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By Manoj Khandelwal, Bright Akuinor Tsidi, Newton Amegbey, Daniel Mireku-Gyimah

This study emphasizes the importance of understanding the factors that influence the throwing of flyrock in mining operations, a significant hazard to both workers and nearby structures. Moreover, it highlights how blasting engineers, when estimating flyrock occurrences and their travel distance, typically rely on input parameters such as powder factor (PF), hole length (HL), subdrill (SD), spacing (S), hole diameter (D), burden (B) and stemming length (ST), while overlooking crucial factors like velocity of detonation (VOD) and charge per bank cubic meter (CPBCM). Through analysis using support vector machine (SVM) modeling in MATLAB, the study underscores the impact of including or excluding VOD and CPBCM on flyrock throw distance.

Jul 1, 2024

By S. K. Sahoo, S. K. Sharma, A. Yadav, B. Behera, G. S. P. Singh

A field representative modeling of the gob behavior is critical for the understanding of the associated mechanics of strata behavior in longwall workings. The findings of theoretical models and laboratory tests on the simulated gob material have shown the strain hardening behavior of the gob material. The double-yield material model, which is a built-in constitutive model in FLAC3D, has significant potential to simulate the gob compaction process as it represents materials which undergo irreversible compaction in addition to shear yielding. Apart from this, several researchers have attempted using modified elastic model or applied nodal force in reaction to the strain in the gob area as per the well-known theoretical relations to meet this requirement. Although these models can simulate the mechanical response of the gob for subcritical and critical extraction widths, they do not apply to supercritical extraction width as the gob stress may exceed the in situ stress level in such cases. For overcoming these limitations, a standard approach has been developed and validated in this paper, which can be used for simulating the gob behavior in supercritical longwall workings as well. The complete study is divided into two stages wherein the laboratory scale compression test on the simulated gob material has been numerically replicated to develop preliminary confidence followed by a parametric study of the double-yield constitutive parameters for a rational estimation of their values in different conditions. The calibrated model has been used for verifying its results by plane strain modeling of two different longwall workings, using well-established empirical and analytical findings and field observations. The approach suggested in this study can be used for the efficient modeling of the gob behavior of longwall panels under varying geo-mining conditions without compromising the accuracy of the numerical model. The meticulously validated model can be helpful in rational estimation of stress redistributions around the gob area apart from load transfer on the barrier pillar.

By Charles D. Litton

This paper presents a description of the hazards of smoke from fires, the detectability of smoke using smoke sensors, and the operational characteristics of such detectors. It places in perspective the advantage that smoke detectors have over other forms of fire detection and also provides a discussion of smoke as a significant fire hazard.

Jan 1, 1992

By E. W. Lindroos, Fred D. DeVaney, R. L. Bennett, R. W. Hernlund, W. R. van Slyke, W. F. McDermott, C. U. Keranen

INTRODUCTION Iron is the world's most widely used metal and constitutes about 5% of the earth's crust. Iron ores are found widely distributed throughout the world. Because of its wide occurrence and relatively low value, an iron ore to be merchantable must contain a relatively high percentage of the metal to be considered as a source of iron. Few ores containing less than 25% iron would be considered as ore unless they existed in large amounts and could be concentrated very cheaply. The grade of the ore, the ease of concentration, and the transportation cost to market are fundamental considerations deter- mining its worth. Iron never occurs as a natural metal except in meteorites. Most iron ores occur as oxides, and to a lesser degree as carbonates. A small amount is recovered as a byproduct from iron sulfides. Iron ore must be reduced (oxygen removed) to metal for use. Iron is marketed in many forms and combinations, such as pig iron and a great variety of steels. Iron is practically never pure as marketed but contains varying amounts of carbon, phosphorus, sulfur, and alloying elements such as nickel, chromium, etc., all of which affect the physical proper- ties of the end product. Please note that in this section all analyses ore on a dry basis and all tonnage figures are on a long-ton basis of 2,240 lb unless otherwise noted.

Jan 1, 1985

By I. E. Odom

An electron optical and microprobe study was made to determine the physical and chemical nature of the silica minerals that occur as natural impurities in bentonite and montmorillonitic fuller&apos;s earth clay products (both granular and powder) and that occur in <10-µm dusts from plants that process these products. The purpose of the study was to determine if discrete "free" silica minerals, such as quartz, could be observed using SEM and EDS methods. No “free" silica minerals could be observed in the SEM images of the granular and powdered products, either among or on the surfaces of the particles. Also, no “free" silica could be identified by either SEM or EDS analyses of the dusts, even though quantitative X-ray diffraction analysis indicated that trace amounts of quartz occurred in all samples. It was determined that these quartz and other silica minerals were encapsulated or coated by montmorillonite. Vigorous washing, however, removed some of the clay coatings, so that small amounts of silica minerals could be seen by SEM and EDS analysis. It was found that all primary crystalline quartz grains in the bentonites have diagenetic overgrowths consisting of a mixture of montmorillonite, opaline silica and possibly microcrystalline quartz. Detrital quartz grains in the fuller&apos;s earth have thin diagenetic overgrowths of opaline silica, also containing montmorillonite. The clay encapsulation of the silica minerals in the products studied, as well as in the potentially respirable (<10-µm) dust produced during manufacturing of bentonite and montmorillonitic fuller&apos;s earth clay products, is considered to be an important factor influencing the potential health effects (e.g.. silicosis) of the contained silica. Even if clay dust containing silica should be inhaled, the montmorillonite encapsulating the quartz and other silica minerals would prevent direct contact of the silica with lung tissue. The clay encapsulation of quartz, as well as the highly hydroscopic nature of montmorillonite, may explain why not a single confirmed case of silicosis has ever been reported among plant workers in more than 65 years of production.

Jan 1, 1998

By L. C. E. FEIO, R. RAMSDEN

In many parts of the world deep level mining relies heavily on the use of refrigeration for providing safe thermal underground conditions. The trend in South Africa in recent years has been to locate the refrigeration plants on surface and to use these plants to chill water that is then distributed throughout the mine. The surface cooling installation normally consists of a pre-cooling tower in which water is first chilled before it is sent to one or more refrigeration plants for further cooling. Presently, the majority of these refrigeration plants use CFC refrigerants; however, to conform with the Montreal Protocol, these plants will have to be replaced or converted for use with alternative refrigerants. The optimum design criteria for installations using the new generation of refrigerants will be affected by their characteristics and are likely to be different from those currently in use. In light of these developments the industry is examining a number of alternative systems for future installations including water vapour refrigeration. Factors that affect the decision on the type of refrigeration system to be used in mine installations are reviewed. INTRODUCTION Gold mining has been carried out in South Africa for over 100 years and most of the reserves close to surface have been mined out; this has led to mining taking place at greater depths, approaching 4000 m below surface. One of the consequences of operating at these greater depths is that it is necessary to install refrigeration plants to cool the ventilation air so as to provide safe and healthy working conditions. With the current low precious metal prices there is pressure on mines to cut costs and in recent years there has been considerable effort devoted to the design of more cost effective and efficient refrigeration plants and cooling systems. In the last 10 years there have been many studies into the use of ice for specific applications (Eschenburg et al., 1986; Hemp, 1988 and Ramsden and Lloyd, 1992) and at present there are three ice making plants in operation on South African mines, each of which produces ice in different processes. In addition, detailed design studies have been carried out on the "split ammonia system" (Napier and Patterson, 1992). In this refrigeration system the evaporator is located underground close to where the cooling is needed, whilst the condenser and heat rejection system is on surface. As far as refrigeration plants are concerned the trend has been to move away from standard package units to custom built refrigeration installations. This has given the design engineer greater flexibility in the choice of key components for refrigeration installations. For example both screw and centrifugal compressors are used, the heat exchangers for the evaporators and condensers may be either plate or shell and tube, and in a number of installations evaporative condensers are used. In addition, the following refrigerants are in use, R12, R22 and ammonia. The acceptance of the Montreal Protocol and its amendments by the South African government will restrict the use of CFC refrigerants and it will be necessary to re-evaluate various refrigeration systems, including those that do not use conventional refrigerants, such as air cycle and water vapour systems, for new surface installations. Although new systems and refrigerants are being developed, only the following refrigeration installations will be compared in this paper: - a refrigeration system using R134a which has properties similar to R12 - refrigeration system using R22 or a new alternative refrigerant with very similar properties - an ammonia refrigeration system - a water vapour refrigeration system The benefits of new cooling systems are not discussed in detail, since they are largely mine specific and outside the scope of the present paper. Since the optimum design for an air cycle system (del Castillo, 1988) is dependent on the mine layout, this system is also not included.

Jan 1, 1993

By Richard J. Leary

At the 1976 Annual Meeting of AIDE, Alan Merson reminded his audience that economic&apos; success comes to those who understand things as they are, rather than as they have been or what ideology says they should be (1). He went on to say that, measured by this standard, the mining industry is in trouble. The mining firms had recognized long ago the importance of effective lobbying in Washington and the state capitals. But, he pointed out, those methods are nearly worthless anymore in coping with many land use problems because the critical decisions are likely to be made at the local level. There the tactics are far different as are the relations of the decisionmakers to the parties before then. Furthermore, in his estimation the trend is to leave land use regulation largely to the local level of government. All who are concerned for access to land for mineral development will be benefited by reading Professor Merson&apos;s paper lie is speaking effectively about the context of decisionmaking for specific cases, the shape of things in the here and now. There is, however, a longer-tern set of considerations that if pursued will be likely to foster the creation of public support for mining developments. At that sane meeting I spoke on two foundations of the long-term relationships: namely, the growth in requirements for minerals implied by projections of increasing consumption; and a rationale for fostering mineral exploration and development. I mentioned in passing the social decisionmaking process for public uses of land (2). This paper is intended as a complement. The growth of requirements for Minerals and the rationale for development based on the Entropy Law will be only mentioned. The main concern will be on the process of social decisionmaking in the context of access to land for mineral development.

Jan 1, 1976

By S. E. Clay, P. R. Taylor

A physical model for the diffusion and flow of gases through porous solids, which accounts for the random nature of individual gas molecules moving through a solid of definite structure, has been formulated. The model was solved using&apos; the Monte-Carlo technique. It was then used to evaluate specific constants related to the dusty gas model and to describe the gas-porous solid diffusion phenomena. Experimental results were used to validate the stochastic model.

Jan 1, 1988

By Russell Dittrich

The Kinross DeLamar Mine has been operated for more than 17 years producing precious metals doré from ores, principally silver bearing with associated gold coproducts. In late 1995, the Stone Cabin ore body was commissioned and the site began a phased shift from silver dominance to gold dominance. The Knelson concentrator was selected to remove gold from a portion of the cyclone under- flow stream in the grinding circuit.

Jan 1, 1998

By R. E. Johnson, L. V. Araújo, B. R. Palmer, L. M. Abrantes, G. W. Warren

The behaviour of chalcopyrite - mineral specimens and a Portuguese concentrate - in 2M chloride solutions prepared from hydrochloric acid and sodium chloride, has been characterized by cyclic voltammetry. The study has shown the influence of the potential and of the presence of impurities in the reaction schemes and their relative extent. The dissolution of chalcopyrite concentrate under potentiostatic control has been investigated and found to confirm the effect of the selected potential on the surface modification/rate of dissolution. An attempt has been made to correlate the information obtained with the response of the concentrate electrode to constant current treatments.

Jan 1, 1993

By S. Lin, Y. J. Wang

Based on systematic definitions of a generalized multiple fan ventilation system, the concepts of an augmented system, its solution curve and the operating points of the original system are introduced. The Quasi-Newton algorithm is applied in combination with the switching-parameter method to ensure a continuous search along an augmented-system solution curve and identify all the possible system operating points on it. Two example networks are analyzed to demonstrate those concepts and the application of the algorithm for identifying operating points of multiple fan ventilation systems.

Jan 1, 1994

By Ingo Kaundinya

"INTRODUCTION Tunnels are important elements of the German federal road network which have to cope with an increasing traffic load especially of heavy goods vehicles. In order to ensure a high availability and safety level for these key infrastructures, big efforts are required by owners and operators in order to keep all infrastructures up to date. This mainly relates to current European regulations regarding tunnel safety (Directive 2004/54/EG) [1] and their national implementation, which must be fulfilled by many existing tunnels. In this context the German Federal Government spends more than 1.1 billion € for the structural and operational retrofitting of more than 120 existing road tunnels. As many of these projects have recently been finished or are still under progress, experiences and best practices could be reported. After the description of current requirements in national standards regarding structural fire protection, the paper gives an example for an upgrade of a tunnel’s fire protection by installing an additional fire protection system, which is a quite costly measure. Another possibility to fulfill current requirements regarding structural fire protection could be the numerical proof of sufficient bearing capacity of a structure. The influence of different fire loads and fire durations on the bearing capacity of typical road tunnel structures have been investigated in a research project funded by the German Federal Ministry of Transport (BMVI) and coordinated by the Federal Highway Research Institute (BASt). The results of this research project have been used to define a procedure how to investigate existing tunnels regarding structural fire protection. Finally, the research results have also been the basis for a current update of national standards for tunnel construction. REQUIREMENTS REGARDING STRUCTURAL FIRE PROTECTION The structural fire protection design of road tunnels is usually based on temperature-time curves of exposure to fire, for simplicity called ""fire curves"". The fire development after fire growth is illustrated with fire curves. Fire curves can be subdivided into a heating-up phase, a fully developed fire phase, and if applicable, into a cooling-off phase (cf. Figure 1). During the fully developed fire, the fire remains at a certain temperature level."

Jan 1, 2016

By Ralph Bernstein

The processing of earth observation sensor data by digital technology is almost two decades old. Only recently, however, has this technology become practical and economical. The reasons for this are interesting to analyze, and they provide some insight into what the future may bring. Some of the earlier activities and their limitations, current systems and their applications, and future technology direction are summarized.

Jan 1, 1979

By E. T. Balazs

Share of developing countries in world bauxite resources mining and processing. Technical assistance of the United Nations Industrial Development Organization to the developing world in the field of bauxite evaluation and processing. Review of selected projects of bauxite research and development aimed at creating indigenous capabilities in developing countries: "Assistance to the bauxite aluminium industry of the People&apos;s Republic of China". Project activities. UN inputs. Achievements. Follow-up. "Upgrading the scientific and technological capabilities of the Jamaica Bauxite Institute". Project activities. UN inputs. Achievements. Expected follow-up. Assistance to the establishment of an "Aluminium research, development and design centre" in India. Project activities. UN inputs. Achievements. Follow-up. "Establishment of an aluminium institute for research and development in Titograd, Yugoslavia". Project activities. UN inputs. Achievements expected. Second UNIDO group training on bauxite evaluation and processing to alumina October - November 1983 in Budapest, Hungary. Programme and coverage. Show of movie picture on organization and execution (15 minutes). Significance of UNIDO&apos;s assistance to the developing world in bauxite research and development. References.

Jan 1, 1984

By R. A. Metz, James W. Clark, William C. Dixon, Kevin L. Wiley, Thomas R. Couzens

This paper is a description of certain geologic aspects of the Inspiration mining operation. It is not a geologic description of the Inspiration deposit nor an exposition of all the geologic work involved in the development and exploitation of the deposit. A few specific examples of the application of geology and geostatistics to the mining process are given to illustrate the progress that has been made in making the Inspiration Mine more efficient and cost effective, and a statement of the philosophy of geologic applications is offered.

Jan 1, 1992

By Michael J. Cruickshank

MICHAEL J. CRUICKSHANK SENIOR AUTHOR Marine mining is a very broad field. It encompasses every facet of terrestrial mining but with the unique addition of a superimposed liquid overburden, which is itself mineralized. It is the unique aspects of marine mining that are discussed in this section. In keeping with the times, a systems approach is taken in the presentation of the material. The environment is first described, and this is followed in logical sequence by the common requirements for working there, then the mineral exploration phase, the exploitation and, finally, the operational restraints imposed by man and nature. Free use is made of tables and figures from original and published works and extensive listings of reference material round out each section. The title "Marine Mining" was chosen in preference to ocean or underwater mining because it is broad enough to include oceans, beaches and inland seas. The selection of terms sometimes may seem odd but they have not been chosen arbitrarily. All things are reduced, where possible, to basics and defined. Applying systems terminology, the word "platforms" is used in the functional sense and includes not only airborne, waterborne and fixed platforms, but man himself in his capacity as a working diver. If there is confusion it is often because of familiarity with a word in a context personal to the reader. For example, what does extraction mean to you-or mining or exploitation? Cost data have been interspersed throughout the section whenever they could be obtained. The use of these data should be tempered with understanding because no two situations are alike and improvements in marine technology are constantly being made. The names of individual authors are listed with each subsection. Where no name is given, the responsibility has been that of the senior author. Acknowledgment is sincerely made to the U.S. Bureau of Mines, and latterly the National Oceanic and Atmospheric Administration, for encouraging and supporting the preparation of the work ; to the secretarial staff at the Marine Minerals Technology Center and to Donald Eisman for editing. To the many other contributors who may not be listed by name, grateful acknowledgment is also due. It is hoped that the chapter will be useful to the practicing mining engineer who suddenly is assigned to marine work or to the experienced marine engineer who is equally suddenly assigned to mining work. It is not possible to put "everything-you -need-to¬-know" into one section, but. the very full bibliographies at the end of the text should adequately fill in for those who wish to know more. 20.1-THE MARINE ENVIRONMENT Marine mining is set apart as a separate discipline by the marine environment. The following subsections describe the environment from the point of view of the physicist, the chemist and the geologist. The interrelationship of these disciplines with the economic mineral potential of the oceans and seas is apparent from the description of the marine mineral resources. The marine environment is an extremely complex natural phenomenon with which the marine mining engineer must learn to live. It can be the source of his wealth or his frustrations and sometimes both. It can never be ignored.

Jan 1, 1973

By Frank F. Aplan

Introduction Heavy media separation (HMS), also called dense media or float¬sink separation, is one of the newer forms of gravity concentration. Though the concept can be traced to the last century, the process has enjoyed its major growth since 1940. Heavy liquid separation is a mutation. The heavy media process is used extensively to clean coal and for the concentration of a wide variety of ores such as those of iron, lead-zinc, chrome, manganese, tin, tungsten, fluorspar, magnesite, sylvite, garnet, diamonds, gravel, etc. It may be used where ever a significant density difference occurs between two minerals, and commercial separations are typically made in the range of 1.3 to 3.8 sp gr. The particle size treated ranges downward from 6-8 in. top size. Particles greater than about 1/16-in. (10 mesh) may be treated in a "static" bath, though for reasons of separation efficiency, + 1/2 -in- feed is usually preferred. For particles less than this size, separation in a heavy media cyclone is generally used. The flowsheet of a typical heavy media process, in this case one using a ferrous medium, is shown in Fig. I. In essence, the process consists of: (1) preparation of the feed usually by wet screening to remove undesired fines, (2) heavy medium separation, and (3) removal and recovery of the medium from the separated products. Many muta¬tions of the basic scheme are possible and numerous options are possi¬ble. HMS offers the following potential advantages:12 1) Ability to make sharp separations. 2) Ability to change the specific gravity of separation quickly to meet changing conditions. 3) Ability to remove products continuously. 4) Ability to treat a broad size range of products. 5) Ease of start-up and shutdown without loss of separating efficiency. 6) Relatively low medium cost and low media losses. 7) Low operating and maintenance costs. 8) High capacity with the use of relatively little floor space. 9) Relatively low capital investment per ton of capacity. The process may be used to produce a finished concentrate, two finished concentrates, or a concentrate and a middling of differing quality, or a preconcentrate by rejection of unwanted gangue. It is an ideal method for the reprocessing of coarse waste dumps. The greatest use for the process lies in coal cleaning and in the preconcentration of ores. The relatively inexpensive heavy media process may be used advantageously to reject large quantities of coarsely crushed gangue. When used in this way, the process will allow: (1) the use of lower cost but less selective mining methods with the "overbreak" material being removed at the front end of the concentrator or preparation plant; (2) a substantial reduction in the quantity of ore that must be finely ground for subsequent mineral liberation and separa¬tion. Since comminution is often the single most expensive step in beneficiation, it is desirable to eliminate as many essentially barren pieces of rock as possible before the grinding step, (3) a decrease in overall plant capital cost per ton of concentrate since the size of the plant from the dense medium step onward will be smaller. Several general references are available,12-18 though much of the technical data on the process is widely scattered in the general litera¬ture. Heavy Liquid Separation Organic Liquids Given sufficient settling time, it is possible to make a perfect separa¬tion between two particles of differing density by placing them in a liquid whose density is intermediate between the two. This means of achieving a perfect separation has proven to be elusive because of problems in feed preparation, particle settling rates, operational considerations, and economic constraints. There are a wide variety of heavy liquids that could be used, most of them halogenated hydrocarbons, and a few typical examples are given in Table 4. These liquids are most commonly used in ore dressing for the laboratory fractionation of ore particles on the basis of specific gravity. Laboratory Separations. Using liquids typified by those given in Table 4, separations are made to develop either the standard washability curves used to estimate the response of a given sample to gravity concentration or to prepare a partition curve to evaluate the effective¬ness of a given gravity separation process or piece of equipment. A typical washability curve is given in Fig. 2.19 Such curves are generated for raw coal, e.g., by treating either the whole or various size fractions of the sample in a series of heavy liquids and analyzing the various specific gravity fractions so produced. The procedure is relatively simple for coal samples because of the ready availability of a wide variety of relatively low cost heavy liquids in the density range 1.2¬-2.0. For ores the problem is much more complicated, because only a few high density liquids, all of rather high cost, are available. Parti¬tion curves are generated in the same manner by treating the separated products in the same liquids. Greater details on the procedures to be used in heavy liquid separa¬tions are to be found in the literature (for coal, Refs. 13, 14 and 19 and for ore, Refs. 20 and 21). For testing coal, calcium and zinc chloride solutions have been used extensively in the past, though today halogenated hydrocarbons (available under the trade name Certigrav) are the preferred media. The liquids shown in Table 4 may

Jan 1, 1985

By Kathleen S. Smith, Katherine Walton-Day, James F. Ranville

Four different scales of observation were used to evaluate the potential effects of a fluvial tailings deposit on water quality along the flood plain of the upper Arkansas River south of Leadville, Colorado. First, we collected surficial material and subjected it to batch water-leaching test. Second, we excavated and leached an intact eight-inch diameter core. Third, we examined the water quality of the shallow ground water beneath the fluvial tailings deposit, and fourth, we monitored water quality along a 5-km reach of the adjacent Arkansas River. Leaching of surficial samples indicates that there is a reservoir of readily water-soluble material yielding elevated metal concentrations and high acidity. However, there are minimal measurable water quality effects of the fluvial tailings deposit on the adjacent reach of the Arkansas River.

Jan 1, 2000

By J. Kern, M. Cain, T. Torries

Until recently, the generation of electricity was regulated in all states, and the value of generation assets in a regulatory environment for ad valorem taxation were based largely on the book value of the plant plus the rate of return on capital allowed by the state regulatory agency. This method of appraising utility generation assets is inappropriate in a competitive environment because past capital expenditures are not relevant in a competitive environment. However, it is not clear what valuation methods are appropriate after restructuring. This paper identifies important problems and advantages of other appraisal methodologies and makes comments on various valuation and taxation methods.

Jan 1, 2003

By B. S. Crocker

Introduction Pebble mill grinding can be defined as wet or dry grinding in a tumbling mill with pebbles used as grinding media instead of metal balls. To clarify the terminology, which may differ in the various mineral industries, it is essential to point out the differences between grinding with partially sized and carefully sized grinding media. When grinding with partially sized media (limited top size only), sizing is usually the setting of the primary crusher and there is no bottom sizing-all fines, chips, sand, and small sizes are included. This is often called run-of-mine or primary autogenous grinding. In this case the entire crusher discharge is fed into the mill and part of it becomes "charge" and the rest is "sand feed." When grinding with carefully sized media (i.e., media that is sized both at the top and bottom of the size range), no fines or chips should be present in the charge to the mill. This is pebble milling and the grinding media consists of ore, rock, Danish pebbles, gravel pebbles, or ceramic pebbles-media that is appreciably lighter than steel balls. The grind¬ing charge is fed to the mill to maintain a load and it does the grinding usually on a classified sand product of suitable size. If the grinding media is the same as the raw material being ground, this type of grinding is also called secondary autogenous grinding. Pebble milling was in common use in the gold mines of South Africa in the early 1900s. It is by far the oldest form of autogenous grinding. It may be used to grind crushed ores from a maximum size of about -% in. to all -10 µm. Pebble milling differs from "run of mine" grinding in many ways, but two big differences are: I) The ore itself is crushed to approximately t/4 in., or it may be further reduced to 8 or 10 mesh in a rod mill or fine crusher. 2) Then carefully screened ore of the correct size is placed in one or more stages of pebble mills to grind the ore to its desired final size. The media size is carefully selected to be the most efficient for the particular size reduction required in each stage. This is a precise practice, and pebble mills can be designed as easily and accu¬rately as can rod mills or ball mills by those skilled in the art. A pebble mill is a rotating cylinder in which the grinding media has been carefully sized for the job in hand. If the mill is doing coarse grinding, the media will be correspondingly large and the mill might be called a primary (or intermediate) pebble mill or a lump mill. If it is to do fine grinding, the media selected is proportionately smaller and the mill would be called a secondary or tertiary pebble mill. The media itself is normally rounded pebbles made from screened ore from the primary-crusher discharge. The charge could be rounded pebbles of the correct size removed from a primary autogenous mill, or the pebbles could be carefully sized gravel, sized beach pebbles, or sized flint pebbles. Usually the specific gravity of a pebble is from 35 to 55% of that of steel balls. It commonly varies from 2.66 to 4.2, depending on the specific gravity of the ore. Primary pebble mills, or lump mills, would be charged with pieces of rock up to 10 in. maximum and 5 in. minimum and would grind crusher discharge with a maximum size of 1/4 in. on hard ores and 3/8 in. on softer ores. Secondary pebble mills could be charged with pieces of rock screened on 4 x 4-in. square mesh and retained on 1 1/2 x 1 1/2 in. These mills would grind material with a top size of 8 to 10 mesh. Tertiary pebble mills could be charged with screened rock that would pass 3 x 3 in. and be retained on 1 x 1 in. and grind 35 to 48 mesh sands. Principles of Operation Media Size To operate a pebble mill efficiently it is imperative that great care be taken in screening the rock feed that is fed into the mill to round up into a range of pebble sizes that are suitable for grinding of the sands that are also being fed into the mill, having regard to the top size and the average size of such sands. Since most mill operators know from experience the correct size of steel balls to be used in a mill to handle different sizes of sand feed, it is convenient to relate the pebble size to a ball size. If the correct ball size is not known, it is relatively simple to conduct batch laboratory tests in 12- to 18-in. laboratory mills to determine the optimum size of steel ball. Also, Bond and others have published formulas for calculating the optimum ball size. When the correct size of the individual steel ball is known, then a pebble of the same weight will perform the same grinding if used in the proper size of mill and under the proper conditions, which will be defined later. This media size, as defined by the average weight of the individual pieces, is most important. The following tests will serve to clarify this point. Laboratory Tests Using Steel Balls or Pebbles. Several labora¬tory grinds were made to compare a given size and weight of steel ball with a pebble of the same weight but larger size, and of the same size but lighter weight. The following loads were used in the 12-in. laboratory mill: All 1 1/4-in. steel balls weighing 122 ± 10 g. All 1 3/4-in. pebbles weighing 123 ± 5 g. All 1 1/4-in. pebbles weighing 45 ± 3 g. To equalize capacity, the test with the steel balls was run only 34% as long as the tests with pebbles. In each case, 2,500 g of primary sands were ground at the optimum gravity for the media being tested at a speed 78 1/2 % of critical speed, and for 170, 340, and 680 revolu¬tions for the steel balls, and 500, 1,000, and 2,000 revolutions for the pebbles. The first test is summarized in Table 74 by showing the 80% passing size in microns for the feed and for each of the grinds. Two other tests are summarized in Table 75 showing the screen analyses

Jan 1, 1985