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By Malcolm J. McPherson

There have been numerous recorded cases of mine explosions being initiated by falls of roof. The actual mechanism of the ignition has often been unknown. One possibility is incendive frictional sparking, particularly when the falling rock contains quartz. Another possibility that has received little attention is the increase in temperature of the air due to adiabatic compression under the falling roof. This paper analyses that possibility and concludes that falls which are extensive in plan area but not necessarily of great thickness of falling rock can, indeed, produce temperatures in excess of those required for the ignition of coal dust or methane.

Jan 1, 1995

By E. R. Mueller

The natural lives of machines and components vary tremendously! Maintenance managers struggle constantly to maximize their equipment lives while minimizing their maintenance costs, but they normally lack the tools and techniques they need to do this properly. This paper proposes a methodology based on probabilities of failure and expected values for repairs that recognizes the variation among individual machines and components as well as the high costs associated with catastrophic failures. It enables a user to determine the optimum change-out intervals for many types of equipment. The approach proposed here is sophisticated enough to be reliable but simple enough to be applied by the maintenance staff on the shop floor. It will complement existing preventive and predictive maintenance programs.

Jan 1, 1995

There are several simulators available and are being used for analysis, improvement and optimal control of the grinding process. However, these tend to be developed on informal software engineering methodologies. The informal approach is not suitable for long term development and a formal design methodology is necessary to develop efficient, error-free, and easy-to-use and to maintain software. In meeting this goal, basic concepts of a software model for describing the mineral process objects has been proposed. The software model forms the basis for software design and efficient implementation of the simulation. This paper describes the development of software models for grinding circuits. The work constitutes the theoretical foundation for the development of an advanced simulator for comminution.

Jan 1, 1995

By S. S. Peng

In this paper, the precise definitions for a number of commonly used subsidence parameters are given. The subsidence parameters have been determined using an integrated approach for 110 subsidence cases collected in the major US coal fields. Attempts have been made to establish empirical formulae for the parameters. The results of this study indicate that some traditionally-held concepts should be changed.

Jan 1, 1995

By F. T. Paleari, J. A. Finch

Radial gas holdup profiles were measured in a 50-cm-diam, 400-cm-high flotation column using a multi-cell-conductivity probe. Three bubble-generation systems, including a system with up to eight vertical filter-cloth spargers, a system with three horizontal filter-cloth spargers and a system with three (horizontal) Cominco-type spargers, were examined. The variables investigated were the number of spargers, the symmetry of their arrangement and the height above the spargers. Batch water containing up to 20 ppm frother was used. Generally, the profiles tended to be uniform (flat) with some profiles exhibiting a slight "wavy" pattern. The vertical sparger arrangement proved to be the most difficult in which to achieve a profile symmetry. The horizontal spargers gave uniform profiles at right angles to their length but gave decreasing gas holdup along their length. The Cominco-type spargers gave slightly less scatter in the data. Switching off selected spargers to simulate a malfunction did not produce strong asymmetry in the profiles.

Jan 1, 1995

By N. Edmonson

An examination of the statistics of the world phosphate fertilizer industry since 1980 reveals an increasing tendency for fertilizer manufacturing activity to concentrate in phosphate rock mining areas. The economic justification for this resource-oriented strategy is clear. The choice between shipping phosphate rock, a comparatively low-value product, and shipping more valuable finished fertilizers will always be resolved in favor of the latter if cost considerations control the decision. The historical existence of substantial phosphate fertilizer manufacturing capacity oriented toward fertilizer-consuming activities and dependent on imported phosphate rock shows that narrow cost considerations have not always determined the industry's location. One is entitled to ask what conditions have changed to precipitate the observed relocations, and how permanent these changes are. These questions are the subject of this investigation.

Jan 1, 1995

By Hanguang Wu, V. Izaxon, Sukumar Bandopadhyay

This paper presents an analysis of the heat transfer process between the mine ventilation air and the frozen rockmass. The heat transfer model takes into account not only by the mutual change in air and rockmass temperatures, but also by the phase change of ice-water- ice. Based on the analysis presented here it is concluded that the mine thermal regime can be regulated by having the air enter the mine through mined-out excavations or development airways.

Jan 1, 1995

By J. E. Murphy, L. A. Walters, F. H. Nehl, G. B. Atkinson

The US Bureau of Mines investigated the selective electrowinning of Ag and Au from cyanide solutions contaminated with Cu with the goal of decreasing the amount of Cu codeposited. Decreasing Cu codeposited will reduce refinery costs. Direct current was applied to the cell in pulsed and square wave voltages at 0.7 A h/150 mL of solution. Times tested for each cycle ranged from 1 to 10,000 msecs. Graphite, lead and stainless steel were evaluated as electrode materials. Square wave electrowinning at 70°C (158° F) gave the best separation of Ag and Au from Cu. Applying 2.5 V during the duty cycle, 0.0 V the rest of the period, a duty cycle of 10% to 30%, and a period of 100 msecs to 1000 msecs gave the following results: Ag concentrations decreased from 34 µg/mL to 0.1 µg/mL. Au concentrations decreased from 54 µg/mL to 0.2 µg/mL and Cu concentrations remained almost constant at 560 µg/mL.

Jan 1, 1995

By M. Polat, S. Chander

Many models have been suggested in the past to obtain a satisfactory fit to flotation data. Of these, first-order kinetics models with a distribution of flotation rate constants are most common. A serious limitation of these models is that type of the distribution must be pre-supposed. Methods to overcome this limitation are discussed and a procedure is suggested for estimating the 'actual' distribution of flotation rate constants. It is demonstrated that the classical first-order model fits the data well when applied to coal flotation in narrow size-specific gravity intervals. When applied to material which is fractionated on the basis of size alone, the use of three parameter models, which were modified from their two parameter analogs such as rectangular, sinusoidal, and triangular, gave most reliable results.

Jan 1, 1995

By L. B. Rothfeld, R. L. Dolzani

This paper presents preliminary results of a US Bureau of Mines (USBM) study of potential costs of compliance with possible federal regulations for copper heap and dump leaching. The regulations could arise under amendments to the Resource Conservation and Recovery Act (RCRA) and/or the General Mining Law. Estimates are given for the cost of compliance with possible technology standards that would require the use of liners and closure caps regardless of terrain and underlying geology and hydrology. This cost is compared to the cost of compliance with 1992 state requirements for terrains typical of those found at existing copper leach sites in the southwestern United States. Incremental compliance costs are found to widely exceed screening criteria used by the US Environmental Protection Agency (EPA) to denote significant economic impact.

Jan 1, 1995

By S. Chander, A. Briceno

The effect of sample storage in argon and in air on the kinetics of oxidation of pyrite from a coal source was studied using accelerated-weathering tests that were performed in laboratory columns. Prior to the weathering tests, the samples were crushed, ground and stored for various periods of time in either argon or air. The test results showed a dramatic difference in the rate of oxidation of the aged samples. The interpretation of the differences was based on the hypothesis that the aging of pyrite led to the formation of surface coatings that changed with time. "Dry" oxidation produced surface coatings that had properties different from those produced by "wet" oxidation. The significance of these results to pyrite/reagent interactions during flotation and in evaluating the acid-mine-drainage potential of pyritic waste materials is discussed.

Jan 1, 1995

Thermal pretreatment can significantly improve industrial comminution operations by reducing energy requirements and increasing mineral liberation. The effect of pretreatment using conventional and dielectric heating followed by quenching on the fracture properties of a number single and multiphase brittle materials has been investigated. The reduction in the impact fracture energies and particle strengths with and without pretreatment has been measured in precise single-particle experiments using the Ultrafast Load Cell. Variations in the fragmentation pattern caused by different pretreatments were assessed by measurements of the progeny size distributions. The differences in the effect of thermal treatment on single and multiphase materials are discussed based on the experimental results.

Jan 1, 1995

By H. Nematollahi

The comminution laws The grindability of the ores or minerals is an important factor that could not be deduced from their mineralogical characteristics (Testut, 1958). Many theories have been proposed to determine the amount of work required in a comminution operation as a function of the nature of material, size and shape of the minerals before and after of comminution. The most important laws are proposed by Rittinger, Kick and Bond. According to Rittinger (1867), the work is directly proportional to the new surfaces produced during the comminution. Kick (1885) believes that the work needed to produce similar modification is proportional to the volume or mass of the material. Bond, during his 25 years of experience and research, reached to the conclusion that on the average the energy required for a known comminution is between the Rittinger's and Kick's laws (Bond, 1952). According to Bond's Third Theory, the work input is proportional to the new crack tip length produced during the particle breakage, and equals the work represented by the product minus that represented by the feed. The work input is given by the following equation: [111W = 1 Owi( FP FF(1)] where W is the The energy input to the mill, (kWh/st); F is the Feed size based on 80% passed, (µm); P is the Product size based on 80% passed, (µm); and W; is the Work Index, (kWh/st) Methods to determine the Bond Work Index Various methods have been proposed by Bond to determine the Work Index. There are some problems with the Standard Bond Grindability Test: •Requiring a special mill. •Requiring a relatively large amount of sample. •Being relatively time consuming and tedious. [ ] During the recent years, numerous attempts have been made to install faster and easier methods to determine the Work Index. A complete description of these methods is presented by Yap et al. (1982). Determining the Bond Work Index by a new size ball mill During the recent years, researchers at the University of Tehran determined the Bond Work Index, using the Anaconda simplified methods by its Denver 305- x 127-mm (12 x 5-in.) laboratory wet ball mill. The equation used to calculate the Work Index for the above mill containing 16.12 kg (35.5 lbs) of steel balls is as follow: [1 _ 1-1W;=0.54x [P [)(2)] In addition, an attempt has been made to determine the Bond Work Index similar to standard Bond ball mill procedure but by a different ball mill, smaller and in consequent portable and less laborious than the Bond ball mill. Method description Just as the standard Bond grindability test, this method is a locked-cycle dry grinding and screening process that is carried out until steady state condition is reached. Test equipment The test is conducted in a special ball mill, called the New Size Ball Mill (NSBM). The mill is scaled down with a coefficient of two-thirds from the standard Bond ball mill. It measures 200 x 200 mm (8- x 8-in.) inside (instead of 300 x 300 mm or 12 x 12 in. in Bond ball mill). Just like the Bond

Jan 1, 1995

By B. K. Parekh, R. H. Mehta

Handling of fine coal is an importance issue for coal as well as the utility industry. Reconstitution in form of pellet or briquet would be desirable if it could be done economically. This paper evaluates the effectiveness of three binders e.g., asphalt- emulsion, corn starch and Brewex, in forming pellets of ultra-fine clean coal. It was found that asphalt emulsion and corn starch were not effective binders for ultra-fine clean coal, however, Brewex provided excellent quality of pellets, which exceeded all the minimum quasi requirements of coal pellets. INTRODUCTION Advanced coal preparation techniques (physical or chemical) are designed to process ultra-fine (minus 200 mesh or 74 pm) coal. The average particle size of the clean coal product is about 25 pm. Currently available dewatering techniques are effective in providing a 25 to 30 percent moisture product of this size coal slurry (Parekh et al. 1989). Handling and utilization of this dewatered ultra-fine size coal is difficult. Reconstitution, which includes pelletization, briquetting, extrusion, etc., of the dewatered coal product provides an ideal answer to handling and transportation problems. However, reconstitution of fines in the coal industry has not been successful due to various reasons. The most important criterion for its unsuccessful application in the coal industry is the cost of the binder (Capes et al. 1965). Various methods of reconstitution, such as briquetting, disk pelletization, extrusion pelletization and roller and die-pelletization, have been tested for fine coal agglomeration (Conkle et al., 1991) (Richards, 1990). Conkle et al., (1992) have provided a detail analysis of each technique in terms of moisture and operating cost requirements. According to Conkle, briquetting requires a product containing very low moisture, whereas pelletization can be done with coal containing up to 30 percent moisture. Currently available dewatering technique provides about 25 percent moisture product, thus, pelletization appears to be an ideal route for reconstitution. In this paper, results are presented on the comparison of three different binders i.e., asphalt-emulsion, corn starch and Brewers condensed solids (BCS or Brewex), for fine coal pelletization.

Jan 1, 1995

By R. R. Klimpel

Over the last six years, several new commercial families of sulfide mineral collectors have been developed. This paper summarizes some plant data and use experiences gained with each of these product families. The first is a series of products (F-series) based on a chelation mechanism of attachment. The second of these families is a series of products (S-series) based on an electrochemically controlled mechanism of attachment. The major advantage of the F-series collectors is the very high metal recovery capability in complex ores involving Au, Pt, Zn, Ni, and mixed Cu minerals. The major advantages of the S-series collectors are the very high pyrite rejection capability along with reduced lime requirements in Cu/Mo plants.

Jan 1, 1995

By E. H. Cho, P. K. Dai

An acid mine drainage sludge sample, containing 3.6% Mn, 0.18% Ni, 0.15% Co and 0.39% Zn, was obtained from central West Virginia. The sample was leached at room temperature with a pH 3 sulfuric acid solution. Air was then bubbled through the leach solution at pH 4.5 to precipitate the iron. The leach solution contained 258 ppm Zn, 105 ppm Ni, 90 ppm Co and 2050 ppm Mn. Three solvent extraction studies were performed to selectively extract these metal ions in a sequential operation with DEHPA. Cyanex 272 and Lix 84. The A/O ratio was one, and the extractants were diluted ten times with kerosene. A study of solvent extraction with DEHPA showed that Zn could be extracted without Ni or Co up to a pH of 2.75. A pH of 2.75 is considered to be the optimum pH to maximize the selective separation of Zn from Mn. The separation factor of Co over manganese suggests that the two metal ions should be extracted together by Cyanex 272. The separation factor of Co over Ni indicates that these two metal ions can be separated most effectively around pH 5.3. Also, Ni can be effectively extracted by Lix 84 above pH 5.5.

Jan 1, 1995

By J. A. Cole

Santa Fe Pacific Gold's Lone Tree mine is located in Humboldt County, NV, 45 km (28 miles) east of Winnemucca. The mine currently operates a 1.74-Mt/a (1.92-million stpy) run-of-mine heap-leach operation and a 2.27-kt/d (2500 stpd) refractory gold¬ore-processing plant. Most of the mill-grade ore reserves are refractory, mainly due to pyrite and arsenopyrite encapsulation of the gold. The autoclave pressure-oxidation circuit was commissioned in February 1994 to process the refractory ore. Mineralogy unique to the Lone Tree deposit allowed for the design of a partial-oxidation process. It takes advantage of the heterogeneous nature of the pyrite morphology. Partial oxidation has resulted in an autoclave plant that is able to achieve low operating-unit costs (Figs. 1 and 2).

Jan 1, 1995

By M. M. Foss

Geophysical methods are gaining acceptance in the evaluation of coal mine ground control condition. The use of in-seam seismic investigations has contributed significantly to the body of information concerning ground control feature identification and analysis. Seismic methods have been used successfully in locating voids, faults, fracture zones, roof and floor rock interfaces, and more recently in characterizing in-seam stress distribution. Seismic tomography is an effective tool in data interpretation for several of these applications, but as is the case with other engineering application of geophysical techniques, there are limits as to how much substantive information can be interpreted from a given data set. This paper discusses some of the applications of in-seam seismic geophysical tomography, and the limits to which the transformation from raw data to information should be taken.

Jan 1, 1995

By J. A. Pontius

The Cresson Deposit is located in central Colorado within the historic Cripple Creek Mining District. The deposit has moved from its initial discovery in 1990 to full production in less than five years (current reserve nearly two million ounces). This accomplishment required a great deal of cooperative teamwork between the exploration, development, permitting and corporate groups. Nowhere was this cooperation more evident than in the construction of the geologic block model. The detailed modeling of gold and its metallurgical characteristics formed the foundation upon which critical financial and design decisions were based.

Jan 1, 1995

By Jorge Rubio, Luis Valderrama, Carlos Pérez

This work describes the effect of the degree of energy transferred to the pulp, during the conditioning stage, on gold flotation fines recovery, grade and kinetics, with three types of frother (Dowfroth 250, pine oil and MIBC) and amyl xanthate as collector. Best results show an increase in 24% recovery and 50% in concentrate grade, depending on the intensity of conditioning and type of frother (better with Dowfroth 250). For low or intermediate energies (0.5-1.5 kWhlm3 pulp), the fine particles adhere to larger ones, increasing the recovery due to a mixture of carrier (pyrite, in this case) and autogeneous carrier flotation and at higher shear (2-3 kWhlm3 pulp), gold fine particles aggregate themselves. These phenomena are demonstrated by measurements of flotation rate, true flotation (flotation by actual particle-bubble adhesion), and by the amount of entrained particles. Alternatives for this type of pulp conditioning and mechanisms involved are discussed.

Jan 1, 1995