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By Alan Zeni

Blind shaft drilling equipment has not changed substantially since the 1970s. Since it resides in a niche market between conventional shaft sinking and raise boring, the relatively small demand has resulted in some stagnation of development. In the 1990s the technology found a larger foothold in foundation and deep piling work for civil construction. WIRTH GmbH from Erkelenz, Germany has been servicing this market for many years with pile top drilling rigs and tools. Frontier-Kemper Constructors, Inc. recently entered into an agreement with WIRTH to develop a new generation of large diameter blind drilling equipment more specifically aimed at mining needs. The, recently completed, DHI-240 is the first rig to apply the most recent technology. This machine made its commercial debut in the fall of 2008. A description of the technology and a short case study of the first project will be described

By Michael G. Zabetakis, Israel Liebman, Henry E. Perlee

Flammability studies conducted to date on stratified methane - air and pentane - air mixtures in air indicate that location and extent of a flammable layer are affected by molecular diffusion , gas motion , and propagation of flame . In initially quiescent mixtures , the lower limit - of - flammability zone may be determined by assuming molecular diffusion alone governs mixing rate . The upper limit - of - flammability zone can be predicted in this manner only for stratified layers with small concentration gradients ( less than 0.2 volumepercent per inch ) ; where large concentration gradients are encountered ( for example , as much as 30 volume - percent per inch ) , the initial - mixture methane concentrations that yield flammable mixtures are larger than anticipated . Flames propagating through heterogeneous quiescent methane - air atmospheres in the direction of the concentration gradient exhibit flame speeds at any point that corresponds to the flame speed of the homogeneous gas mixture having the same gas composition as that of the point in the heterogeneous system . In flowing mixtures with large concentration gradients , the degree of mixing prior to ignition does not appear to affect the flame velocity and emitted radiation appreciably in a 6.5 - ft - diameter gallery .

Jan 1, 1964

By Karoly (Charles) Kocsis, Kyle A. Scalise

Understanding the climatic conditions in underground mines is necessary for efficient ventilation design, cost savings, and to ensure the health and safety of mine workers. Large volumes of ventilation and climatic data including air volume, barometric pressure, dry bulb temperature, and relative humidity were collected at active underground precious metal mines in Nevada, which allows for the determination of wet bulb temperature and other key parameters. Through the utilization of neural networks, the wet bulb temperature at the bottom of the intake shafts is predicted, while taking into account the “thermal flywheel effect” (TFE). Wet bulb temperature is one of the most important climatic parameters to model and understand because it significantly affects the work conditions and the cooling capacity of the ventilating air. The accurate prediction of the dry bulb and the wet bulb temperatures at the bottom of intake shafts is critical when assessing the climatic conditions in future underground mines and deciding onwhether a cooling system is needed to assure adequate working conditions throughout themine. By utilizing accurate predictions of wet bulb temperatures and other climatic parameters, mine personnel will be safer as reported by Bluhm et al. (2003), and a more accurate ventilation design can be achieved resulting in major cost savings for underground mines.

Mar 3, 2021

By S M. C Verryn, K Deysel

In heavy mineral sands ilmenite, rutile and zircon are present in economic concentrations. These minerals are important sources of Fe, Ti and Zr for various industrial uses. The beach sands of northern KwaZulu-Natal, along the Zululand Indian Ocean coastline in South Africa, have been mined for 30 years. The various processes involved in mining and processing these minerals have been controlled by time-consuming and labour-intensive analysis methods which, in some cases, can be replaced by X-ray powder diffraction (XRD)-Rietveld analysis. Heavy minerals are concentrated in the plant to produce a heavy mineral concentrate (HMC). The heavy mineral extraction plant is controlled and informed of the proportions of minerals in the HMC according to their magnetic susceptibility. The different fractions are called magnetic, paramagnetic and non-magnetic fractions and quality control is based on their magnetic fraction weights combined with X-ray fluorescence (XRF) elemental analysis. This current quality control process is very time consuming and labour intensive. We have aimed to show that we can, by XRD, identify and quantify all the minerals in the HMC and also in the magnetic separations. These results are then compared with results obtained by Mineral Liberation Analyser (MLA) as well as with the results from the physical separation. It was possible to detect all minerals present in the material by XRD and use the Rietveld method for a full bulk mineral quantification. A maximum of 15 minerals can be included in the Rietveld refinement due to computational restraints. Major minerals detected are rutile, ilmenite and zircon and minor minerals are quartz, epidote, anatase, amphibole, monazite, magnetite, titanite and augite. The XRD and MLA results correlate relatively well and detailed results will be presented for HMC and magnetic fraction samples. Small differences were observed for some minerals due to the different ways in which minerals are detected or due to preparation methods within each method. Rutile seems to be more abundant in the XRD analysis and the detection of Anatase is possible by XRD, but not MLA. Leucoxene (consisting of mainly microcrystalline anatase, rutile and titanite is not detectable by XRD since it is not a mineral, but ilmenite alteration phases are observed instead. There is, however, a correlation between anatase, titanite and rutile detected by XRD and leucoxene analysed by MLA.

Jan 1, 2008

By E. Esmaeilzadeh

There arc 8 primary ball mills in the concentrator plant and their task is material crushing. At the time of balls charge to mills, a lot of noise is produced. This noise would affect all grinding areas. The aim of this research was a reduction about 16 dB at source sound power level (SWL was 129.1 dB). With considering two noise control principles, noise producer sources and propagation paths were specified. For removing and reducing of noise producer sources, rubbers with a thickness of 5-20 mm were used on the all inside surfaces of charge box, ball transferring box and exit canal. For increasing transmission loss and damping coefficient of charge box body, rubber with thickness of 1 cm at outside surface of box was used. For reduction of noise propagation a secondary metal cover was designed and made. This cover includes two parts and was set around the charge box. Glass wool (75 kg/m3) with 10 cm thickness was used as acoustical absorbent between free space of two body's layers and from secondary cover side. The of the rest space was left empty. After installing of secondary cover on the ball charge box no.4, SWL showed a reduction about 16 dB. For studying of control system effect on noise reduction at points far from of charge place, measuring results at these points showed the maximum reduction 12.1 dB and the minimum 11.8 dB.

Jan 1, 2007

By R. Khanna, M. Dubikova

A variety of carbonaceous reducing materials are being used for the production of ferroalloys. Reductant materials include not just carbons such as coke, coal, char but also bio-carbons in form of charcoal produced from different types of wood. This paper discusses critical properties and reactions of carbonaceous materials and their significance in dictating reductant performance. Main considerations include among others, gasification of reductant, dissolution of carbon into molten metal, and direct reduction of slag by solid carbon. These are discussed in relation to material properties such as structural order, mineral matter content, composition, physical characteristics etc. A brief review of both conventional and novel approaches towards characterisation of carbonaceous materials has been included. Due to complexity and heterogeneity of carbonaceous reducing materials and a wide range of process requirements, the selection of the most appropriate reducing agent will depend on a number of considerations such as the availability of raw material and associated costs, product and process requirements and environmental aspects of waste gas emissions.

Jan 1, 2004

By Marshall D. Himes

The Pima mine, 39,000 ton per day copper mine, is located 17 miles south of Tucson, Arizona. The mine is in a sequence of Paelozoic and Mesozoic sediments striking east-northeast and dipping southeasterly intruded by a Tertiary quartz monzonite porphyry. The Paleozoic Permian(?) dolo¬mites, limestones, and sandstones have been altered to talc-silicate skarn, marble and quartzite. The overlying Mesozoic Triassic(?) clastic sediments have undergone some recrystallization and hydro¬thermal alteration. The clastic metasediments are divided into three distinct lithologic units, lithic arenite, arkose, and inter¬bedded and overlying black argillite. Three types of hydrothermal alteration in the clastic rocks are propylitic alteration (epidote, chlorite and quartz, with or without sericite), quartz-sericite alteration, and potassic alteration (K-feldspar and quartz, with or without sericite and chlorite). The porphyry has undergone potassic alteration. Dominant structures are an east-west post mineral fault in the western part of the pit, and strong low angle faulting and shearing which truncates the ore body at depth. Two joint sets, one parallel to bedding and the other at right angles to bedding are prominent in the mine. Faulting is in two dominant directions, one striking northwest, dipping northeast and the other striking northeast, dipping northwest. Mineralization is predominantly disseminated pyrite and chalcopyrite, and molybdenite-quartz veinlets. There are lens important amounts of magnetite, hematite, sphalerite, galena, tennantite and bornite. Mineralization is believed to be both structurally and chemically controlled.

Jan 1, 1972

By D. R. Tesarik, M. K. Larson, D. K. Denton, J. B. Seymour, R. L. Rains, T. J. Williams

Because backfill has occasionally collapsed into an active working area, posing a hazard to miners, engineers from the Spokane Research Laboratory of the National Institute for Occupational Safety and Health and Hecla Mining Co. installed instruments in a cemented, backfilled, stope-ramp intersection at Hecla’s Lucky Friday Mine, Mullan, ID. The purpose was to measure stress and strain changes in the backfill and reinforcing members during undercut mining. The instruments were monitored for 6 months while three successive cuts were mined below the intersection. Readings showed induced loads up to 3450 kPa (500 psi) in the backfill as stope walls converged 2.5 to 12 cm (1 to 5 in). The backfill then deformed against the top and bottom plates of the 2-m- (6-ft-) long vertical rock bolts installed as reinforcement, producing loads to 177 kN (40,000 lb)on the rock bolts. We hypothesize that a compressive zone was created in the backfill that allowed the backfill to remain stable as long as the compressive zones from adjacent rock bolts overlapped. This hypothesis is presented in graphical form. Of particular interest was the effect of loading on trusses installed to augment the vertical rock bolts and wire mesh typically installed in backfill. Data from the instruments indicate that wall closure perpendicular to the vein induced loads in truss legs parallel to the vein and in the rock bolt driven through the center of the truss, but, because they are designed to function under tension, truss legs perpendicular to the vein supplied in significant support as a result of compressional forces from wall closure. Based on this study, use of trusses was discontinued, and an alternative support system of wood beams and posts was installed as needed to ensure the safety of miners working beneath the backfill in stope-ramp intersections.

Jan 7, 2001

By Román Tejada R

El presente trabajo señala los resultados del sostenimiento de labores subterráneas a través del proceso de pre-inyecciones e inyecciones de contacto y consolidación. De acuerdo a este estudio, se encontró que la mina San Vicente contiene estructuras denominadas "Brechas de Colapso", compuestas de material suelto y deleznable que originan un problema en el laboreo minero. En este sentido, se busca un método que se adecúe a estas estructuras como el "Inyección de cemento"donde no se requiere de muchos aditivos.

Nov 8, 1984

By Irina A. Pulyaeva

Hydrogenetic Fe-Mn crusts are ubiquitous in the ocean basins and play an important role in marine mineral-deposit research because of their widespread occurrence and high concentrations of valuable and rare metals. Directed research, which started in the last quarter of the 20th century, has provided significant results, with substantial amounts of data produced on Fe-Mn crusts structure, composition, and distribution in the global ocean. Detailed study of several seamounts has given confidence about the scale of Fe-Mn crust development and their characteristics. Now, commercial interests gain more and more attention. However, at the same time some theoretical questions related to Fe-Mn crust history including controls on metal sources and concentrations have not yet been answered. Here, we present the overview of age, composition, distribution, and geological setting of hydrogenetic Fe-Mn crusts from Pacific and Atlantic seamounts and discuss the geological evolution and conditions that led to the formation of potentially economic concentrations of metals. The age, composition, and distribution of Fe-Mn crusts in the Atlantic and Pacific oceans were determined by using a Fe-Mn deposits database compiled from our original data, published papers, published databases, and other international sources. Comparative analysis of Fe-Mn deposits from the Atlantic and Pacific oceans shows that there are similarities in basic characteristics of crust formation in both oceans. Simultaneously, there are significant differences in the scale of Fe-Mn crust distribution and composition that can be explained by differences in geological settings and characteristics of the deposition environment.

Jan 1, 2011

By Izabela Diniz Duarte

Perdas térmicas decorrem do armazenamento do aço líquido em panelas. O controle da temperatura do aço líquido é condição essencial para se obter um produto final de alto nível de qualidade. Este trabalho buscou avaliar a transferência de calor a partir do aço para o refratário da panela e para a camada de escória, utilizando um modelo numérico (Computational Fluid Dynamics). As perdas de calor dão origem a correntes de convecção dentro do banho e a consequência típica desse fenômeno é a estratificação térmica. Foi avaliada a influência de parâmetros tais como espessura da camada de escória, tipo de revestimento refratário e a temperatura de encharque. As perdas térmicas para o refratário lateral e do fundo da panela se mostraram maiores que as perdas térmicas para a camada de escória. Nas condições estudadas a espessura de camada de escória não apresenta influência significativa sobre as perdas, ao contrário do tipo de refratário e temperatura de encharque.

Aug 16, 2017

Andy H. Vassiliou – Introduction Ladies and gentlemen,.good evening and welcome to the Paul F. Kerr Memorial Symposium dinner. I believe we are all here to honor the memory of a great teacher and advisor, and a true friend -- Professor Kerr, or Dr. Kerr, or to most of us who were his students, "Pappy" Kerr. Most of us who attended the Paul F. Kerr Memorial Symposium earlier today at the New York Hilton had the opportunity to read, or listen to, or discuss scientific papers presented in honor of Dr. Kerr. In addition, all of the speakers had the opportunity to reminisce about their association with Dr. Kerr, who was credited as having contributed substantially to their graduate education and the development of their careers. Dr. Kerr's two daughters -- Ruth and Nancy -- who attended the Symposium at the Hilton are also here with us tonight. I think we should keep tonight's festivities on an informal basis since we are all old friends or former students of Dr. Kerr. Some of us may want to say a few words about our experiences at the time we were students at Columbia, and hopefully include some anecdotes relating to Professor Kerr. I would like to invite everyone to do so, but first we have arranged for two of us to be the lead speakers, whom I will introduce in a few moments.

Jan 1, 1985

What has happened to the basic coal industry during the past year? Has it been a better year for coal than 1961? What striking new developments have occurred in mining, preparation and utilization? Are there positive signs that production will go up? What has happened in the utility market, the metallurgical market and the export market? Is the controversial question of coal transportation nearer to a solution? What progress has research made for coal during the year? The answer to these and many other important questions are offered in the following review of the Coal Industry-1962.

Jan 2, 1963

By Robert H. King

An analysis of low-voltage shielded trailing cables is performed to develop a method for calculating the differences in operating costs between shielded and non-shielded cables. Two types of shield constructions are considered (SH-C and SH-D) for the three major underground coal mining methods. Data from laboratory and in-mine testing are presented in addition to an analysis of safety benefits. Recommendations for further investigations are proposed. Example calculations with sensitivity analyses are presented. Recommendations for further investigation include costs impact considerations.

Jan 1, 1980

By William M. Camp

An extensive design phase testing program in which 10 piles and 2 drilled shafts were instrumented and statically load tested to failure, was undertaken to establish relevant foundation design parameters for two bridges in Charleston, South Carolina, USA. The bearing strata at both test sites was a calcareous, overconsolidated, firm to very stiff, slightly sandy, clayey silt (locally known as Cooper Marl). Confirmation dynamic and static loading tests on production piles indicated only 50 to 60% of anticipated pile capacities; including consideration of time effects. Supplemental subsurface explorations did not reveal any significant variations in marl properties. Ultimately, 22 static and 75 dynamic tests were performed on production piles for a 2 mile section of the project. Pore water pressures in the marl were also monitored during and after pile driving. A study of unit skin friction in the marl as a function of time revealed two distinct trends: piles driven in areas with a sand overlaying the marl gained more capacity significantly faster than those driven through a clay or silt. Although the insitu marl properties are apparently similar across the site, the material immediately above the marl evidently has a large effect on pile capacity. This effect of overburden on pile capacity was verified by testing piles over a period of two years.

Jan 1, 1993

By R. P. Gardner

Results of work on the development of a phenomenological model for the continuous flotation process using short-lived radioactive tracer techniques for the determination of model parameters is reported. A model that incorporates a flotation first-order rate constant and a residence time distribution based on the cross flow finite state (CFFS) model has been derived and investigated. The short-lived radioactive tracer techniques necessary to extract the model parameters have been developed as have the necessary nonlinear search methods. The conditions under which all the model parameters can be extracted from a single radiotracer experiment are detailed and illustrated. A microcomputer-based unique data logging system has been designed, assembled, and implemented for collecting the necessary transient radiotracer data. Results are reported for a continuous laboratory flotation system for the two systems mica ore and coal.

Jan 1, 1985

By E. W. Pehrson, J. W. Furness

THE method of presentation in the accompanying charts is based upon the well-known formula: production plus imports minus exports equals apparent consumption. Thus for each area for which data are shown there are two columns of equal height, one representing production plus imports, the other apparent consumption plus exports. Usually no account has been taken of increases or decreases in stocks. All columns and flow lines are drawn to a linear scale which is supplemented by figures showing the actual quantities represented. Owing to the small area occupied by the European countries on the world map it was impossible to show the details of the trade of the individual countries. All of Europe (except Russia), therefore, is shown as a single unit. However, statistics of production and consumption indicated elsewhere on the charts show the degree of self-sufficiency enjoyed by the major European powers.

Jan 1, 1936

By Antoni Kidybinski

Mechanical properties of the nether roof rocks play a substantial role in maintenance of mine openings. On longwall faces too low strength is a cause of rock- falls which bring about delays in face advance, and hazard to the miners, while too high strength may result in over hanging of roof rocks in gobs, sometimes over a large area, initiating dynamic periodical pressures. Hence, efforts have been undertaken to develop methods of caveability evaluation for proper selection of roof control system and of longwall-face supports. DESCRIPTIVE CLASSIFICATIONS One of the basic qualitative criteria for classification of roof types with different caveability is the criterion of geological structure. The 26-stages scale worked out for conditions of Donetz Coalfield. USSR, by E.T. Proyavkin et al.(l) may be an example of that type of classification. This scale is presented graphically in Fig. 1. The first types of roofs according to this scale are easily caving immediately after exposure of a small surface, while the last be- long to the strongest roofs which over-hang in waste and require artificial fracturing. Observations and measurements carried out also in Donetz mines lead W.T. Davidyanc (2) to following classification: Class I - weak and densely bedded rocks very easily caving after each advance of supports. The roof subsidence over a waste edge is: [ ] where: h is roof subsidence, m M is thickness of the seam being mined, m S is width of an opening, m Class II - medium caveability and small main roof movements [ ] Class III - hardly breakable rocks, thicker beds with higher strength: caving by large blocks. After exposure of larger roof surfaces the movements of the main roof strata may affect an opening. [ ] Class IV - very hardly breakable rocks - thick, monolithic sandstones: they cause considerable difficulties in working with caving. [ ] Class V - thin-bedded rocks featured by the capacity of a smooth subsidence without disintegration into loose blocks with keeping contact between the particular blocks in a bed. In this classification mining with full roof caving is recommended for class I-III and only exceptionally for class IV. In many coal fields, qualitative and quantitative observations of roof rocks behavior have been carried out: as a result relations and characteristics found in certain mines are given. As an example, Kizielov Coalfield data in the USSR given by B.S. Vinokur (3) may be mentioned. In working thin pitching seams man- less system of undercutting the seam by means of rope saw is used there. The author separated four classes of roofs and gave, for every class, the empirical relation between the extracted area and the time of roof stability. The size of the stable area of exposed roof for particular classes is as follows: 120 to 150 m2 (1291 to 1614 ft2 ), 170 to 200 m2 (1829 to 2152 ft2), 250 to 280 m2 (2690 to 3013 ft2 ) and 350 to 400 m2 (3766 to 4300 ft ). For Kuznetz coalfield in the USSR the following classification of roofs was developed at the WIMI Institute in Leningrad: Class I - completely unstable: impact strength in- dex of rock f = up to 1.5 - the strip of roof 1.4 to 2.0 m (4.59 to 6.56 it) wide, exposed after shearer's cut holds stable during the time not longer than 5 min. Class II - low stability: f = 1.5 to 2.5 - the strip of roof 1.4 to 2.0 m (4.59 to 6.56 ft) wide and up to 4.0 m (13 it) long, exposed after shearer's cut holds stable during 5 to 20 min. Class III - medium stability: f = 2.51 to 3.5 - the strip of roof 1.4 to 2.0 m (4.59 to 6.56 it) wide and 4.0 to 25 m (13 to 82 ft) long, exposed after shearer's cut holds stable from 20 min. to 2 hours. Class IV - stable: f = 3.51 to 4.5 - the strip of roof 1.4 to 2.0 m (4.59 to 6.56 ft) wide and 25 m (82 it) long exposed after shearer's cut is stable during the time longer than 2 hours. Class V - entirely stable: f = over 4.5

Jan 1, 1982

By Manara Nogueira Quintino

No processo de produção de ferro e aço, uma grande quantidade de resíduos é gerada. Esses resíduos e coprodutos contêm consideráveis quantidades de ferro e carbono, que podem ser reaproveitados, ajudando a reduzir a poluição ambiental que pode surgir devido ao descarte inadequado. Este trabalho tem como objetivo avaliar o potencial energético contido em um resíduo siderúrgico, quando misturado à biomassa. Por meio de análise termogravimétrica e cinética (método Coats – Redfern) amostras com diferentes proporções de resíduo metalúrgico, biomassa de capim elefante e carvão vegetal foram estudadas. A amostra com menor tamanho de grão e maior quantidade de resíduo foi a que apresentou melhor perda de massa ao final do experimento, apesar de não ter apresentado o melhor resultado antes da faixa isotérmica do experimento. Contudo, a quantidade de capim elefante se mostrou relevante na performance da queima.

Oct 30, 2017

By Bunting S. Crocker

EBBLE grinding at Lake Shore is not a temporary BlE wartime substitute. The tube milling plant, with a 1000 ton per day capacity, grinds a hard siliceous ore to 90 pct — 325 mesh. The plant, prior to using pebbles, was consuming 4.3 lb of 11/4-in. grinding balls per ton of ore, which amounted to 785 tons of balls per year. At September 1951 prices, $132.60 per ton, this steel cost amounted to $104,400 per year, or $0.285 per ton milled. By the Lake Shore method of substituting screened rock for this steel, all of this cost is saved. This is one of the major economies in Lake Shore mill practice. Regardless of the ultimate price of grinding balls, a change back to steel balls is not considered. The present pebble plant is more flexible than a steel ball plant and equally efficient. For example, if it is desired to change the size of grinding media, the pebble charge in any mill can be changed completely in 4 to 5 days, as against 76 days to change completely a 1-in. steel ball charge. To change pebble size it is necessary only to change two sets of screens and clean out the rock feed storage bin. This will take 8 to 10 days as against 3 to 4 months to clean out the customary supply of grinding balls kept on hand. Also it has not always been possible to purchase all desired sizes of balls at any price. In an analysis of savings effected by the use of the pebble mills, the flexibility of the Lake Shore grinding plant should be discussed, as it has a direct bearing on these savings. The plant has always used several units to handle tonnage rather than sending all the tonnage through a single unit. This principle may result in using small diameter mills, but no objection to that is seen. At no time has any advantage been found in cost per ton ground in large diameter mills. Both the capacity and the power of any mill varies as the diameter raised to the 2.6th power. Consequently large or small mills are equally efficient, and a plant, should be designed to use as many units or combination of units as is consistent with reasonable operating practice. Mills under 5 ft diam are harder to reline, etc. To define the case at Lake Shore, 2600 tons per day formerly were milled in seven 7x6-ft ball mills and twelve 5x16-ft (and two 6x16-ft) tube mills.* This gave an excellent test plant and an extremely efficient one. In this plant the ratio of ball mills to tube mills was 1:2. When the much cheaper pebble mills were substituted for the tube mills, this ratio was changed to one ball mill to four pebble mills to take the greatest possible advantage of the cheaper operating mill, i.e., the pebble mill. This flexibility without loss in efficiency has been an important item in the cost savings. It is interesting to note that the use of pebbles for fine grinding was proved first in the laboratory in a 12-in. ball mill. In fact, since 1934 all testing on — 8 mesh material has been done in this 12-in. mill. Scope of the Tests A paper on fine grinding at Lake Shore Mines was published in July 1940.' This paper covered 7 years of intensive research on fine grinding as well as sizing methods and equipment, plant scale grinding tests on 5x16-ft tube mills with and without grate discharges both with 1-in. and -in. balls, the use of laboratory mills to evaluate plant changes, and several reports on classifiers and classification. In the following July the addendum report2 was added in which the idea of series-circuit grinding was introduced, and the results of running five stages of tube mills and bowl classifiers were shown. Since 1940, the ball milling end of the plant has been altered extensively as a result of tests on the use of 3-in. rods in 7x6-ft mills and the use of the Tyler repulping screen with from 7 to 14 mesh screens. These tests are lengthy and may be covered in a separate report later. The scope of this report is confined to ore ground by rod milling and ball milling until it passed through an 8 mesh Tyler Ty-rod screen. The —8 mesh screen undersize then was pumped to a primary bowl classifier in open circuit and the sands from the bowl sent to the primary pebble mills, see Fig. 1. In the pebble mill circuit the ore is ground to 90 pct —325 mesh (24 pct + 28 microns). In studying the flowsheet, attention should be paid to the efficiency of the classification equipment used. The Tyler repulping screen is an efficient machine on the 8 to 10 mesh separation, and the bowl classifier is equally efficient at the 325 mesh separation. Efficient classification is a necessity for series-circuit stage grinding. The ore is hard siliceous porphyry, 60 pct SiO,, 80 pct insoluble. Its grindability at different meshes has been shown near the top of the list in F. C. Bond's grindability tests.' Lake Shore is not shown, but an adjacent mine with identical ore, Wright-Hargreaves, is. Reasons for the Changeover Since 1936 grinding balls have been rising steadily in price with no sign of stopping. For a mill that used 4.5 to 5.2 lb of grinding balls for every ton of ore ground this rise represented an alarming increase in grinding costs. In many cases the quality of the grinding balls fell off as the scrap steel became more difficult to obtain. The ratio of tube mills to ball mills increased with the use of the Tyler repulping screen in the ball mill circuit. Originally only 3.0 lb of balls per ton were used in the tube mills, and this

Jan 1, 1953