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By Emmanuel John M. Carranza, Martin Hale

Abstract- An application of logistic regression to mapping of gold potential in the Baguio district of the Philippines is described. Categorical map data such as lithologic units and proximity classes of curvi-linear features, based on spatial association analyses, are quantified systematically and used as independent variables in logistic regression to predict the probability for presence or absence of gold mineralization. Regression experiments to compare between using all independent variables that are associated spatially with the response variable and using only statistically significant independent variables are performed. The results of the regression experiments are similar; however, the use of all independent variables produces slightly optimistic results but better prediction rates for the known gold deposits in the test district. At least 68% of the ‘model’ large-scale gold deposits and at least 76% of the ‘validation’ small-scale gold deposits were predicted correctly. The predicted geologically favorable zones are also similar to delineated geochemically anomalous zones. The technique presented using logistic regression as a data integration tool is effective for geologically constrained technique of mapping mineral potential.

Jan 1, 2001

By J. D. Stewart

One of the first important questions that should be settled on the opening of a mine or the development of a new deep is the system of haulage to be used, since the layout of the haulage-ways at the pit bottom, or, in case of a slope mine, the arrangement of the surface works, depends largely upon the type selected. Should it later become desirable to install a system of haulage of a type other than that for which the mine roadways were originally laid out, considerable difficulty would likely be encountered in its installation and with the result that it would in all probability be most unsatisfactory and expensive in its operations. It therefore becomes very necessary that full consideration be given the haulage problem at an early date in order that the type best suited to the mine conditions and the required outputs can be intelligently planned for. It is sometimes found, however, that no matter how well the mining engineer explores the seam to be developed, or how accurately he anticipates its eccentricities, there will still remain some unrevealed or unsuspected conditions, or shall we say obstacles, placed by Nature to prevent the too easy winning of her treasures and incidentally to frustrate or modify the application of the plan so painstakingly laid down on paper, and to adversely affect the cost estimate.

Jan 1, 1928

By C. J. Mwenifumbo

Preservation of groundwater quality is a major environmental concern. The disposal of hazardous material from urban waste dumps, milling wastes in tailings ponds, and industrial wastes in injection wells may contaminate potential sources of potable groundwater. The containment or migration of contaminants in the subsurface depends on bedrock characteristics and type of overlying sediments. Fractured bedrock and permeable sediments that may provide pathways for the transport of potentially harmful contaminants should be avoided when selecting a disposal site. The role of borehole geophysics in hazardous waste-site investigations is reviewed in three categories: (I) site selection; (2) determination of the extent of contamination at existing sites; and (3) long-term monitoring of migration of contaminants. In the first category, borehole geophysical techniques developed for lithology and fracture identification, and for porosity determination, provide some of the necessary information for understanding the subsurface hydrogeology. Lithological information can be obtained from gamma-ray, electrical resistivity, induction, magnetic susceptibility, and sometimes induced polarization logs; fracture detection and characterization can be accomplished with the use of caliper, density, resistivity, temperature, and acoustic televiewer logs. Estimates of porosity are derived primarily from neutron-neutron, resistivity, density, and soniclogs. Borehole geophysical techniques used in water quality determinations include self-potential, fluid resistivity, Eh (redox potentials) and pH measurements, and these can be used to map the extent of contamination at an existing waste site. Cross borehole and borehole-to-surface electrical methods can also be used in this application. Long-term monitoring of contaminant migration in boreholes can be accomplished with techniques that respond to changes in electrical conductivity, temperature and fluid chemistry caused by the presence of contaminants.

Jan 1, 1993

By Russell Spry

THE cyanide plant of the Cariboo Gold Quartz Mining Company, Limited, at Wells, B.C:, in the Barkerville section of the Cariboo mining division, was erected during the latter part of 1932 and placed in operation in January, 1933. The initial capacity of the plant was about 50 tons daily. This was increased to 100 tons at the end of 1934 and stepped-up to 150 tons in September, 1935. It is expected that further plant extensions, for milling a total of 200 tons daily, will be required during the early part of 1936. Recent additions to the mill have been so planned as to permit ready expansion to 275 tons daily when underground development warrants the milling of this tonnage. It is the purpose of this paper to outline briefly the steps taken to bring the plant to its present capacity and to comment on possibilities for further plant expansion. The gold in Cariboo Gold Quartz ore occurs as free gold in the quartz and on pyrite surfaces. Some of it is very finely divided, specks l/10,000 inch having been observed microscopically, according to Dr. G. Hanson. In addition to pyrite, which comprises probably 15 to 20 per cent of the mill feed, galena and sphalerite are present, with small amounts of arseno-pyrite and chalcopyrite. Sulphobismuthites-chiviatite, galenobismutite, cosalite, etc.-occur more or less frequently in certain sections of the mine. Following tests on Cariboo ore by the Mines Branch, Department of Mines, at Ottawa, treatment of the ore by straigh t cyanidation was recommended and a 50-ton plant employing continuous counter-current decantation was erected. This plant was built as a pilot mill, the intention being to erect a larger mill after metallurgical treatment had proved successful over an extended period. This original installation, however, has been enlarged and retained as an operating unit, additional equipment being added to it as capacity increases have been required.

Jan 1, 1936

By Fernando Canales

In the last decade a great dealt of attention has been paid to the chemistry of polynuclear gold derivatives with a central heteroatom, not only from the experimental and structural but also from the theoretical standpoint (1). Interesting hypercoordinated species of the type [C(AuPR3)5]+, [C(AuPR3)6]2+, [N(AuPR3)5]2+, [P(AuPR3)5]2+ and [P(AuPR3)6]3+ have been described (2), apart from other complexes with more conventional stoichiometry, and all have in common the presence of gold-gold interactions of ca. 3 Å. Usually, the chemistry of the first row elements of the p-block is known to follow classical rules of bonding, and only in cases of extreme electron-deficiency has the traditional electron count to be reconsidered to account for special types of molecular or solid state structures. Many of these heteroatomcentered complexes are electron-deficient and the gold-gold interactions provide a significant contribution to their stability. The attraction between these closed-shell systems has recently captured the attention in inorganic and organometallic chemistry (1). P. Pyykkö has summarized the available theoretical and experimental evidence of intra- and inter-molecular interactions between heavy metal atoms, such as Au(I) (3). In general, when heavy metals are involved in such interaction, the closed-shell attractions are of metallophilic or van der Waals type, as has been found for d8, d10 coinage-metals, and d10s2 centers. Experimental evidence for the aurophilic Au(I)???Au(I) attraction is available from crystallography, NMR and Raman spectroscopies, and optical spectroscopic measurements of the interaction strength (4). It is manifested in molecular conformations with relatively close Au???Au interactions of ca. 3 Å, whose strength has been estimated at ca. 25-50 kJ/mol (5). These attractions are weaker than most covalent or ionic bonds, but stronger than other van der Waals bonds and comparable in strength to typical hydrogen bonds. The phenomenon cannot be satisfactorily explained by classical theories such as hybridization (3). From a theoretical point of view, the attractions close to equilibrium distances are caused by dispersive interactions together with relativistic effects (6). The attractive contributions are dominated by pair-excitations from the heavy metal, such as gold, strengthened by relativistic effects. In all the cases where there are inter- or intramolecular metal-metal attractions, the correlation effects are essential; if they are omitted, at the Hartree-Fock (HF) level, repulsive interactions are obtained (6). Hence, it is necessary to use at least second-order Møller-Plesset level (MP2) methods for the dispersive forces, which are included in the electronic correlation effects (7).

Oct 1, 2003

The use of roof bolting gave promise of substantial economic benefits to Continuous Miner operations at Michel Colliery. As the first step in the adoption of bolting, 142 anchorage tests, employing six types of anchor shells, were made in three mines. Significant differences were found in the anchorage capabilities of the various shells and the various roof rocks. Introduction S PORADIC attempts at roof bolting had been made at Michel Colliery over a period of ten years. Results were indifferent or inconclusive. A concerted effort was not made, because a compelling need for bolting did not exist. By 1962, however, the advent of Continuous Miners had brought a new significance to roof bolting and, at the same time, the increasing price and scarcity of mine timber made bolting materials more competitive in cost. A fresh appraisal of bolting, therefore, became justified and necessary. The potential benefits of roof bolting at Michel are several.

Jan 1, 1963

By Robert M. Hopkins, Craig C. Monahan

A corrosion survey was performed on ten selected H-piles which support a reinforced concrete wharf located at Trepassey, Newfoundland. Approximately 900 thickness measurements were taken at twelve different locations in each off our zones on the steel piles: the splash (atmospheric) zone, the intertidal zone, the low tide zone and the submerged zone. Results show that over the 2o-year life of the wharf, corrosion has been most severe in the splash, low tide and submerged zones with average corrosion rates for the flanges of 6.95, 9.45 and 6.86 mils per year (mpy}, respectively, and average corrosion rates for the webs of 5.13, 4.12 and 3.45 mpy, respectively. Minimal attack was found in the intertidal zone where the average corrosion rate was only 1.66 mpy on the flanges and 1.37 mpy on the webs. Such a pattern of attack is frequently observed f or steel piles partly submerged in seawater. The abnormally low corrosion rate associated with the intertidal zone is generally explained as being due to cathodic protection induced by the high corrosion rate in the adjacent low tide zone. Analysis of the corrosion data obtained for the Trepassey wharf would suggest that this explanation might be incorrect. An alternate hypothesis has been brought forward which better accounts for the extremely low corrosion rate observed in the intertidal zones of these piles.

Jan 1, 1990

By C. Bourgoin

"The limited development and application of new technologies to narrow-vein mining is currently impeding progress in the industry to a point where productivity, working conditions and operating costs have reached a critical stage in many Canadian mines.In an effort to foster the emergence and application of new technologies, Université Laval, together with other organizations, has carried out a R&D project focussed on the need of identifying new development pos5ibilities and of facilitating the transfer of technology in matters related to underground mining methods and machinery.The first phase of the study consisted of an in-depth assessment of current practices and problems in narrow-vein mining. The second phase was a state-of-the-art survey of mining methods and equipment, together with proposed modifications and innovations. The third phase of the project concerns the experimental development as well as the principal application of various proposed technologies.This paper presents an overview of the organizational, technical and strategic considerations which guided the University group and it serves as well to introduce a newly-published brochure, entitled ""Underground Mining Techniques of Vein Deposits"", which summarizes such work to date. More specifically, a perspective of technological developments applicable to narrow-vein mining is presented."

Jan 1, 1994

By L. Xu, A. Vazirnegad, R. Bhaskar

"Respirable dust samples are collected by the U.S. Mine Safety and Health Administration (MSHA) and mine operators as part of the requirements of the Federal Coal Mine Health and Safety Act of 1969. About 1.85 million respirable dust samples collected by MSHA were analyzed for the period 197/ to 1990 f or the states of Alabama, Colorado, Illinois, Kentucky, Ohio, Pennsylvania, Utah, Virginia, and West Virginia. Trends in coal dust exposure of continuous miner operators, helpers, and roof bolters were analyzed. The reported production in each of the nine states was analyzed. The relationship between average dust exposure and coal production was evaluated. In addition, the differences between mine operator and MSHA inspector collected data were examined for each state. The average exposure levels for the three occupations - miner operator, miner helper, and roof bolter were also compared.The data indicate that the dust exposure levels have significantly dropped over the last twenty years, while at the same time production has increased, attesting to the ability of the U.S. mining industry to adapt successfully to the stringent dust control regulations. In terms of coal production, it appears that some states, such as Utah, Illinois, and Kentucky, were able to increase their production significantly relative to others, such as Pennsylvania and Virginia. The relationship between dust concentration and coal production is, at best, tenuous. 11 implies that existing dust control technology has assisted several mine operators to meet dust standards, while, at the same time, enabling mines to produce more coal than before. Comparison of data collected by MSHA and mine operators indicate that MSHA inspectors recorded higher dust levels and lower shift production relative to mine operators."

Jan 1, 1993

By D. S. Robertson, G. E. Nutter, T. P. Podolsky

"IntroductionBasal Proterozoic quartz-pebble conglomerates are found both west of Elliot Lake, in Duncan, Jarvis and Aweres Townships, north of Sault Ste. Marie and to the east and northeast of Sudbury, Ontario. Locally, while well packed and pyrite-rich, they carry little to no uranium. Locally, they carry gold. These features generally are ascribed to variations in the source areas of the rocks.In two other areas of Canada, similar basal Proterozoic units contain quartz-pebble conglomerate beds with uranium.Sakami LakeLocation and HistoryThe Sakarni Lake uranium deposit is found in a narrow, northeast-trending, Archean greenstone belt located 95 miles southeast of Fort George, on James Bay, and 240 miles north of Matagami, Quebec (Fig. 1).The deposit was detected by Inco in 1971, during an airborne EM survey which carried a newly developed Inco spectrometer system flown at 450 feet. Ground investigation, in 1972, resulted in the discovery and sampling of quartz-pebble conglomerate which yielded assays as high as 0.19% U3O8.Extensive field exploration was conducted between 1972 and 1975. Sixty-five diamond drill holes, to a total of 13,969 metres, served to define the extent and economic potential of the conglomerates which extend over a total strike length of about eight kilometres."

Jan 1, 1986

By F. Charette

The paper reviews the requirements of the instrumentation required to monitor mining excavations in a high stress environment, and the development plan to instrument the D-Bolt in such a fashion as to be able to follow the behaviour of the bolts in relationship to the ground movements and support performance. Recent field observations of D-Bolts under static and dynamic conditions are also presented. Also discussed is the assumed mechanical behaviour of the D-Bolt. D-Bolts are used in two (2) Canadian underground mines operating in high stress environments and tested in three (3) other operations.

Jun 1, 2012

By A. R. Kinkel JR.

"The Buffalo Ankerite mine is in the southern part of the Porcupine gold district of northern Ontario. The mine has a history of exploration and development reaching back to 1911, and has been operated continuously since 1932. From 1932 to 1942 a total of 510,874 oz. gold was produced from the mine. In 1941 the production was 71,654 oz. from 448,621 tons of ore.With few exceptions, the ore-bearing veins did not crop out. They were discovered during the prospecting of the prominent outcrops of barren or low-grade quartz-carbonate breccia near the veins.GEOLOGICAL SETTINGThe Porcupine gold district lies in an area of folded and sheared Precambrian lavas, sediments, and intrusives. The Buffalo Ankerite mine is in the southern part of a large fold. The structure appears to be a continuation of the Vipond cross anticline and accompanying syncline described in earlier papers (1, 2). The orebodies lie along both flanks of the synclinal part of the fold, and are known as the north and south vein systems, respectively. No mineable veins have been found on the axis of the syncline (Fig. 1).The rocks found in the mine area are pillow lava, dacite, conglomerate, quartz porphyry, tuff, and fragmental lava. The tuff and fragmental lava are now largely altered to chlorite schist. Productive veins have been found only in pillow lava, dacite, and conglomerate, but a minor amount of gold occurs in some of the quartz porphyry. The Porcupine Creek fault, a zone of intense shearing over 1,000 feet in width, lies immediately south of the mine. A spur fault branches to the northwest from the Porcupine Creek fault, separating the north from the south vein systems ( Fig. 1) ."

Jan 1, 1954

By Y. Dong, L. Li, L. Yu

Converter slag has become an important secondary raw material. Consequently, vanadium bearing converter slag is of great interest for the recovery of vanadium. Solidified converter slags are made up of three main mineral components: dicalcium silicate, calcium ferrite and wustite. Vanadium in modified slow cooling down converter slag of Magang is found mainly concentrated in a separate mineral phase. The mineral phase with high content of vanadium (24%, wt, V205), concentrating 83% of vanadium, can be separated from the slag for economical vanadium recovery. Some characters of the mineral phase were discussed comparing with dicalcium silicate, one of the typical mineral phases in converter slag studied well.

Jan 1, 2004

By S. I. Vobnout, J. Wm. White, J. P. Olson

"Mine management now recognizes the value of computer-based truck-shovel dispatching in open pit mines. The key tangible benefits are substantial productivity increases with a given fleet of equipment or, alternately, significant reductions in equipment required to meet production quotas. In this paper, the authors review the literature, report on efficient, optimal algorithms for haul truck dispatching in real-time, and briefly discuss methods used by various mines to evaluate productivity improvements over manual methods. Actual results obtained at thirteen mining operations around the world are also presented.IntroductionModular Mining Systems manufactures, installs, and supports its computer-based DISPATCH system at mines worldwide. DISPATCH is a large scale, computer-based mine management system which controls the dispatching of all haul trucks in any open pit mine (Fig. 1). When increased production is important, DISPATCH gets the needed production increase with fewer new or recommissioned trucks. DISPATCH also helps operations meet its blending objectives while minimizing rehandle and maintaining production. Furthermore, DISPATCH includes a powerful, eventdriven simulation subsystem. With the simulator mine management can pose ""what if' situations and see the predicted effect on production"

Jan 1, 1993

By H. E. Frimmel

Most of the gold in the Late Archean Witwatersrand Basin is of detrital origin but, in its present form, it is metamorphosed. The only relict detrital features preserved are the morphological characteristics of some placer gold particles. Regional burial metamorphism in the shallower and deeper levels of the Witwatersrand Supergroup reached temperatures around 300°C and 350°C at pressures between 2 kbar and 3 kbar, respectively, estimated from phase relations, chlorite-chloritoid exchange thermometry and chlorite thermometry. Metamorphism of the gold involved compositional homogenization of individual gold particles and partial recrystallization. Various infJ.1tration events caused the partial mobilization of the gold at slightly lower temperatures, between 240°C and 300°C. These infiltrations can be related to crustal extension during the deposition of massive carbonate sequences of the Transvaal Supergroup (around 2.5 Ga), and later to the emplacement of the Bushveld Complex, and to the Vredefort event around 2.0 Ga ago. The mobility of the gold was dependent on the sulfur fugacity prevailing in a particular horizon which, in tum, was determined by the amount of pyrite (and other sulfides) present in that horizon. With the sulfur fugacity being controlled by stratigraphy, low fluid rock ratios in a pyrite-rich horizon were most conducive for the mobilization of the gold. Thus, long-range gold mobilization in the Witwatersrand Basin can be expected to be the exception rather than the rule.

Jan 1, 1994

By Yongteng Dong, John P. Quinn, Bradford Wesstrom, Guikuan Yue

With increasingly low-grade copper ores being mined, it is difficult to handle growing concentrations of As, Sb, Bi and/or Fe in the whole flowsheet, especially during copper electrorefining. To better elucidate the effects of those impurities on the quality of cathode deposit and/or current efficiency, a detailed speciation study of the electrolyte under industrial conditions is required to quantitatively describe their distribution in the solution, thereby facilitating their effective control and removal. In the present study, a thermodynamic model of complicated iron-arsenic-copper-sulfuric acid electrolyte under industrial conditions was developed. The main species of those elements involved in the electrolyte were first identified, and their thermodynamic data were collected and critically assessed. Those data were then used to estimate the activity coefficients of species, and to calculate the equilibrium constants from 25°C to 70°C. The developed model provides a mathematical tool that is capable of quantifying the concentrations of free ions and complexes in terms of temperature and solution composition. Finally, oxidation-reduction potential measurements were employed to validate the model. INTRODUCTION In copper electrorefining, impurities like arsenic, antimony, bismuth and/or iron can partially dissolve in the electrolyte and accumulate gradually, and eventually cause problems such as bad cathode deposit (co-deposition with copper or mechanical inclusion from the floating slime), lower current efficiency, etc., which eventually increases the operating costs. The presence of impurities may substantially reduce properties of copper products such as conductivity, corrosion resistance, mechanical properties, and leads to more energy consumption (Schlesinger et al., 2011; Jafari et al., 2017). To remove these impurities, various methods have already been proposed and applied in industrial copper production, such as solvent extraction, ion exchange, chemical precipitation, absorption by activated carbon, etc. (Cunningham et al., 1997; Navarro et al., 1999; Toyabe et al., 1987). In addition, adding reducing agents such as sulfur dioxide and trivalent arsenic could also precipitate arsenic, antimony and bismuth from copper electrolyte, by changing the distribution of impurities with various valence (Braun et al., 1976; Xiao et al., 2007). The disadvantages of those methods include high energy consumption, serious contamination, lower selectivity and efficiency, and so on.

Jan 1, 2019

By R. A. Mantz, V. N. Balbyshev, A. N. Khramov, N. N. Voevodin

Corrosion protection properties of the hybrid organo-silicate coatings on aluminum substrate can be significantly improved by means of encapsulation of organic corrosion inhibitors within the coating material. In order to achieve sustained release of corrosion inhibitor and long-term corrosion protection, a secondary chemical equilibrium of either ion-exchange interaction with the coating matrix or cyclodextrin-assisted molecular encapsulation of the inhibitor has been employed. Several organic corrosion inhibitors, such as mercaptobenzothiazole, mercaptobenzimidazole, mercaptobenzimi¬dazolesulfonate, and thiosalicylic acid have been examined by a group of electrochemical methods including potentiodynamic scan, electrochemical impedance spectroscopy, and scanning vibrating electrode technique. It has been found that the formulations that contain ß-cyclodextrin demonstrate superior corrosion protection capabilities as the complexation equilibrium results in a continuing delivery of inhibitor to corrosion sites. The approach is considered a promising way for active corrosion protection of high-strength aluminum alloys when integrity of barrier coating is compromised.

Jan 1, 2004

By A. L. Rutledge, R. S. Nichols

"Waste dump development at the Fording River Operation s in British Columbia began in 1971. In 1981, several unique as well as unplanned situations provided an opportunity to study water flows through waste dumps.The key to the drainage is naturally segregated rock sizes .created by end dumping from the crest of a pile. The largest size material accumulates at the toe. Flow rates along the base of the studied dumps varied from 2,800 to 13,500 m3/day or 0.032 to 0. 156 m3/s (1.2-5.5 cfs}. Sediment concentrations in the water ranged from 1 to 1,800 mg/I.This paper describes water flows through dumps of varied lithology and maturity. The results suggest that neither long-term water flow nor contained sediments in small streams are jeopardized when crossed by end-dumped fills.IntroductionThe Fording River Operations are located in southeastern British Columbia approximately 115 km north of the United State s and 10 km west of Alberta, within the medial range of the Rocky Mountains. All production is carried out at elevations ranging between 1,600 and 2,200 m above sea level.The property is within a continental climatic zone, with temperature extremes of - 40°C in January to +35°C in July. The average annual precipitation of 335 mm of rainfall and 390 cm of snowfall feeds the numerous seasonal, small drainage courses along the Fording River valley.Since start-up in 1971, 149 million bank cubic met res of waste from truck-shovel and dragline mining have been placed in numerous dumps or ""spoils"". The dumps were developed to the point that several unique situations could be studied. During the peak run-off period of May-June 1981, water flow studies were performed on dumps of varied lithology and maturity. This paper describes the results of the study."

Jan 1, 1982

By Jorgen Elbrond, Yve Lizotte

"A mathematical procedure for determining the underground mining level layout which minimizes the total costs of excavation and haulage is described. This procedure has the advantage over previous graphical procedures of being easily amenable to microcomputers. The paper demonstrates through examples how the procedure can be applied to minimizing discounted costs of level layout as well as providing the optimal location of a production shaft.IntroductionIn recent years there have been numerous publications in mining journals concerning the minimization of mine haulage costs. With the rapid escalation of energy costs during the last decade it is not surprising that several studies applying queuing theory, simulation and mathematical programming to mining related transportation problems have been successfully undertaken. Because of the increasing availability of various sizes of computers these techniques have gained considerable popularity as evidenced by the numerous commercial packages presently being marketed.The publications concerned with haulage cost minimization are generally concluded along the lines of ' ... haulage costs at production rate x will be minimized by assigning y units to location z...'. Details about the route followed and whether it guarantees a minimization of costs are seldom considered. Route data has to date been manually resolved and used for input to developed programs. It would appear that no program has been developed to date which simultaneously minimizes costs by both fleet assignment and routing network resolution.Another important factor in transportation problems is the investment cost associated with the routes; road construction costs or, in the case of underground levels, drift excavation costs. Except for special cases the transport network which minimizes haulage costs only will not provide a minimum excavation cost solution.Zarnbo(1) illustrates by graphical and mathematical procedures how central 'facilities' can be located to simultaneously minimize investment in the transport network and haulage costs. Unfortunately these procedures and others recently applied to mine haulage problems do not provide solutions to the generalized problem. Also because these procedures are geared to graphical solutions and hand computations whereby certain possibilities can be 'visually' eliminated, they can be tedious to program and present storage capacity constraints with current microcomputers."

Jan 1, 1985

By R. W. Thompkins

The two largest contributors to silicosis in machine men are dust from drilling and dust from blasting. With the advent of the shatter-cut method of blasting, the evils of blasting dust were greatly reduced. The problem of dust from drilling has been attacked from two different angles: (1) The adoption of dustless frontheads or enlarged water tubes, both of which have proved capable of reducing dust concentrations; and (2) The adoption of better ventilation. Many operators are loath to adopt dustless frontheads or enlarged water tubes on Leyner drills because of the increased repair costs on drills, and also because it has been found difficult to enforce 'the proper usage of dustless machines: for example, workmen sometimes plug the vents in the fronthead.

Jan 1, 1943