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By K. K. Kelley

This work is a continuation of a program of study of the measurement and correlation of fundamental thermodynamic data inaugurated several years ago by the United States Bureau of Mines. Previous bulletins in this series were concerned with (1) The Entropies of Inorganic Substances (66),3 (2) High-Temperature Specific-Heat Equations for Inorganic Substances (67), (3) The Free Energies of Vaporization and Vapor Pressures of Inorganic Substances (68), (4) Metal Carbonates-Correlations and Applications of Thermodynamic Properties (71), (5) Heats of Fusion of Inorganic Substances (69), and (6) A Revision of the Entropies of Inorganic Substances-1935 (70). The present bulletin has as its primary purpose the correlation of the thermodynamic properties of elementary sulphur and its inorganic compounds and the presentation, after careful consideration of all available information, of a self-consistent system of thermodynamic relationships for these substances. It is believed that such a set of relationships, concerning substances of such great metallurgical importance as sulphur, sulphur oxides, hydrogen sulphide, metal sulphides, and metal sulphates, is of considerable value in indicating directions of possibly profitable experimental work and in clarifying subjects of technical interest upon which direct experimental information is difficult and expensive to obtain. It cannot be doubted that many problems of technical importance that at the outset appear almost hopelessly complex or obscure either may be solved, if enough information is available, or the specific lacking information clearly indicated by the adoption of the thermodynamic viewpoint. This has been demonstrated many times in the work of the Metallurgical Division, Pacific Experiment Station, Berkeley, Calif. Even when only crude basic information is available, a semiquantitative picture often may be developed by thermodynamic means in which general trends and approximate limits of a proposed process or modification of practice may be clearly portrayed and thus used as guides in further considerations and experimentation.

Jan 1, 1937

With the burgeoning opportunities for new gold mines to be developed the opportunity now exists for a reassessment of the optimal processing path for these gold ores. Projects have traditionally followed the path of whole ore cyanidation with either Carbon In Pulp (CIP), Carbon In Leach (CIL) or Merrill Crowe. The challenge ahead is whether these traditional processing methods are the best option for stakeholders, or to go beyond CIP and use a combination of currently available technologies that can be used to provide gold recovery at a significantly reduced capital cost and minimal deleterious impact on the environment. Either stand alone or combinations of gravity concentration and flotation, can produce a gold bearing concentrate that can then be fed directly to a batch or continuous intensive cyanidation unit. Treatment of a gravity/flotation concentrate by intensive cyanidation may be processed by the InLine Leach Reactor (ILR) developed by Gekko Systems of Australia. The advantages of the technology combining gravity/flotation followed by intensive cyanidation are significantly reduced capital costs; improved safety due to reduced cyanide inventories; the majority of tailings un-contaminated by cyanide; less energy required due to an opportunity for an increase in grind size û reducing green house gas emissions; detoxification of concentrate tailings can minimise tailings disposal and containment requirements.

Jan 1, 2004

By D. R. Wilburn

This summary of international mineral exploration activities for the year 2010 draws upon available information from industry sources, published literature and U.S. Geological Survey (USGS) specialists. The summary provides data on exploration budgets by region and mineral commodity, identifies significant mineral discoveries and areas of mineral exploration, discusses government programs affecting the mineral exploration industry and presents analyses of exploration activities performed by the mineral industry. Three sources of information are reported and analyzed in this annual review of international exploration for the year 2010: 1) budgetary statistics expressed in U.S. nominal dollars provided by Metals Economics Group (MEG) of Halifax, Nova Scotia; 2) regional and site-specific exploration activities that took place in 2010, as compiled by the USGS, and 3) regional events including economic, social and political conditions that affected exploration activities. These were derived from published sources and unpublished discussions with USGS and industry specialists.

Jan 1, 2011

The Bureau of Mines is active in programs pertaining to solid waste disposal and land reclamation. Nine reclamation projects of mining companies and six projects of the Commonwealth of Pennsylvania were investigated to provide information on the procedures, costs, and results obtained from reclamation projects of bituminous coal-waste disposal areas. Refuse disposal costs of mining companies ranged from 9 to 28 cents per ton-mile, and quantities disposed of ranged from 600 to 4,500 tons per day. Spreading and compacting the refuse at the dump site ranged from 3 to 20 cents per ton. Cost of covering with soil and planting ranged from $750 to $1,646 per acre. Total costs of reclamation for the Pennsylvania projects ranged from about $1,800 to more than $15,000 per acre. Refuse preparation costs ranged from $772 to $5,550 per acre. Soil covering and planting costs ranged from $1,083 to $5,086 per acre. The best results were obtained by soil preparation and conditioning prior to planting. The type and depth of soil and effect of climate were also factors.

Jan 1, 1973

By K Eglinton

The extractive industries response to community engagement due to social and political pressures in the 1980s and 1990s was the starting point of development leading to the modern function of corporate social responsibility.Challenged by communities who have greater access to information, demand greater transparency and responsibility of corporate organisations and who possess greater awareness of the communities they participate in, the mining sector faces another step change in how it approaches and integrates corporate social responsibility, transparency and engagement.Mining is seeking reserves of lower grades, in more sensitive environments and in countries with minimal transparency and governance. Not only are the locations changing but so to the stakeholders to engage with and issues to contend with. An increasingly savvy opposition, including not just the ‘green hippies’ but the Gen-Ys interested in the wider sustainability debate and Baby Boomers moving to life style environments, are now questioning mining’s behaviour and encroachment into ‘their space’. Issues including effect on lifestyle amenity, property value, energy efficiency, social provision; concepts such as ‘doing more with less’, participatory engagement, increasing transparency and balancing economic, environment, cultural and social aspects are becoming the norm.The traditional model of supporting local communities, communicating plans and providing employment, community investment and indigenous support programs is not enough. Our industry faces challenges on multiple fronts. It must now be able to address these from a personal/community perspective if it is to remain relevant within communities and play a role in addressing the global sustainability issues.This paper draws on corporate social responsibility trends, integrated functions and implementation approaches through the application of activities within Newmont operations across Asia Pacific.The approach presented works on the philosophy that communities do not necessarily have to like what we do, but should be able to identify and recognise the way in which we involve them in decision making, communicate our intentions and respond to issues.

Nov 22, 2011

By R. C. Bates

Uranium mine workers? daily exposure to radon daughters was estimated by two methods of sampling, detailed and spot-check. Both methods involve time- weighting measured exposure levels to arrive at an average exposure figure. Random sampling, or sampling without regard for the duration of exposure, has proved inadequate for estimating average exposure levels. During the first phase of the study, the breathing zones of eight workmen in three underground uranium mines were sampled for an entire shift; 20 to 36 radon-daughter samples were collected per man-shift, Analysis of these data indicated that a suitably accurate estimate of the full-shift exposure could be made by time weighting a considerable lesser number of representative radon-daughter samples. This analysis led to the spot-check method of estimating full-shift exposures to radon daughters. Additional detailed studies were made in seven mines to Learn more about the full-shift radon-daughter exposures and to test the spot-check method, In all, 617 radon-daughter samples were taken in 10 underground uranium mines during 26 man-shifts, and the spot-check method was tested 35 times.

Jan 1, 1962

By Parker JB

The Channar Iron Ore Mining Project (the Project) is a joint venture involving the Hamersley Holdings group (Hamersley) and the China Metallurgical Import and Export Corporation (CMIEC). The Project represents the first new iron ore mine development in Australia since 1971/72 and is the first major overseas mining investment by the People's Republic of China (PRC). The Channar mine is being constructed in the Pilbara region of Western Australia and will operate for more than 20 years supplying the PRC with 200 M tonne of high grade iron ore. The Project is configured to take advantage of Hamersley's existing production and support facilities. The Joint Venturers will invest A$280 million to achieve an initial production rate of 3 M tonne/yr and an additional A$120 million over the next 8 years to achieve full capacity of 10 M tonne/yr. Financing the Project to meet the requirements of both Hamersley and CMIEC, with the dependance of the Project on Hamersley's infrastructure, and in a depressed iron ore market, required an innovative financial structure to be arranged in a limited period. This paper describes both the financial structure and its implementation, and draws a number of conclusions regarding the financing of resource projects.

Jan 1, 1989

By J. Walden, J. Rhoades, R. E. Miller, R. Gibbs, P. Boman

Accelerometers and pressure transducers, mounted in suspension components, can be used to monitor the ride of haulage trucks. Presently, it is difficult to tell what caused a jolt to the truck using either pressure or acceleration data alone. When information from a global positioning system (GPS) is recorded at the same time as shock pressures or acceleration data, the exact location of an event can be determined. Mine management can then determine what caused the jolt to the truck. This research is part of a National Institute for Occupational Safety and Health (NIOSH) project called "Engineering controls for reduction of jolting/jarring injuries in surface mines." NIOSH is investigating how the work environment of haulage truck drivers can be improved. The US Mine Safety and Health Administration (MSHA) notes that, between 1986 and 1995, 60% of the back injuries were to haulage truck drivers in metal/nonmetal surface mining. Personnel from Phelps Dodge suggested that combining acceleration data with information obtained from a GPS could generate results with a variety of uses. Therefore, researchers at the Spokane Research Laboratory (SRL) began investigating how to tie acceleration and GPS data together. It was originally thought that this tool would be used primarily for road and truck maintenance. However, as research progressed, it became apparent that it would also be useful in providing feedback about equipment operations and identifying unusual causes of jolting. Experimental approach Available components that function well by themselves were linked for the purpose of proof of concept. Using a Modular Mining Systems (MMS) product called Vital Signs, SRL researchers took an Aero-Marine Products (AMP) acceleration recorder and hardwired it to MMS Dispatch equipment already installed on a truck. AMP is one of several manufacturers of acceleration recorders frequently used to monitor freight during shipping. MMS Dispatch is used for production accounting and truck dispatching and uses GPS tracking. Through Vital Signs, an analog or digital alarm signal can be recorded. Steve Rhoades, of MMS, wrote unique code that links the Vitals Signs data and GPS location and stores this data in an ASCII log file. SRL researchers installed the equipment in a Caterpillar 793 haul truck. The result was a working prototype. Figures 1 and 2 show the instrument installation in the truck. Discussion From 8:30 pm on Sept. 14, 1998 until 6:50 am on Sept. 15 1998, 899 jolts above a threshold of 2 g (1 g is the acceleration of gravity. It is either 9.81 m/s2 or 32.17 ft/ s2.) were measured on the test truck. Some sections of the haulage road produced most of the events, while

By Bowen Li, Matthew D. Andriese, Jiann-Yang Hwang, Zhiwei Peng

"Magnetic iron sulfide ore particles heat rapidly by microwave (MW) irradiation generating plasma during oxidation of surfaces. The high temperatures produced by the exothermic event decompose local portions of samples forming a differentiated flux of partially melted silicate (si-mt) with entrained metal-sulfide (sul-mt) droplets. The si-mt forms crystals of olivine (01) in Ca-rich pyroxene (Cpx) melt that resorbs a portion of iron sulfide (Po). The immiscible sul-mt droplets of (Fe,Ni,Cu)1-xS contain magnetic subsolidus Ni-pyrrhotite (Ni-Po) with Cr-bearing ferrospinel (Mag). Sub-micron sized flecks of Au are found on the surface of a sul-mt droplet with scanning electron microscope (SEM).IntroductionSulfide ore deposits supply the world's mineral resource in the production of non-ferrous metals. Particularly, Cu-Ni deposits are chemically enriched in metals from high sulfur saturation during magmatic formation that creates the proper geochemical environment for metals enrichment [1, 2]. The sulfur and oxygen fugacity control redox conditions of a differentiated silicate/sulfur magmatic melt with the concentration of iron playing a dominate role in the chemical partitioning of metals in sulfide liquid [3, 4]. The initial magnetic crystallization of (Fe,Ni)i-xS monosulfide solution (mss) from metal-sulfide liquid has an exclusive effect on the fractionation of mineral assemblages (zoning) [5].Phase equilibria of sulfide melt (sul-mt) components are related by a miscibility gap with mss forming Ni-rich pyrrhotite (Ni-Po) followed by the crystallization of (Fe,Cu)1-xS intermediate sulfide solution (iss) [6]. Precious metals form stable chacogenides and are retained in solution until the final liquid solidifies. The redistribution of metals now occurs by subsolidus cooling of mss and iss that exsolve low-temperature Fe-Cu-Ni bearing mineral assemblages [7]. The proper interpretation of ore genesis increases processing capabilities by isolating chemically enriched zones during extraction and should be used to aid in sustainable mine site development [8-11]."

Jan 1, 2013

By R. Loots

High temperature creep resistant alloys are used in power plants as steam pipes and headers where they have to withstand super heated steam temperatures of up to 600ºC for the lifetime of the plant. Frequently certain portions of pipes may be subjected to excessive temperatures and consequently suffer from creep damage. Damaged portions then have to be replaced by welding new steel to the service exposed steel. Such welds will have to be subjected to a post weld heat treatment (PWHT) to relieve residual stress. During such a PWHT it is then possible for the steel to crack in the heat affected zone (HAZ) by what is known as Reheat Cracking (RHC), also known as stress relief cracking. For the present purpose the RHC susceptibility of service exposed steel was compared with that of new steel by reproducing the temperature cycle to which the HAZ will be subjected during welding in a weld simulator. This was followed by PWHT where the influence of residual stress was studied by applying a constant stress to such a weld-simulated specimen in a creep testing machine. A sharp spiral was machined along the length of the cylindrical specimen to simulate the presence of a possible weld defect. In this way it is possible to subject all portions of the simulated HAZ to the same stress intensity. For the purpose of the investigation 1/2Cr-1/2Mo-1/4V steel known to be susceptible to RHC was chosen. During testing a constant tensile stress was maintained while the temperature was increased to follow the thermal cycle prescribed for PWHT. It was found that the new steel in all instances failed by RHC along the grain boundaries in the high temperature heat affected zone. In the case of the service exposed steel the creep strength of parent metal was lower than that of new steel as a result of exposure to a steam temperature of 540°C for about 15 years. Consequently, all of the failures during testing occurred in a ductile manner by necking outside the HAZ rather than by RHC. When service exposed steel is welded to new material, it is expected that the creep strain associated with stress relaxation will mostly be concentrated in the parent steel outside the HAZ. Accordingly, it should probably not be necessary to make any extra allowance for reheat cracking during repair of steam pipes manufactured from 1/2Cr-1/2Mo-1/4V steel other than that applicable for welding new steel.

Jan 1, 2004

By Forest H. Majors

"INTRODUCTION The Brutch sulfur deposits were explored by the Bureau of Mines during the spring and summer of 1944. Old workings were rehabilitated and sampled, test pits and shafts were sunk, and trenches were cut through the surface alluvium.ACKNOWLEDGMENTSIn its program of exploration of mineral deposits, the Bureau of Mines has as its primary objective the more effective utilization of our mineral resources to the end that they make the greatest possible contribution to national security and economy. It is the policy of the Bureau to publish the facts developed by each exploratory project as soon as practicable after its conclusion. The Mining Branch, Lowell B. Moon, chief, conducts prelimi¬nary examinations, performs the actual exploratory work, and prepares the final report. The Metallurgical Branch, R. G. Knickerbocker, chief, analyzes samples and performs beneficiation tests. Both these branches are under the supervision of Dr. R. S. Dean, assistant director.Special acknowledgment is due S. R. Zimmerley, regional engineer, and Paul T. Allman, principal engineer, Bureau of Mines, Salt Lake City, for their supervisory aid, and to the metallurgical staff, Salt Lake City, for beneficiation of samples."

Oct 1, 1946

By Woodcock J. T, Jackson N

Wattle Gully Gold Mines N.L. of Chewton, Victoria, has decided to replace its present battery plant by a new mill to treat a free milling gold ore, after fine grinding, by concentrating coarse gold in a jig concentrate for amalgamation, and fine free gold and auriferous sulphides in a flotation concentrate for cyanidation. This paper outlines the economic justification and advantages of this step. In a review of the present position at the mine, a history of the company is followed by a description of the ore and its occurrence.Present treatment practice in two separate battery plants is described. Treatment tailings assay 1 dwt/ton gold and a substantial part of this gold is finer than 200 mesh.Experimental work showing how recovery may be improved is discussed and alternative methods of concentrating coarse gold mentioned. It is decided that the most suitable method for recovering gold from the ore is by jig concentration plus flotation.The merits of various flowsheets incorporating these techniques in the existing plant and in a new plant are assessed. It is concluded that there is no advantage in extending or converting the existing two battery plants, and it would be more economical and satisfactory to build a new plant.An economic study shows that for a capital expenditure of £120,000 a new plant could be erected to treat 70,000 tons p.a., working a 5-day week, and return an additional 14.2/- per ton, or £50,000 p.a., in revenue over the existing plant. It is estimated the plant cost can therefore be recouped out of savings inside three years. Probable ore reserves are sufficient to cover this period and additional ore will certainly be developed.INTRODUCTIQNSince 1934, Wattle Gully Gold Mines N.L. has been mining a lode gold ore at Chewton, near Castleniaine, and treating it by stamp milling and plate amalgamation, plus table concentration of auriferous sulphides. In 1954, following two years of unprofitable operation, a rehabilitation programme of all sections of the mine and plant was commenced. In the treatment section, two, major prospects for improvement...

Jan 1, 1959

By Rashmi Ramesh

Zinc oxide nanoparticles and nanostructures form an active area of research in the recent years. It can be doped to modify electronic, optical and other properties. Synthesis and characterization of zinc oxide is of extreme importance in order to understand the structure property relationship in this material which is now being considered of importance next to carbon nanotubes. Microstructure of zinc oxide nanomaterial is extremely sensitive to the preparation conditions. Nanocrystalline pure and Mn and Mg doped zinc oxide samples were prepared by wet-chemical method under different growth conditions. The samples were investigated for crystalline phase, microstructure, morphology and luminescence characteristics. The effect of Mn and Mg doping on the characteristics of ZnO have been discussed.

Jan 1, 2010

By Matthew E. MacGregor, Hugh T. Kelly

I.H. 635 (LBJ Freeway) was constructed as a loop freeway around the city of Dallas in the late 1960s and early 1970s with a design capacity of 160,000 vehicles per day (vpd). Today’s facility is carrying over 270,000 vpd. To help mitigate community concerns with regard to right-of-way issues, noise, safety, and traffic disruption, the Texas Department of Transportation (TxDOT) is planning a number of significant improvements. The West Section improvements include the construction of managed HOV/Toll lanes. Within this 8-mile section, TxDOT is planning two mined tunnels(1.5 and 1.9 miles long) beneath the LBJ Freeway. The mined tunnels will connect to cut-and-cover box sections in both the east and west directions. This paper presents an overview of the project development from the design schematic, through the geotechnical investigations, into the current design stage. The project is being planned for implementation by use of a Comprehensive Development Agreement.

Jan 1, 2005

By Peter White

A bridge reconstruction project involved driving pipe piles for the new foundations. Disturbances within soil layers at depth caused sinkholes to open up within the construction site. After a near fatal accident, the pile driving installations were suspended. The solution involved ground im­provement through cement grouting with sleeve pipes and special grout additives to minimize migration of cement grout.

Jan 1, 1997

By J. E. House, C. J. Lewis

In the solvent extraction process, also referred to as liquid-liquid extraction, the clarified uranium-pregnant sulfuric acid leach solution is contacted with an organic extractant dispersed in kerosene; the uranium enters the organic phase and associates itself with the organic extractants. Simultaneously an-ions or cations, as the case may be, leave the organic extractant phase and enter the aqueous phase. The two phases are then allowed to separate by virtue of their immiscibility. Based on the data available, a generalized procedure for the recovery of molybdenum from the sodium carbonate scrub liquor of acid leach uranium mills using secondary and tertiary amine extractants is presented, as is another generalized procedure for the recovery of molybdenum from the bleed stream of alkaline leach uranium mill circuits. Such economic appraisal as can be made at this stage of the development is also indicated. Molybdenum occurs in most uranium ores. Whether the uranium is solubilized by the acid leach route or by the alkaline leach process, some of the molybdenum present in the ore is solubilized along with the uranium. Molybdenum in uranium mill pregant solutions may cause processing difficulties, among which may be mentioned the poisoning of resins, the precipitation of organo-molybdenum complexes, or the build-up of molybdenum in recycle streams. While the uranium milling industry has been successful in applying measures to overcome the problems arising from the presence of molybdenum in the circuits, such measures nevertheless add to the cost of producing uranium yellow cake concentrate. Table I shows the analysis of typical uranium pregnant leach solutions. Uranium recovery by solvent extraction is presently practiced only in conjunction with acid leach circuits, and it may be noted that of about 16,000 tons of uranium ore daily processed by sulfuric acid leach, more than half of this tonnage follows the solvent extraction route. Uranium mills leaching ore with sodium carbonate-sodium bicarbonate solution (the so-called alkaline leach circuit) do not use solvent extraction. In this case the uranium pregnant solution is sufficiently pure to permit the direct precipitation of uranium concentrate with subsequent recycle of the alkaline leach solution. While solvent extraction procedures may be developed which are economically attractive for the alkaline leach uranium process, this has so far not been the case. The earlier uranium mill solvent extraction circuits used organo-phosphate extraction reagents, among which di-2-ethylhexyl phosphoric acid was popular. It should be borne in mind that uranium in its sulfuric acid leach solution exists in an equilibrium involving uranium in both anion and cation forms: uo + 2S0 = [uo2(so4)2] For this reason uranium may be extracted by either cation extractors or anion extractors and this is likewise true of molybdenum. However, the organo-phosphate extractants also load ferric iron which is usually present in the uranium acid leach solution. For this reason, it is necessary to reduce this solution substantially to the ferrous state ahead of solvent extraction using organo-phosphate reagents. The amines, being anion extractors, selectively reject iron and thus render the iron reduction step unnecessary. In the solvent extraction process, also referred to as liquid-liquid extraction, the clarified uranium-

Jan 1, 1961

By Harry: Lobb Verakis

Mining operations have made tremendous strides in improving the safety of blasting as shown by the significant and continual decrease in fatalities and serious injuries. Advances in blasting technology and blasting practices have played a significant role in reducing fatalities and injuries. However, even with the decline of fatalities and injuries blast area security continues to be by far the leading factor in blasting accidents. Even though there continues to be an increase in the use of industrial explosives and blasting agents in the U.S.A., the decline in fatalities and injuries from blasting accidents has persisted.

Jan 1, 2010

By R. Ordóñez Olivares

This manuscript focuses on the design and development of steels for the railroad industry and methods of testing them. Here are summarized the results of research focusing on alloy development and design, with particular focus on advanced rails and wheels. The high-performance steels are characterized for their mechanical and service characteristics. The relationship between properties and cleanliness is discussed and compared to the regular railroad steel. Some of the tests presented are standard tests that are complemented with the nonstandard test recommended by the Association of American Railroads (AAR) and recommended modifications of this. The paper presents the results of metallurgical analysis, mechanical, and residual stress tests. The laboratory methods include microstructure, residual stresses, tensile, hardness, fracture toughness, and microcleanliness tests.

Jan 1, 2013

By C. Jack Powell

TVA's involvement in air quality programs is perhaps a bit unique. The Tennessee Valley Authority was established in 1933 as a regional resource development agency and therefore must view clean air and clean water as essential resources for the overall development of the Tennessee Valley region. TVA operates certain resource development and production facilities, including power-generating facilities, which are potential air pollution sources. The remarks in this paper will be restricted to air pollution control technology at power-generating stations. Perspectives of Air Pollution Control The art of pollution control must be viewed from several perspectives: 1. It must be viewed in a time perspective--what is available now for air pollution control, what might be available in the near future, what are the likely technologies for the more distant future, and what the future trends in availability of resources might be. 2. Each air pollution abatement scheme must be viewed in the perspective of its total environmental consequences--not just its ability to clean up the air. The generation of electricity from coal produces waste gases, waste particulates, and waste heat. These wastes must be disposed of either in water, in air, or on land. If the polluting wastes are removed from the gas streams going into the air, then they must be disposed of either on land or in water or, hopefully, converted into a product which has some beneficial use. As an example, removal of S02 from stack gases could be more readily accomplished if one could dump waste sulfur products in water. He would solve an air pollution problem by creating a water pollution problem. 3. Techniques for air pollution control must be viewed in the perspective of regional, national, and world resources with their national security and economic implications. The power industry might reduce air pollution by switching generating plants to burning desulfurized fuel oil or natural gas.

Jan 1, 1972

By Dan Armour

Bolting solutions traditionally offered only wet drilling or dry drilling options; each with advantages and disadvantages. Joy Mining Machinery has developed a Mist Drilling System that incorporates the best of both types of drilling. Compared to wet drilling, mist drilling offers as much as a 90% reduction in water use. As a result, floor and roof conditions are tremendously improved. This offers a safer and more comfortable work environment for operators. Mist drilling can offer a significant reduction in noise levels compared to dry drilling and in recent trials, bit life has been greatly improved. Holes are cleaner and operators experience less exposure to respirable dust.

Jan 1, 2005