.% Bi, the morphology of primary Mg2Si precipitates was altered from coarse dendritic to polygonal shape, and their mean size was significantly decreased from more than 20 µm to about 8 µm. Moreover, the rod-like eutectic Mg2Si in Bi modified Mg-Si alloy solidified much finer than that of the base alloy. Such improved microstructure of the modified alloy resulted in 10% tensile strength and 20% elongation improvement. The isotropic growth manner of primary Mg2Si particles in modified alloy was attributed to the poisoning of growth in preferred directions by dissolving Bi atoms in Mg2Si lattice. The reduction in size of eutectic Mg2Si was found to be the result of Mg3Bi2 compound formation around eutectic Mg2Si rods.

Oct 1, 2011

By Mark S. Stagg, Stephen A. Rholl

The Bureau of Mines is studying blast delay timing influences on rock fragmentation in a series of tests that started in 3-ft concrete blocks, progressed to 45in-bench reduced-scale tests in massive dolomite, and has now included 22-ft-bench full-scale tests in a thin-to-thick horizontally bedded dolomite. This paper discusses the full-scale test results and how they compared to the concrete block and reduced-scale studies.

Jan 1, 1988

The recent installation of top gas temperature probes in blast furnaces at Australian Iron & Steel Pty. Ltd. - Port Kembla, has provided the means for studying radial gas flow distribution in blast furnace shafts. This paper describes the top gas temperature probes and outlines their development and results of initial studies aimed at obtaining a suitable relationships between temperature profiles and furnace parameters. The effects of various combinations of burden materials have been outlined by the results reported. The use of probes has also helped to clarify the effect of casting cycles on furnace operation leading to increased emphasis on the reduction of the interval between gun-up and drill-out. Areas for future studies are also broadly Outlined.

Jan 1, 1975

By P Moser, P Schimek, F Ouchterlony

This paper summarises the findings of 27 model-scale bench blasts in magnetic mortar, which were conducted from 2011 to 2014 at the blasting site of the Montanuniversitaet Leoben.The aim of the study was the investigation of the influence of blasthole delay times in small-scale bench blasts on the face characteristics and cracking damage created in the surrounding rock, which affects the fragmentation in subsequent rows.The test specimens or blocks, which were produced out of magnetite concrete, were roughly 1:100 in scale to normal bench blasts. The confinement of the testing blocks at the sides and back ensured the similarity to normal bench blasts with regard to the wave transmission into the surrounding rock. The specimen contained three or four rows with five blastholes each and were shot row by row, with virgin (undamaged) material in row 1 and increasingly damaged or preconditioned material in the following rows.The tested delay times varied between 0, 28, 73 and 140 µs, which corresponds to 0, 0.4, 1.0 and 2.0 ms/m of burden.The evaluation of the material properties showed that the use of magnetite concrete eliminated the inhomogeneous behaviour of rock, but some unexpected differences of the individual production cycles were still detected.The characteristics of the bench after each blast was evaluated along three horizontal lines out of a 3D model of the bench face. The tested delay time sequences of the virgin (first row) and single preconditioned blasts (second row) produced more overbreak for longer delays, while the third-row shots resulted in a smoothened surface independent of the chosen delay.The introduced damage after every blast was evaluated by an observation of irregularities on the newly blasted bench face. The visible cracks at the top of the testing blocks were documented and divided into several crack families according to their angle, length and origin. The remaining block was broken out of the blasting site after blasting, cut into several horizontal and vertical slices and dye penetrant spray was used to visualise the cracks created. Some of the detected crack families showed an influence of the delay time. The calculated mean crack density and mean crack intersection density showed that longer delays resulted in a higher degree of damage.The fragmentation results were well reproduced by the basic three-parameter Swebrec function. The observed improvement of fragmentation in the second- and third-row blasts was linked to the preconditioning of the burden for the following row while blasting the actual row. With the exception of the simultaneously initiated blastholes, the blasts showed coarser fragmentation for the shortest delays, while the longest delays generated the finest fragmentation.CITATION:Schimek, P, Ouchterlony, F and Moser, P, 2015. Influence of blasthole delay times on fragmentation as well as characteristics of and blast damage behind a remaining bench face through model-scale blasting, in Proceedings 11th International Symposium on Rock Fragmentation by Blasting, pp 257–266 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Aug 24, 2015

By T. Marin, J. E. Seccatore, M. Cardu

The most energy-intensive process in the mining industry is particle size reduction – comminution. The first phase of this process is blasting to fragment the rock from the initial Boussinesq’s halfspace size. Scientific literature demonstrates that the output of a blast in terms of fragment size distribution can be manipulated by varying the specific charge (powder factor) and charge distribution. This study focuses on the influence of blasting on the internal resistance of the blasted fragments; in particular the influence of the spatial distribution of charges. Small-scale blasts were performed on 14 marble blocks using different powder factors and charge distributions. Three control blocks were fragmented by mechanical means for comparison. The results show the correlations between charge distributions and the specific energy of mechanical comminution. Opportunities for adjusting charge diameter and distribution in opencast blasts to improve the performancet of comminution circuits are discussed.

Aug 8, 2024

By R. K. Singhal

In open pit mining every effort is made to maintain as steep a pit slope as possible. Pi t slope angles are influenced by numerous factors some natural that are amenable to limited or no control at all and other mining technology related where a greater degree of influence can be exerted by the engineer. One such area where control is possible is the method of drilling and blasting in the pit. This paper discusses the influence of blasting techniques on the economics of pit slopes and demonstrates how balanced blasting can be used with advantage to pit wall control and improve fragmentation (thus reducing material handling costs). Microprocessor based instrumentation to monitor drill and blast performance has also been described.

Jan 1, 1988

By Gordon M. Matheson, David K. Miller

Sinkhole development in karst terrains is a serious problem and risk to facilities built in these areas. The potential for sinkholes are a function of many things including the chemistry of the underlying rock, the location of water conductive fractures and fissures in the rock, the natural fluctuation of the ground water table, vibrations from various sources and surface regrading activities. When sinkhole open near quarry operations the occurrence of these features is occasionally attributed to vibrations from quarry blasting. The purpose of this paper is to review the evidence regarding the development of sinkholes as a result of quarry blasting. In general, it has been found that sinkholes result from collapse or piping of overburden soils into cavities or caverns developed within underlying carbonate rocks that have been in existence for many thousands of years. The overburden soils in karst areas are typically clayey and as a result are not subject to densification or liquefaction as a result of vibrations. The primary cause of an increased rate of sinkhole development can be attributed to changes in the ground water table or local surface water drainage. These changes may be caused by seasonal fluctuations, ground water level lowering in drought periods, dewatering of excavations or quarries, or local changes in surface drainage resulting from grading activities.

Jan 1, 1998

By L. L. Oriard

In order to predict the influence of blasting on slopes, one must first understand the action of explosives, the manner in which rock is broken or displaced, and how seismic waves are transmitted, and the nature of these seismic waves. In this paper, the author describes the action of high explosives on rock masses, beginning at the source of the explosion, extending through the zone of rupture to a distance where only elastic waves of low energy persist. The paper describes the relationships between explosives charge quantities and distances, including the character and intensity of the seismic waves in various types of terrain. In looking at the analysis of slopes subjected to seismic waves, it has been common practice to assume a simplified model in which the anticipated acceleration is applied horizontally to the slope as an equivalent static force, or a model in which there is a rigid base subjected to mechanical shaking. Such models do not accurately portray blasting activity taking place within or near the slope in question. In dealing with blasting effects, it is important to under- stand seismic wave types, wave lengths, attenuation and transmission paths, as well as vibration intensity. In most cases, those slopes that are the least stable under static loading will also be the least stable under dynamic loading, although there may be rare exceptions. Thus, the more that is known about a slope statically, the better will be the assessment of it dynamically.

Jan 1, 1983

By H. G. Naidu, Raj K. Singhai, S. P. Singh

Introduction The economics of open-pit mining de¬pend heavily on the cost of overburden removal. It is necessary to maintain a pit slope as steep as possible. Steeper slopes lower the volume of overburden to be handled, thus lower the cost of material handling. There is, however, a limit to which the pit slopes in any given situation can be steepened. Steepening beyond this point could jeopardize the pitwall stability, ren¬dering it unsafe. It might even result in curtailment of pit production. Several factors affect the stability of pitwalls. Some are natural, such as topogra¬phy and regional and local geology. Others are man made, such as pit geometry, ground water management, equipment, drilling and blasting, and the type of excavation. Greater control can be exercised in the latter with proper planning and by following a set of procedures based on field experiences. This article deals with one such control¬lable factor - drilling and blasting. The eco¬nomics of steeper pit slopes is discussed, followed by a record of drilling and blasting practices in western Canadian surface mines. Microprocessor-based instrumentation and its application to blasthole drilling is also described. Economics of steeper pit slopes The economics of steeper pit slopes is not a new issue. It should be reinforced, though, that pit slopes must be as steep as possible to ensure maximum recovery of minerals at a minimum stripping ratio. For operating, economic, and safety reasons, different pit slopes are maintained during different phases of pit development. Figure 1 shows a hypothetical ore body, cylindrical shape, with a radius of 225 m (740 ft) and located 30 m (100 ft) beneath the surface. The mining operations are planned at two different slope angles of 25° and 40°. The ultimate pit depth was established on the basis of the criterion of "break even pit." The variation of the stripping ratio at differ¬ent mine levels is shown in Fig. 2. Table I summarizes the eco¬nomics involved for two pit slopes, 25° and 40°. Figure 3 presents the profits during the life of the mine for these two pit slopes. This study shows that steeper slopes for an open pit operation are far more advanta¬geous than flatter slopes. But, what is the optimum slope at any one point in the life of a moving pit? And how can the stable slopes be maintained over the short- and long-term? Canadian blasting practices A survey of blasting practices in western Canadian surface coal mines revealed that, at most mines, hard consolidated overburden is drilled and blasted. Blastholes are drilled with rotary crawler-¬mounted rigs using bits in the 220 to 310 mm-diam (8.5 to 12 in.-diam) range. Typi¬cal hole depths are 12 to 18 m (40 to 60 ft) to accommodate the pit bench heights of 12 to 15 m (40 to 50 ft). For a 12-m (40-ft) high bench, a typical hole will be 14.6 m (48 ft) deep including the subgrade drilling of 2.6 m (8.5 ft). Half of this hole will typically be a powder column and half will be stemmed. For a 15-m (50-ft) bench, a typical hole will be up to 18 m (66 ft) deep including the subgrade drilling. Drilling patterns are usually 9 m (29.5 ft) spacing and 9 m (29.5 ft) burden. De¬pending on local geology and hole diameter selected, drilling patterns of 8 x 8 m, 8.5 x 7.3 m, and staggered patterns of 11 x 5.5 m

Jan 1, 1989

By L. Dorador

"In block caving projects, the rock mass fragmentation process plays a controlling role in the design and success of the operation. Poorly understood, however, is the secondary fragmentation that occurs as blocks of caved ore move through the draw column and interact with one another. The initial size of these blocks can be considerable, as can their strength and stiffness as the presence of non-persistent joints and veining will be variable. Intuitively, more planes of weakness within the blocks should facilitate early fragmentation, but this assumption could depend on additional factors such as contact type between adjacent blocks and loads imposed through varying heights of the draw column. This paper reports the findings from a laboratory investigation examining the role of block strength and planes of weakness on secondary fragmentation. Testing was carried out on small-scale concrete cubes of uniform size, with and without embedded planes of weakness. The results obtained demonstrate the significance of veining and other planes of weakness on secondary fragmentation under one-dimensional compression, and allow for the development and constraint of empirical guidelines.INTRODUCTIONThe fragmentation process during block caving involves the initial (primary) fragmentation of rock in the cave back through stress-induced fracturing, release and fall of individual blocks onto the broken ore muck pile. This is followed by secondary fragmentation involving splitting and rounding of blocks as they move downwards through the draw column towards the draw point. The degree of secondary fragmentation achieved involves many variables related to the mechanical properties of the rock blocks as well as different operational factors. With respect to the block properties, the presence of planes of weakness in the form of veining and non-persistent joints is a topic of special interest given the significant influence it potentially has on the overall fragmentation process. However, the inherent heterogeneity and variable size of individual blocks generated from the cave back makes it difficult to develop any empirical relationships relating secondary fragmentation to compressive loading based on insitu observations and mine data. More preferable would be to keep constant block parameters, such as shape (aspect ratio and angularity) and size, through controlled laboratory testing. This work presents an efficient technique to fabricate concrete cubes in order to keep constant different block properties. These are then tested under one-dimensional compressive loading. The objective of these tests is to emulate a tight packing condition of single blocks such as that which would occur when the air gap between the cave back and broken ore in the draw column is small. When the air gap is small, detached blocks are unable to fall and rotate but instead maintain a very tidy arrangement with minimal voids left between adjacent blocks."

Jan 1, 2015

The paper highlights the conceptual developments in rock mechanics applied to quantify the effect of bolts in the stabilization of joints and yielding soft rocks. NOTATION Aa = stress change after excavation Eä EE, Eä = moduli of intact rock, single joint and jointed rock Eb = elastic modulus of the bolt k, = normal stiffness of joint Ab = cross section area of the bolt A, = area of influence per bolt ab = axial bolt stress e, = axial strain of the bolt eä e,, ej, = strains of intact rock, joint and jointed rock Ai = area of the joint ,r+ = shearing resistance of the bolt (at the joint) a = normal stress at the joint plane aäa3 = major and minor principal stresses c;, 4i; = cohesion and friction angle of the joint 0 = angle between the bolt and the joint a = inclination of the joint to the vertical axis P = bolt density parameter u, = convergence of unsupported tunnel section u, = convergence of bolted tunnel

Jan 1, 1992

By Buddhima Indraratna

The paper highlights the conceptual developments in rock mechanics applied to quantify the effect of bolts in the stabilization of joints and yielding soft rocks.

Jan 1, 1992

By AK Chakraborty, AK Raina

Burden of drill hole is third most sensitive parameter effecting the ground vibration in blasting apart from maximum charge per delay and distance. Various types of burdens can be seen in practical bench blasting situations. Burden can be expressed as drilled burden and initiation burden. Drill holes may have various front row burdens (toe burden), which may be different from the designed burden. In order to ascertain the impact of front row burden as well as the initiation burden on intensity of ground vibration and air overpressure experiments were conducted in sandstone overburden benches of an opencast coal mine and limestone quarry. Blast trials were carried out in cognisance of both drill and initiation burden while keeping all other parameters constant. Effect of front row burden was also extensively studied by keeping almost all other parameters constant. The results of the study reveal that there is substantial influence of front row burden on the vibration and air overpressure. The statistical analysis of vibration and air overpressure data with drilled burden and initiation burden gives significant evidence that the level of peak particle velocity of vibration (Vmax)) is reduced by reducing effective burden with modification of initiation. Data analysis carried out for air overpressure versus initiation burden shows that reducing effective burden with modification of initiation results in increasing the level of P. The study reveals a qualitative assessment of the influence of burden on the blast induced vibration and air overpressure. The study also recommends to modify the initiation pattern to reduce the effective burden in order to reduce the vibration, without disturbing the fragmentation.

Jan 1, 2004

By Claudio Gutiérrez

The action of quebracho in the selective flotation of fluorite and calcite has been studied. It has been found that the unavoidable ions produced by dissolution of these minerals can precipitate calcium oleate which profoundly influences the adsorption of oleate. The depressing action of quebracho is due, at least partly, to a decrease of the oleate adsorption. The depression of flourite flotation by quebracho in the presence of calcium is due to the fact that precipitated calcium oleate can become only physisorbed, its adsorption being strongly prevented by quebracho. It is concluded that adsorption experiments with flourite must be conducted with a flotation-size mineral, which has a lower dissolution rate than when finely ground.

Jan 1, 1980