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By Jack Parker

Critical width Before we go further into pillar design, we should look into the matter of "critical width," because of its effect on pillar behavior. Critical width refers to the greatest width that the rocks above the mine can span before failure or, if there are pillars, the width to which we must mine before the pillars accept the full weight of the overlying materials. There are theories to help calculate that probable width, and they usually boil down to one-quarter or one-third of the depth of cover; but they don't always fit, and it would be better for us to understand the factors that control local conditions. Critical width may be controlled by the thickness or strength of one or more of the strata above the mining horizon. In a gold mine, we might have a thick granite or basalt above us, perhaps capable of bridging across most or even all of a small ore body. In that case, pillars might have to support nothing, or perhaps just the weaker or damaged rock below the strong main roof. Obviously it would be wrong to design for tributary load on pillars based on some lab-determined rock strength. In some coal mining districts in Utah, huge steps can be seen in the mountain sides, with slopes on weathered shales and vertical cliffs 100 or 200 ft high in thick, hard sandstones. The sandstones are capable of bridging across hundreds of feet, often making mining deceptively easy at first, but it becomes more difficult as greater areas are opened. Sometimes such a mining sequence leads to violent bumps and bursts as the wide spans of sandstone sag and bear down on the working faces, like giant nutcrackers, or as bridges eventually fail, releasing much pent-up energy.

Jan 1, 1993

By C. J. Bise

One area of study for the Coal Mine Injury Analysis project was to illustrate how loss control can be used as a foundation for mine safety training. After an initial review of the principles of loss control, the instruments often used for worker/worksite appraisal, such as incident reporting, analysis, and investigation, workplace observation, worksite inspection, and safety audits and checklists were evaluated for their effectiveness. The findings were then applied to the continuous-miner worksite at several mines to modify the techniques for use in a highly dynamic man/machine work environment. Before the information gathered from the review of the various continuous-miner worksites was presented at the annual retraining sessions at five mines, a trainability analysis procedure was developed to separate training shortcomings from those problems resulting from improper equipment design or unsatisfactory working environments. Also, several microcomputer-based training programs were developed to aid in conveying the loss-control concepts to the trainees. Finally, the results from the implementation and evaluation of the loss-control-based training programs were documented, including the complete training script for one of the case-study mines. The procedures outlined in this study will aid mine training specialists in developing focused, pro-active accident-prevention strategies and training programs that are based on sound loss-control principles.

Jan 1, 1993

By Kerry Spickelmier

A traveler driving across desolate central Nevada to the Round Mountain property can easily identify with turn- of-the-century prospectors seeking a fortune, or at least an existence, from the high desert. But now a shadow passing overhead is as likely a sophisticated fighter aircraft from Stillwater Naval Air Station as a circling bird, and the famous trails west show only as historical markers along the highway. Even the approximately 400- km (250-mile) trip southeast from Reno or northwest from Las Vegas takes hours now, not days. And the camps of Round Mountain’s early prospectors have been replaced by one of the world’s largest gold heap-leaching operations. While Round Mountain’s uniqueness begins with its scale, it doesn’t end there. Consider the mine’s low cutoff grade of 0.2 g/t (0.006 oz/st) gold, made possible by a new dedicated (not reusable) leach pad. Also, the operation’s degree of automation means that ore dropped in the south crusher system is not manually handled until it is stripped from the pads. There is the bonus of a geothermal field that is tapped to heat the leach solutions in the winter. And, of course, there was that “unexpected” hitch expected in all mining operations. For Round Mountain, it meant buying a gravity plant to process the gold nuggets discovered last summer in a fault gouge.

Jan 1, 1993

By Angove JE, Johnson GD

Sulphide concentrates are produced in a wide range of mineral processing operations. In the case of base metals they are further treated by smelting and/or hydrometallurgical techniques. In the case of refractory gold ores, sulphide concentrates containing gold are roasted or oxidised by various hydrometallurgical processes. This paper discusses the development of the Activox(R) Process which can provide an alternative method for treating sulphide concentrates. This process consists of fine or ultra-fine milling, followed by mild pressure oxidation to produce solutions of base metals or to liberate refractory gold. Base metal solutions (eg of nickel and cobalt) can be further processed by solvent extraction and electrowinning, while refractory gold can be recovered by cyanidation. The economics of the Activox(R) Process are often better than alternative oxidation techniques.

Jan 1, 1993

By Peter A. Bevan

"In massive and semi-massive deposits, it is essential to take samples continuously for specific gravity determination as well as for assays of the various metals of interest. Failure to use specific gravity in mixed deposits of high density contrast will result in incorrect determinations of the average grade.Graphs and linear equations of specific gravity against the grade of one metal are not considered accurate enough in a multi-mineral deposit, but appear to be very satisfactory in a theoretical one-sulphide/gangue mix. Specific gravities based on base-metal values only in massive sulphide deposits will result in underestimating the effect of low grade areas on the average grade and also cause a reduction in the calculated tonnage.IntroductionBy definition, a base-metal assay is the percentage of metal by weight contained in a sample, whether it be a soil or silt sample, a grab, chip or channel sample, or a section of diamond drill core.In those cases where there is a series of contiguous samples, say in diamond-drill core, the average grade may be required of the various assays that are above a desired or required cutoff grade.Internal low grade or waste can be included if the assays one side or the other are used to ""carry"" the lower grade material and the resulting average grade is above the cut-off grade. To determine the average grade, the individual assays are ""weighted"" by multiplying these values by their respective lengths, adding up the cumulative assay times length and dividing by the total length. This method is essentially a weighting by volume, because the core diameter is constant and is common to all the samples being considered for estimating the average grade . If some of the core samples have much higher specific gravities than others, and the specific gravities are not used in the weighting process, the resultant average grade will be over-estimated or underestimated, depending on the grade of the heavier sections. When core assays of approximately the same specific gravity or weight are averaged by length, the error in the weighted estimation of the average grade is negligible."

Jan 1, 1993

By Donald W. Gentry

It appears that the competition for increasingly scarce international mining investment capital will continue among developing countries. Indeed, there is growing evidence this competition may be on the verge of becoming destructive and cutthroat in nature. While this may be viewed as good news by short-term, speculative investors, it could have dangerous repercussions for international minerals producers over the long-term. It seems there has been a continuous parade of traveling foreign delegations touting the virtues of mineral investment in one country or another. These pitches all contain the politically correct key words, such as macro-economic reform, political reform, privatization, liberalization of laws governing mining and related investment incentives, reconciliation of ownership and right-to mine issues, and tax reform specially tailored for the minerals sector. Unfortunately, some of the information being trumpeted is based on plans, concepts or hopes for change rather than actual reforms approved by all necessary governmental entities. In yet other situations, the information provided consists of half-truths at best. Too often, potential minerals investors, faced with metals and minerals prices hovering near their recession lows and the need to offset old, high-cost operations, desperately want to believe all they hear about investment alternatives cast in a near-utopian setting. However, the virtues being proffered for mining investment abroad do not eliminate the need for careful scrutiny of subtle variations of traditional risks associated with foreign investment that can drastically affect the economic viability of mining projects.

Jan 1, 1993

By Y. K. Yen, J. D. Miller, C. L. Lin

A PC image-based, on-line coarse particle size analysis system is being developed for the control of crushing and grinding circuits. Initial results had indicated that the system is accurate when tested with irregularly shaped particles which are stationary and do not overlap with each other. Now the system has been successfully tested in the dynamic mode with a moving belt conveyor. On-line size analysis for actual plant operations is discussed and includes: (1) the video system with respect to lighting and data acquisition requirements for a rapidly moving belt and, (2) image sampling, specifically the number of observations needed for accurate sampling and the total time required to perform the calculations for a representative data set.

Jan 1, 1993

We live in a world in crisis. One where the tides of change run ever faster. Where the only thing we can be sure of is change itself. The challenge we face, therefore, is not how to resist the forces that move us nor even to try to predict where they will lead us, but to harness their energy and use their momentum to find new and innovative solutions to problems that can seem intractable. Indeed, crisis is often the catalyst which energises a business to change its technology and to form a partnership with a company which specialises in pioneering new technology.

Jan 1, 1993

By Ruoming Gong, Arindam Samanta, Ragula Bhaskar

Three studies conducted at the University of Utah relating to the effect of scrubbers on face airflow patterns, the resulting dust exposure of operators and the development of a throat scrubber for continuous miners are summarized in this paper. The first study involved full-scale laboratory tests to determine the potential dispersion patterns and transport of dust at a continuous miner face, using different ventilation configurations and a scrubber. Concentration maps were developed and potential exposures at operator positions were examined. The second study involved the development of a 3-D numerical model to predict airflow patterns generated by a scrubber at a continuous miner face. The model was designed to evaluate the role of various face parameters on airflow patterns at a miner face and to complement the full-scale testing. The third study involved the design and fabrication of a low profile hydraulically powered continuous miner throat scrubber. The scrubber uses three hydraulic motors and small diameter fans to draw about 3000 cfm (1.42 m3/s) of air and uses a flooded bed scrubber to filter dust particles.

Jan 1, 1993

By Henry A. Russell

During the last ten years Parsons Brinckerhoff has become a leader in the area of tunnel rehabilitation. A significant part of any tunnel rehabilitation program is the control of groundwater infiltration into the structure. This leakage over time, causes deterioration of the structures, and creates numerous operational problems for the tunnel operator. This paper will address current procedures for identification, documentation, and recommendations of repair methods for the control of groundwater infiltration in rail and highway tunnels.

Jan 1, 1993

By G. Heinrich

Size by size recoveries and pentlandite liberation data from image analysis were used to evaluate plant tests of pentlandite flotation and pentlandite/pyrrhotite middlings flotation from pyrrhotite in a circuit treating low grade magnetic intermediates at the Strathcona Mill. Raising the pH with lime gave improved selectivity for liberated pentlandite but there were major losses of middlings unless the circuit was pulled hard indicating the need for finer regrinding. However, size by size recoveries at the normal regrind showed that, while the pentlandite recovery improved in the finer ranges, the pyrrhotite recovery also increased leading to little selectivity in any size range. A much finer regrind gave greatly improved selectivity in all size classes suggesting that 'surface polishing' as well as liberation effects were being seen. Using liberation data from image analysis of the products, surface polishing accounted for over 99 % of the improvement in selectivity. Unreasonable recoveries of more highly liberated particles were observed for liberation analysis by volume fraction but showed the expected trend in recovery when the analysis was based on surface liberation. The surface liberation data showed that the improved pentlandite recoveries after finer regrinding were to more than 90 % caused by improved liberation with the remainder contributed by surface polishing effects. The practical implications of this work are that i) the comparison of particle grades with product grades allows to estimate the effect of regrinding on co-flotation/entrainment, ii) surface liberation models should be used for evaluating recoverabilities by flotation and iii) regrind circuits may be set up to ensure that all particles go through the mill or that only the coarse fraction is reground depending on the surface scrubbing found to be required.

Jan 1, 1993

By J. M. Mutmansky

Introduction Since the Hardy Cross numerical algorithm was applied to the solution of mine ventilation networks, numerous developments have taken place in ventilation network analysis procedures. The Newton-Raphson method, the linear theory method and the second-order approximation method are three of the additional approaches that have been applied. The efficiency of a ventilation network analysis procedure depends not only on the method of solving the system of equations derived from ventilation networks but also on the method of deriving the equations through selection of the meshes. This paper investigates the impacts of three mesh selection methods on the efficiencies of four ventilation network procedures. The minimum-resistance spanning tree method is the most popular procedure for selecting meshes. The shortest-path method was suggested by Epp and Fowler (1970) for improvement of the Newton-Raphson method. The minimum-resistance-path method is suggested here as the third method of selecting meshes. The effects of these three methods on the Hardy Cross method, the Newton-Raphson method, the linear theory method and the second-order approximation method are analyzed in this paper. These network analysis methods have been summarized in the paper by Kim and Mutmansky (1991). Mesh selection methods A procedure for mine ventilation network solution based on mesh equations typically consists of two steps: • the mesh selection step and • the network solution step. The mesh selection step establishes a set of independent equations to solve the network. The resulting equations are nonlinear. One method of solving nonlinear equations is to linearize the equations and iteratively solve these linear equations. The four analysis methods previously discussed in this paper use this approach. The linear equations required for solution can be written in matrix form as: P•dQ = dF (1) where P is the pseudo-resistance matrix, Q is the quantity matrix, and F is the Jacobian matrix (Kim and Mutmansky, 1991). The efficiency of the solution method depends on the characteristics of the P matrix, which in turn depend on the method of selecting meshes. To understand the impacts of the mesh selection methods on the efficiency of the ventilation network analysis procedures, the characteristics of the P matrix must be understood first. The diagonal elements of the P matrix, m in number, are the sums of the pseudo-resistances of the branches contained in the m meshes necessary for solution. The off-diagonal elements are the sums of the pseudo-resistances of the branches shared by two meshes. For example, P12 (first row, second column element of the P matrix) is the sum of the pseudo-resistances of the branches common to mesh 1 and mesh 2. When solving the system of equations expressed in Eq. (1), three of the iterative procedures (the Newton-Raphson, the linear theory and the second-order approximation methods) use the direct factorization method (Burden and Faires, 1985) on the P matrix. When the direct factorization method is used, the computational efficiency is enhanced by larger diagonal elements and a sparser P matrix. Generally, the sparsity of the P matrix is more important than the size of the diagonal elements. There is no known mathematical method of predicting the efficiency of these procedures, but heuristic methods can be used to study the efficiency. The Hardy Cross algorithm applies the Gauss-Seidel method of splitting the P matrix and applies the Newton-Raphson procedure. The efficiency of the Hardy Cross method depends on the spectral radius of the matrix[[D-L]-1U], where D is the diagonal, L is the lower triangle and U is the upper triangle of the P matrix for the Newton-Raphson method (Ortega, 1972). The smaller the value of the spectral radius, the faster the convergence. As with the other three methods, larger diagonal elements and a sparser matrix aid in faster convergence. With the Hardy Cross method, however, the size of the diagonal elements is more important than the sparsity of the matrix. Remembering that each of the diagonal elements of the P matrix is the sum of the pseudo-resistances of the branches in a mesh and that each off-diagonal element is the sum of the pseudo-resistances of the branches shared by two different meshes, the following strategies can be used: • Avoid having branches with larger resistance values shared by multiple meshes. This increases the size of the diagonal elements and reduces the chance of off-diagonal elements being large.

Jan 1, 1993

In April, 1990 the Sydney Water Board invited bids for the construction of the Hazelbrook Carrier Section 3 being part of the Blue Mountains Sewerage Transfer Scheme. In December 1990 the contract was awarded to Costain Resources Limited, Mining Services Division for the sum of $A15.3 M. The works consisted of excavation of 9 520 linear metres of tunnel, 3.35 metres diameter with installation of permanent ground support in the form of rockbolts, mesh, steel sets, shotcrete and in situ concrete

Jan 1, 1993

By J. E. Hoffman, Massimo Maccagni, T. J. Keefe, Marco Olper, O&apos

The problem relating to the use of insoluble anodes in acidic and highly aggressive environments has still not been definitely solved, and it is therefore a recurring subject of discussion. The fields of application in which insoluble anodes play a fundamental role are mainly in the electrolytic extraction of metals, in cathodic protection with impressed current, and in the production of oxidized halogenated salts. Based on the foregoing, B.U.S Engitec Servizi Ambientali (BESA) developed an electrode suitable for the use in the above-mentioned system. The basic component of the BESA anode is a bimetallic wire made of a copper rod (core) covered by a thin layer of valve metal, to be chosen between tantalum (Ta) and niobium (Nb). The wires, coated with suitable catalyst (PbO2, Pt or others) are inserted in a plastic frame and fixed to a current feeder by means of a screw, ensuring an optimal contact. Experimental data on the operating conditions, such as kind of electrolyte, current density, electrode potential, corrosion tests and fields of possible application, are described.

Jan 1, 1993

By Rolf J. Wesely

Recent events in The Peoples Republic of China indicate that a major effort is underway to increase trade with the western world. Gold production contributes to China's ability to participate in international trade. Consequently, the Chinese government places a high priority on increasing gold production. Many of the operating gold mines in China contain refractory ores that to date have not been efficiently treated. Figure 1 illustrates the description and location of a few of China's major gold mines. China recognizes that the technology necessary to economically treat this ore has been developed in the United States and other western countries. The country has therefore begun to seek assistance in evaluating several potential projects. Organizational structure Gold mining and processing in China is directed by the China National Gold Corp. (CNGC), headquartered in Beijing. The agency has a director in each province who is responsible for exploration, mining and processing. CNGC uses the services of two engineering organizations for metallurgical testing, process development and design engineering for their projects. The largest engineering organization is the Central Engineering and Research Institute for Non-Ferrous Metallurgical Industries (ENFI), also located in Beijing. The second engineering organization is the Changchun Gold Design Institute (CGDI), located in Changchun Jilin province.

Jan 1, 1993

By Coates J. R, Firth I. C

The Imperial Smelting Furnace at Cockle Creek, NSW, was constructed in 1961 and is the oldest facility using the Imperial Smelting process in operation. In recent years there have been significant improvements in many areas of the operation, including production, safety, hygiene and environmental performance. Steps taken to achieve these improvements are described. Plans to secure a profitable future while meeting environmental requirements are outlined.

Jan 1, 1993

By Y. Topkaya, R. E. Johnson, D. Buttinelli, B. R. Palmer, A. Geveci, L. Stoppa, C. Lupi, F. Pochetti

Representative samples of a composite mixture of three typical Cu/Zn complex sulphide ores, namely Black-, Yellow- and Altered-ore, from the Cayeli deposit in Turkey, were used for leaching tests in 2.2 M FeCl3, or 1 M CuC12-2 M NaCl solution in the temperature range from 313 K to 363 K (40O to 90O C), with and without the presence of an organic solvent for elemental sulphur, such as CCl4. The leaching rates of copper and zinc were quite high, as expected, in FeCl3 solution and complete recovery of both metals was obtained at the highest temperature when the solvent was present. However in CuC1, solution, extraction yields of the order of 95% only for Cu and Zn were achieved and the positive effect of the solvent was not clearly shown. The kinetics curves of Cu leaching by FeCl3 indicated a model of reaction rate depending on the particle surface, as in a chemically controlled reaction, in tests with the presence of CCl4. Without the solvent, or in CuCl2 media, the model of the diffusion controlled reaction seemed to prevail. Values of apparent activation energy, in both of the leaching processes, were determined and compared with those reported in literature.

Jan 1, 1993

By Manlapig EV, Lawson F, Clarkson CJ

A low grade middling sample from Mount Isa, Australia, assaying 31.7 per cent zinc, 12.9 per cent lead and 14.5 per cent iron was leached under atmospheric pressure in sulphate and sulphate-chloride solutions with ferric ions as oxidants. About 96 per cent of the contained zinc was dissolved into a 0.5 M H2SO4 - 0.5 M Fe2(SO4)3 solution at 90¦C within three hours from a five per cent by weight slurry density mixture. The leaching process is very temperature-dependent, suggesting a surface chemical control. In the latter stage, the leaching rates levelled off and the leaching was most probably controlled by both surface chemical reaction and diffusion processes. The addition of chloride ions in the leach solution resulted in an increase in the leaching rate. About 95 per cent of the lead sulphide contained was converted to lead sulphate which precipitated and remained in the residue. Essentially no lead remained in solution and the low chloride concentration did not affect the precipitation of lead sulphate. Half of the iron orginally present remained in the residue as pyrite although essentially all the cadmium and 90 per cent of the copper initially present were dissolved. Some three per cent of sulphide sulphur was oxidised through to sulphate, about 67 per cent was converted to elemental sulphur which can be recovered by solvent extraction, the rest remained in the residue mainly as pyrite. Ferric sulphate and sulphate-chloride solutions can be used to effectively leach finely ground lead-zinc middlings and other bulk sulphide concentrates. A complete separation of zinc from lead and silver has been demonstrated.

Jan 1, 1993

By J. A. Brierley, C. L. Brierley, K. A. Prisbrey, R. W. Bartlett, J. G. Harrington

Bacterial oxidation of gold-encapsulating sulfide minerals in low-grade refractory ores was investigated over a period of one year. Crushed ore samples were held in wet, bacteria-inoculated, aerated columns. Sulfide mineral oxidation data were correlated with subsequent gold cyanide extractabilities. Experimental results for both sulfide conversion and gold extraction were compared with a biooxidation kinetics model that incorporates rock size, mineral grain size, and heterogeneous pyrite reaction kinetics. Using the model, extrapolations were made over three years for biooxidation of ore in large stockpiles. Ferric ion (oxidant) diffusion in the solution-filled pores of the rock matrix does not control the biooxidation kinetics of ore crushed to -25 mm.

Jan 1, 1993

By Vogwill R. I J

The Boddington Gold Mine, operated by Worsley Alumina Pty Ltd, is located about 100 km south of Perth and is one of the largest-producing gold mines in Australia.Multiple open pits are planned and several extend below the water table, through the deeply weathered lateritic profile and into the underlying Archean bedrock. Groundwater related mining problems, such as poor trafficability, wet floor conditions and slope instability began to occur early in 1990.The mine is located within Archean greenstones which are steeply dipping and faulted. Rock types include mafic to felsic volcanics, pyroclastics and sediments. These rocks have been regionally deformed and metamorphosed and are intruded by quartz micro-diorites and post-mineralisation dolerite dykes. Deep weathering has produced a typical oxide profile consisting of laterite, clay and saprolite zones underlain by weathered to fresh bedrock.

Jan 1, 1993