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By M. Honarmand, Z. Moezifar, H. Ranjbar

Abstract - The Sar Cheshmeh area covers an area of 640 km2, and is located in the southern part of the Iranian Volcanic-Sedimentary Belt. ETM+ images have been used for alteration mapping. The Crosta method was found useful for enhancing exposed areas containing hydroxyl and iron oxide minerals. However, field checking has shown that this method is not able to identify all the known areas of hydrothermal alteration. Many of these altered areas are also not clearly resolved in the geophysical data. In order to improve resolution of such areas, integration of remote sensing and geophysical data may be helpful. The Darrehzar Cu deposit was chosen as a control area. The data have been divided into two sets: explanatory and target variables. The explanatory data are Landsat hydroxyl and iron oxide images and airborne geophysical data (K, U, and Th total counts, and total magnetics). The target variables are rock geochemistry and ground geophysics. Data analysis and integration of geophysical and remote sensing data were performed using directed principal components. This technique is found to be useful for the delineation of hydrothermally altered areas with more confidence.

Jan 1, 2002

"The Schist Lake mine, owned and operated by the Hudson Bay Mining and Smelting Co., Limited, is situated on the northwest arm of Schist Lake, 4 miles southeast of Flin Flon, Manitoba.The deposit was discovered early in 1947, during a diamond drilling program exploring small copper showings along the west shore of the lake. The orebodies came to surface between the mainland and the north end of Stitt Island.Production began in August, 1954, at an initial rate of 300 tons per day. Underground workings have been carried to the 1,700-foot level, where exploration work is now being done.General GeologyRocks of the area are Precambrian lava flows, with associated pyroclastic breccias and tuffs, and related minor intrusions. Their general strike is northwest and their dip is nearly vertical. Shearing is common and is related to the activity which caused intense folding and much faulting in the area. The larger faults and the fold axes trend a few degrees west of north. The lavas are mainly andesitic but a layer of sericite-carbonate schist, which is probably derived from a fine-grained, grey quartz porphyry, occupies most of Stitt Island and extends into the mine area. It is within this layer of schist that the ore deposit occurs (Figure 1).The OrebodiesThe ore occurs as tabular to lens-shaped bodies which strike northwest, dip nearly vertically, and plunge at 60 degrees to the southeast. The ore zone is 800 feet long and has been traced for a distance of 2,000 feet along the plunge (Figure 2). The maximum length of continuous ore within this zone is 200 feet. Widths of individual ore lenses vary from a maximum of 35 feet to a minimum of 2 feet.The ore consists of three distinct types: massive sphalerite-pyrite, massive chalcopyrite-sphalerite, and disseminated chalcopyrite-pyrite. The No. 1, or eastern, orebody is predominantly massive sphalerite-pyrite. The western part of the No. 2 orebody is massive chalcopyrite-sphalerite. The disseminated ore forms the eastern part and the lower end of the No. 2 orebody. All three types of ore, with patchy distribution, are found in the smaller lenses below the 1, 100-foot level.Metallic minerals, in order of deposition, are pyrite, arsenopyrite, chalcopyrite and sphalerite, galena, enargite, and gold. Both massive types of ore exhibit banding in many places, probably as a result of the selective replacement of schist. The disseminated ore was formed as a filling of the interstices in a brittle chert rock which appears to have been crushed by the same forces which produced the schist."

Jan 1, 1949

By Tetsuya Yumoto, Toshikazu Shiratori

Information on the elements contained in E-Waste is important from the viewpoint of recycling. We analyzed several components of waste electrical and electronic equipment (WEEE) collected from citizens. The distributions of the minor metals in WEEE were determined with elemental mapping and chemical analyses. The annual change in elemental composition was investigated by analyzing equipment manufactured in different years. The information will be of use for separation techniques required for future advanced recycling.

Jan 1, 2011

By A. G. Lylie

Introduction At the Britannia Mining and Smelting Company's mill, the equivalent of 2,500 to 2,800 pounds of copper sulphate crystals per day is made from copper which is precipitated from the mine-water. This copper sulphate is used in the mill for the recovery of sphalerite by2 flotation. From 1932 until 1936 it was produced by exposing the precipitate to air for thirty days and then dissolving the oxidized copper with dilute sulphuric acid. In 1936, the flotation of sphalerite was discontinued and, when it was resumed in 1947, steps were taken to try to improve the former method. The present paper outlines the way in which this was done, and describes the plant and procedure by which copper sulphate is now being made, and by means of which an appreciable saving is being effected. Aboratory Tests The previous work had established that the precipitate, which consists mainly of metallic copper, must be oxidized before the copper would dissolve in dilute sulphuric acid. Various methods for oxidizing it were tried but in the early laboratory work the only suitable one found was to heat the precipitate to 600°C. and hold it at this temperature for two hours. When this was done, approximately 90 per cent of the copper became soluble in .acid. This percentage was considered satisfactory for practical purposes, but the cost of roasting was prohibitive because only a small amount of roasted precipitate was required each day.

Jan 1, 1951

By Marcel Vallée

"This text consists of two parts, comments on "Classification and Reporting of Mineral Resources for High-Nugget Effect Gold Vein Deposits," by S.C. Dominy, P.R. Stephenson and A.E. Annels, Volume 10, No. 3, pp. 215-233, and related comments on some features of the JORC Code.This paper is, in many respects, a very elaborate and valuable account of the problems encountered in the estimation and classification of high-nugget gold deposits. However, in some key areas, it appears to fall short of its objective to help "resolve satisfactorily most issues" and "contribute to improved estimation and classification" (abstract). One must realize that high-nugget effect gold deposits constitute a very narrow field, despite the occasional bonanzas. This is a field of mostly smaller sized deposits, where limited success and a significantly higher rate of failure occur relative to larger deposits of mostly lower grade. This is shown by the six deposits described in the paper. Thanks for contributing to greater understanding of these complex and nevertheless challenging deposits."

Jan 1, 2002

By W. A. MacWilliams, R. G. Reynolds

"This paper describes, for the first time, a patented method of extracting sodium sulphate from a natural deposit in Western Canada. The method resembles, but is not, strictly speaking, solution mining. The fact that a plant based on this method has been in successful operation since late 1969 will be of added interest. The deposit in question is located near Metiskow, Alberta, and conventional surface recovery methods could not be applied because of heavy concentrations of mud. Considerable pilot work went into the separation and recovery process on which the plant operation is based. The history of the industry is covered briefly as background to this development. Today it is clear that the method has broader application within the industry and, in concept, may generate interest in many other applications."

Jan 1, 1973

By D. A. Sloan

THE Young Davidson mine is of particular interest because of the extremely low costs and also because of the novel method devised for pillar recovery. The outstanding feature of the mining system is its simplicity and regularity. The successful operation of this property was de-finitely assured by the rise in price of gold in 1934. The mine is situated in the southeastern part of Powell township, about forty-five miles southeast of Timmins, Ontario. The local town, two miles distant, in Matachewan, which is twenty-five miles northwest of Elk Lake, the terminus of a branch line of the Temiskaming and Northern Ontario railway. Mining is confined to one of a group of five claims. The ore occurs in a body of syen.ite porphyry which is intruded along a contact of Timiskaming conglomerates with Keewatin altered ruffs and greenstones. This intrusion extends in a north-easterly direction from Davidson creek for 3,300 feet, of which length 700 feet lies within the Matachewan Consolidated property. The maximum width of the mass is 700 feet; it tapers off as it extends east-ward. The Orebody.-The syenite porphyry is of three types, probably all differentiates of the same magma, but only one of them is gold bearing. This is a cherry-red, coarsely crystalline type which forms the central part of the mass. It is more brittle and more fractured than the others, and consequently has allowed easier access to the gold-bearing solutions. The gold has been brought in by a branching system of quartz veins and along minute fractures. It is mainly in the form of very fine 'free' gold, but also occurs in pyrite, which is the chief metallic mineral.

Jan 1, 1942

By J. Beland

The Rimouski-Matapedia region is along the southwestern extension of the central Siluro-Devonian synclinorium of Gaspe Peninsula. As in Gaspe about 30,000 feet of Siluro-Devonian strata rest on a "basement" of tightly folded Ordovician rocks. The central synclinorium is divided up into two northeasterly trending belts each of which is characterized by distinctive lithologies and patterns of folding. The northwestern belt is gently to moderately folded and includes lithologies which point to a stable environment. The southeastern belt on the other hand is tightly folded and the lithologies indicate a less stable environment. A marked angular unconformity separates the basement from the Siluro-Devonian cover near the north-western margin of the northwestern belt. At the southeastern margin of the southeastern belt the relationship of cover to basement has not been as-certained because of a fault. The northwestern belt has several features significant in the search for petroleum; the Silurian strata are exposed in broad open anticlines

Jan 1, 1962

By James H. Cobbs

Blind boring of shafts is becoming an increasingly important part of shaft sinking. Blind-bored shafts have reached diameters of 25 It in Europe and 16-1/2 ft in the United States. Much of the technology developed for blind boring has evolved from the technology of drilling wells for oil and gas production. Blind boring offers economic advantage in a number of situations where conventional shaft sinking either is extremely difficult or very expensive by virtue of large-capacity aquifers which must be penetrated, unstable ground or where very rapid sinking is required. In sedimentary rocks, blind boring is cost competitive with conventional sinking, in diameters of about 12 ft or less. Blind boring is not competitive with raise boring where conditions are favourable for raise boring. Blind boring does, however, offer greater flexibility than raise boring and can be performed in situations where raise boring is inefficient or impossible. Combination machinery has been developed to perform both blind boring and raise boring, so that the machine can be used effectively in situations where only the near-surface portions of the shaft are unfavourable for raise boring. Development work is being supported by the United States Bureau of Mines for blind boring equipment capable of boring shafts up to 30 feet in diameter.

Jan 1, 1979

By Ana Adé

The use of gold compounds in homogeneous catalysed organic reactions has been undervalued for many years due to the preconceived opinion that gold is an expensive and extremely chemically inert metal. However, recent reports have changed this assessment and gold salts in small amounts are known to display high catalytic activity.[1] Thus, it has been reported recently that anhydrous AuCl3 catalyses the formation of C-C and C-O bonds (usually mediated by hydrochloric and other protonic and Lewis acids).[2] Some examples include Michael type additions, selective cross cycloisomerisation/dimerisation of propargyl or allenyl ketones, benzannulation between o-alkynylbenzaldehides and alkynes or cyclisation of alkenyl furans. It is noteworthy that some of these reactions[2a] can be also catalysed by silver salts but with much lower yields. The 1,3-dipolar cycloaddition of nitrones to alkenes is a powerful synthetic method that has found numerous applications in the synthesis of a variety of carbon frameworks.[3] Several laboratories have focused on the development of Lewis acidic reagents to modulate the selectivity of the reaction.[4] Despite the extensive study performed, there are not many examples in which a metallic complex catalyses the dipolar cycloaddition. Most of these reactions require the presence of a metallic complex and nitrone in a 1:1 molar ratio. Recently, we have carried out stoichiometric reactions of the N-benzil-C(2-pyridil)nitrone, 1, with different dipolarophiles in the presence of zinc and silver compounds.[5] Apart from an increase of the reaction rate and the selective formation of one of the possible isomers, we have shown the formation of zinc and silver-nitrone complexes during the reaction, which behave as the intermediates in the cycloaddition process. As suggested by the results obtained with silver complexes, we decided to test gold derivatives (in the oxidation states of I and III). Interestingly, we have found that they can be used not only as stoichiometric reagents but they can also mediate these cycloadditions in catalytic amounts (5 to 10 mol % respect to the nitrone). Here we present the results on the 1,3-dipolar cycloaddition reaction of the N-benzil-C(2-pyridil)nitrone (2-PyBN), 1, and methylacrilate, 2, (Eq. 1) catalysed by gold(III) derivatives.

Oct 1, 2003

By I. E. Puddington, F. W. Meadus

The beneficiation of barite in mill tailings using an agglomeration technique has been demonstrated. Barite was selectively absorbed from a conditioned aqueous slurry into a collector oil by mechanically mixing slurry and oil. The resulting oil-bonded barite-containing agglomerate was washed, to remove gangue material, and further mechanically contacted with dilute H2SO4 This treatment resulted in the transfer of barite into the aqueous phase. Grades as high as '97% BaSO4, with recoveries of 80%, have been obtained using this technique.

Jan 1, 1973

By N. Aslin, G. Heferen, H. DeWaal

"Mount Isa Mines (MIM) acquired a reputation for the successful application of R&D to develop break-through technologies for the mining industry starting in the 1978’s through until the early 2000’s. The ISAPROCESSTM tank-house technology has been licensed to copper refineries throughout the world, and a significant per cent of the world’s copper is refined using this technology. Since development in the late 1980’s more than 20 ISASMELTTM copper and lead smelting furnaces are now installed in countries around the world. Jameson Cell flotation technology developed jointly by Mount Isa Mines and Professor Graeme Jameson is widely used in the Australian coal mining industry and increasingly in the base-metal and gold industry. The IsaMillTM’s developed at Mount Isa and McArthur River made it possible to develop the McArthur River and George Fisher orebodies and has been successfully implemented into base metal fine grinding applications around the world. The most recent commercialised innovation is the atmospheric leach Albion ProcessTM with its supersonic HyperSpargeTM gas sparger, is being adopted as a solution to the increasing complexity of orebodies. MIM’s contribution to the industry was significant given the size and the remote location of its operations with Townsville Copper Refineries more than 1350 km and Mount Isa 1800 km from the nearest state capital of Brisbane. This paper will briefly discuss the development of each of these technologies and why MIM – now owned by Glencore - was so successful innovating and developing such technologies over a period of nearly 40 years.INTRODUCTION Mount Isa is located in the Gulf Country region of Queensland about 1800 kilometers North West of Brisbane (see Figure 1). It came into existence because of the world class mineral deposits found in the area. In 1923 the orebody containing lead, zinc and silver was discovered by the miner John Campbell Miles. Mount Isa Mines Limited (MIM) was founded in 1924 to develop the minerals discovered by Miles, but production did not begin until May 1931. It paid its first dividend in 1947 after 16 years of troubled production. In 1954 the 1100 copper orebody was discovered and with rapidly rising reserves during the 1950’s and 1960’s led to the construction of new concentrators to treat lead/zinc/silver ores in 1966 (#2 concentrator) and copper ore’s in 1973 (#4 concentrator). The difficult nature of the Mount Isa lead-zinc orebodies has meant that the company had always needed to be at the forefront of mining technology. In the 1970’s through to the 1990’s, it became a world leader in developing new mining techniques and processing technologies as a response to declining metal prices and rising costs. Mount Isa has been smelting copper since 1953 and lead since the early 1930’s. Copper Refining at Mount Isa’s fully owned subsidiary of Copper Refineries Proprietary Limited (CRL) had commenced operations in 1959."

Jan 1, 2018

By Mark Sherman

One of the main functions of a process technology and engineering services provider is to select grinding equipment that will meet the client?s throughput rate target. Prudent risk mitigation strategies would suggest that throughput rate estimates be cross-checked using a variety of techniques. Whilst some comminution circuit designers have estimated specific energy requirements via the manipulation of the Bond specific power equation e.g. Barratt?s Grindpower and OMC?s FSAG approach, other designers have opined that the Bond Indices have no role to play in the calculation of SAG mill specific power. As a result, they have developed their own specific energy estimating methodologies, e.g. SMCC?s Drop Weight based power methodology, SPI, SAGDesign, etc. Each method has its supporters and its critics. The challenge facing the engineering provider is how to nominate an SAG mill based circuit throughput rate estimate when faced with significantly disparate specific energies calculated by the different methodologies. This challenge is exacerbated via the continuing evolution of contemporary circuit configurations such as two stage pebble crushing some of which incorporate HPGR technology as the second stage of pebble crushing. The methodology presented in this paper shows that the Bond Indices are an excellent tool for estimating the specific energy requirements for SAG mills in SAB, SABC and SSS circuits. The method accommodates primary crushed feeds, partial and or fully secondary crushed SAG mill feeds, and emulates the impact of pebble crushing on SAG specific energy. As a result, the new Bond Index based method has provided an additional tool for increasing Fluor?s confidence when predicting SAG mill based circuit throughput rates. Based on this outcome, the author proposes that the decades old conundrum of using Bond Indices to calculate SAG specific energies for SAG mill based circuits has been solved.

Jan 1, 2012

By H. O. Olsen

"GeneralTHE IRON ORE at Marmora is a replacement deposit of Precambrian limestone and is overlain with Palaeozoic limestone. It is a low-grade magnetite deposit and the ore will require concentration be-fore it can be used economically for the production of iron and steel.Present plans estimate an annual production of 500,000 tons of concentrate in the form of pellets. This will require mining approximately two million tons of ore and\ waste annually. Shipment of pellets from Marmora to a ship-loading dock at Picton will be made by rail on a year-'round basis. Shipments from Picton to the Lackawanna plant of the Bethlehem Steel Company at Buffalo, N.Y., will be carried on during the Lake shipping season.LocationThe mine, located a mile southeast of Marmora village in Hastings county, Ontario, is 31 miles north of Belleville and 3•7 miles east of Peterborough. Marmora is 130 miles from Toronto and 150 miles from Ottawa. Highway No. 7 passes half a mile north of the mine and Highway No. 14 half a mile west of the mine. The Canadian National railways' branch line between Trenton and Bancroft passes through the property just east of the open pit.HistoryThe Ontario Department of Mines conducted an airborne magnetic survey of the district during 1948 and 1949. Maps of this survey were released to the public early in 1950. Bethlehem representatives carried out ground surveys in the Marmora area shortly thereafter and in June, 1950, commenced diamond drilling."

Jan 1, 1955

By M. Timmins

Campbell Mine operates a 100 tpd gold autoclave that treats sulphide concentrates at 305 psi and 200 °C. Autoclave feed consists of fresh pretreated concentrate, an acidic oxide slurry recycle as well as other high arsenic sources. The brickwork that is used to construct the five compartments of the vessel must be inspected and repaired periodically. Shutdowns are scheduled to inspect this brickwork and at times replace entire sections of the brick walls or lining. It also provides an opportunity to repair or replace other internal features such as agitator blades or oxygen spargers. Mechanics and contractors are scheduled around the clock to perform the maintenance requirements. There are many safety considerations and precautions that must be taken in order to protect the well-being of the workers and minimize damage to sensitive pieces of equipment. Heat, arsenic and lead contamination and confined space issues are just a few of the hazards involved. The following paper will describe how these safety concerns are addressed and will serve as a guide for future autoclave shutdowns.

Jan 1, 2004

"THE Canadian Advisory Committee on Coal Research, which is commonly known as CACCR, has been in existence for the past five years and has held annual meetings during that period. This Committee is comprised of representatives from: the Mines Branch, Fuels and Mining Practice Division, Ottawa; the Geological Survey of Canada; the Dominion Coal Board; and the Provincial Research Councils and Foundations. It embraces all coal research in Canada that is sponsored, for the most part, by Government funds.To bring the country's coal research activities to the notice of the Canadian coal industry, the Coal Research Committee of the Coal Division, C.I.M., passed a resolution at the 1964 business meeting in favour of recommending the publication of an annual research report prepared by the CACCR. The first such report was published in the September, 1964, issue of the C.I.M. Bulletin; the second appeared in the November, 1965, issue of the same publication."

Jan 1, 1966

By R. E. Evans

This paper contains an analysis of the geography and geology of the Black Lake area, Eastern Townships of Quebec, an outline of the work to be performed, an explanation of the design concept and how it was developed, and a discussion of the actual stripping operation, including details of the machinery being used, etc.

Jan 1, 1967

By I. Caraconcea, M. Cousineau, S. Gariepy

"Several base metal plants operate in low grade/high tonnage conditions which impose strict cost control on all fronts for their operations to remain profitable. This leads to decisions regarding plant design, procurement, operations and maintenance that often favor cost minimization over value optimization. This may hinder the representativeness of the metal accounting systems while this should be a priority in a low margin environment. Such poor representativeness not only creates confusion in plant scheduling but can also hide suboptimal metallurgical performance. Furthermore, this situation can run undetected for a while since one dominant feature of a poor metal accounting system is its poor self-awareness. During their practice, authors have particularly identified measurement bias and reproducibility, as well as their impact on metal recoveries as areas of concern. These issues have not been extensively reviewed in the specific context of precious metals in base metal plants. Pragmatic solutions are proposed either at the design stage for a new project or to retrofit an actual operation. These solutions should be based on a business analysis that set a value of information matching the contribution of precious metals as well as the potential to use such information to act toward increasing plant profitability. Conclusions are that trade-offs are an inherent part of the design/audit/retrofit of any metal accounting system in any plant processing either base or precious metals. However, compromises must be evaluated on economic/technical ground.INTRODUCTION Every mineral processing plant must produce metal accounting reports (Morrison, 2008) where estimated saleable metal flows and inventories are published. These metal accounting reports are delivered to a variety of stakeholders who use them for internal management purposes (day-to-day plant production planning, plant operation optimization, capital spending, etc.) as well as for external management (royalty settlements, tolling agreements, etc.). Hence, there should be no doubt that state of the art metal accounting is instrumental for mining companies to reach and sustain high-quality corporate governance (Gaylard, Randolph, & Wortley, 2014). However, producing a metal accounting report involves much more than just printing it (Lachance, Leroux, & Gariépy, 2015a). The underlying metal accounting system (Figure 1) is responsible not only for the reliable production of the metal accounting reports but also for the quality of the outcome. As such, the crucial tasks of its design/audit/retrofit should neither be seen as ancillary nor be left as an afterthought."

Jan 1, 2017

By Yves Gratton

The International Commission on Large Dams (ICOLD) is a non-governmental International Organization which provides a forum for the exchange of knowledge and experience in dam engineering. It is the world's leading professional organization dedicated to advancing the technology of dam engineering and supporting socially and environmentally responsible development and management of the world's water resources. ICOLD was founded in 1928 and has National Committees in 82 countries with approximately 7,000 individual members who are engineers, geologists and scientists from governmental and private organizations, consulting firms universities, laboratories and construction companies. Its original aim was to encourage progession in planning, design, construction, operation and maintenance of large dams and associated civil works by collecting and disseminating relevant information and by studying related technical questions. Since the late sixties, focus was put on subjects of current concern such as dam safety, monitoring of performance, reanalysis of older dams and spillways, effects of ageing and environmental impacts. More recently, additional subjects have included cost studies at the planning and construction stages, harnessing international rivers, information for the public at large, and financing.

May 1, 2002

By D. E. M. Plaumann

"The wet grinding of asbestos has attracted the attention of engineers for many years as a means of improving fibre liberation from the ore. Today, added impetus for wet grinding has resulted from a need to minimize worker exposure to asbestos dust in conventional dry mills using air, as well as a need to provide a cleaner, technically superior fibre for use by product manufacturers. However, high viscosities of asbestos slurries create transport and grinding difficulties which have been a factor impeding use of a wet grinding process.Laboratory evaluation of reagents to reduce viscosities of asbestos slurries found that one reagent, XFS-4272.00, reduced the viscosity of a 35% (by weight) slurry by 75%. The impact of XFS-4272.00 on actual grinding parameters were evaluated using a pilot plant rod mill. Slurries containing 78% solids and XFS-4272.00 were successfully milled and asbestos fibres from that milling compared favourably in liberation and relative surface area to asbestos fibres obtained from milling a 4% slurry without additives. IntroductionAlthough the use of asbestos has been declining over the last few years, the large deposits of chrysotile in eastern Canada (Quebec in particular) remain very important. Asbestos production in Canada stands ninth in volume for mineral production with over 740 000 tonnes produced in 1985. Of this quantity, 722000 tonnes were exported with a value of $446 million. Canada is second in world production of asbestos behind the U.S.S.R. The milling of asbestos ore has been done primarily by a series of steps involving crushing, dry-grinding and aspiration using large quantities of air to release the fibres. Contrast this process to those practiced in the purification of virtually every other mineral. Ores other than asbestos are crushed and milled using an aqueous slurry in rod, ball or pebble mills. The finely ground mineral, still as a slurry, undergoes beneficiation by froth flotation, heavy media separation, hydrocyclones or any of the other aqueous techniques currently employed in mineral processing.One of the driving forces behind the use of air instead of water as a means of separating asbestos is the nature of the aqueous asbestos slurry. While most slurries of minerals contain as much as 80070 solids and can still be pumped and transported, asbestos slurries become highly viscous with as little as 10070 solids. These high viscosities reduce the pumpability of the slurry and create severe handling problems. Mills become prohibitively large and uneconomical due to having to handle slurries containing only 10% rock and 90% water.Health-related issues have affected the use of asbestos in many countries. In Canada, the asbestos-producing provinces have introduced stricter occupational legislation to improve safety in the workplace. This primarily means strict control minimizing release of air-borne asbestos fibres . Quebec and Ontario have set limitations of airborne asbestos at 2 (2) and 1(3) fibre per cubic centimetre of air. With present milling technology, maintaining these low levels in the workplace requires constant vigilance."

Jan 1, 1988