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By F. D. DeVaney

Manganese-bearing deposits f the Leadville, Colo., district are of two distinct types. Manganosiderite, a combined or intimately mixed ferrous and manganous carbonate, occurs in unaltered zones as the predominant mineral. From 50 feet below the surface to as deep as 900 feet, psilomelane and wad, both black manganese oxide minerals, are found in intimate contact with hematite and limonite. A typical analysis of the black oxide ore shows 20.08 percent Fe, 18.64 percent Mn, 15.91 percent insoluble, 14.91 percent moisture, 0.94 percent Zn, and 2.13 percent Pb. A considerable tonnage of the oxide ore has been mined for production of spiegeleisen and for other purposes.

Jan 1, 1940

By F. D. DeVaney

Manganese-bearing deposits of the Leadville, Colo. district are of two distinct types. Mangano siderite, a combined or intimately mixed ferrous and manganous carbonate, occurs in unaltered zones as the predominant mineral. From 50 feet below the surface to as deep as 800 feet, psilomelane and wad, both black manganese oxide minerals, are found in intimate contact with hematite and limonite. A typical analysis of the black oxide ore shows 20.08 percent Fe, 19.64 percent Mn, 15.91 percent insoluble, 14.91 percent moisture, 0.94 percent Zn, and 2.13 percent Pb. A considerable tonnage of the oxide ore has been mined for production of spiegeleisen and for other purposes.

Jan 1, 1940

By Jon C. Volkwein

In This Chapter [Instruments for measuring dust How to calculate the amount of dust from a source How to get a valid concentration measurement Sampling to assess control technology effectiveness] When there is more than one source of dust, sampling may be required to find which dust sources are most significant. Then, efforts to reduce dust can be concentrated where they will have the most impact. This chapter explains how to perform dust source sampling. It describes two kinds of instruments that are available and discusses their limitations. It explains how environmental variables such as concentration gradients, dust dilution, and production changes can impact dust measurements. It also suggests practical ways to improve the validity of dust source measurements under adverse conditions, such as high-velocity airflow or the presence of water mist in the air. [Dust source sampling at coal mine longwalls and at tunnels is more complicated. Chapters 3 and 7 have more information on sampling in those circumstances.] TWO KINDS OF INSTRUMENTS FOR MEASURING DUST Gravimetric samplers. The conventional gravimetric sampler is a good device for measuring dust because it is the instrument used for compliance measurements. This dust sampler consists of an air pump, a small cyclone that separates out the respirable size fraction of the dust cloud, and a filter to collect the respirable dust. In coal mines, the Mine Safety and Health Administration (MSHA)-approved gravimetric sampler uses a 10-mm Dorr-Oliver cyclone operating at an airflow of 2.0 L/min [30 CFR78 74 (2002)]. A correction factor of 1.38 is applied to make the results consistent with the U.K. MRE sampler, the instrument on which the 2.0 mg/m3 coal dust standard is based.

Jan 6, 2003

By James J. Snodgrass

The Bureau of Mines has investigated vibration levels produced by blasting at four underground sites to establish how such factors as type of explosive, delay blasting, charge weight, and geology affect amplitudes of ground motion. A summary of the work is presented and the results of further analysis of the data are discussed. Square root scaling was found applicable to two of the underground sites and could be applied with minor error to all the sites. Comparison of empirical propagation equations in the different rock types indicates that although the site effect is apparent, the combined data may be used as a basis for engineering estimates of vibration amplitudes from subsurface blasting in many different rock types. Recommendations for predicting and minimizing vibration amplitudes from underground blasts are given.

Jan 1, 1974

By J. Ritchey

This report describes the engineering feasibility and economic advantages of using a mobile bridge-type cross-pit belt conveyor system in conjunction with a large dragline to excavate and transport spoils in a typical Western surface coal mine. The studies show that such a system could be built to survive operational and climatic mining conditions, and that its implementation would offer a reduction in both acquisition and operating costs versus conventional mining units of comparable rapacity. Reclamation costs can also be significantly reduced, and the total reclamation process accelerated, making the cross-pit conveyor system attractive to mine Owners and environmentalists alike.

Jan 1, 1976

By Lawrence J. Froisland

To aid in minimizing adverse environmental impacts of mining and mineral processing, the Bureau of Mines studied changes in soil characteristics in topsoil stockpiled during mining operations. Topsoil stockpiles in arid areas of New Mexico, Utah, and Wyoming were sampled over a 1- to 8-yr period to determine any changes in physical, chemical, or biological parameters in the top 15 ft of soil. Parameters examined in this study included soil moisture, pH, electrical conductivity, and cation exchange capacity; total nitrogen, organic carbon, carbon to nitrogen ratio, extractable potassium, and soluble phosphorus levels in the soil; general levels of bacteria activity in the soil; and productivity of the soil as measured by shoot yield in growth tests. Data were analyzed by multiple linear regression to determine possible effects of stockpile age and depth. Bacteria population counts, used as an indicator of goner l bacterial activity, was the only parameter found to be significantly correlated with stockpile age and depth at more than one site. Bacterial activity increased with depth at the two sites where it was measured. Activity increased with age at one site and decreased with age at the second. Significant changes in soil pH, electrical conductivity, and moisture were significantly correlated with stockpile age and depth at single sites. This study did not show significant, consistent deterioration in soil characteristics or growth yield of topsoil stored in stockpiles.

Jan 1, 1987

By Forrest T. Moyer

To keep the mineral and allied industries informed of trend: in the causes of accidents and to point out the need for corrective measures, the Bureau of Mines collects, analyzes, and publishes, periodically health and safety statistics. This report includes data for 1961 on injury experience, with related employment statistics, at coal mines in the United States. These data are presented in tabular form under the following headings: General Injury Experience Tables 1 to 6: Current data on number of injuries, degree of injury, frequency and severity rates, average severity, and related employment and production data by work location.

Jan 1, 1963

By A. G. Mayton

Vehicle vibration exposure has been linked to chronic back pain and low back symptoms among agricultural tractor drivers. The objectives of this study by the National Institute for Occupational Safety and Health (NIOSH) were to assess driver whole body vibration (WBV) exposures and recommend interventions to reduce the risk of back related injuries, particularly relative to vehicle jarring/jolting (the transient mechanical shock components of WBV). The methodology included collecting, from two independent samples, ?eld data and health and work history data of farm equipment operators. Data were collected during mowing, raking, baling, chiseling, tilling, and road travel for different model tractors. Spraying using a sprayer and shrub removal with a skid steer loader were also included. Based on ISO 2631 (1985), the American Conference of Governmental Industrial Hygienists, threshold limit values, presents 0.5 m/s2 as the action level recommended by the Commission of European Communities for overall weighted total RMS acceleration (vector sum for axes x, y, and z) [ACGIH, 2006. Threshold limit values and biological exposure indices. Cincinnati, OH]. WBV measured at the operator/seat interface exceeded this action level. The roughest rides and highest vector sum accelerations occurred with small utility tractor mowers (3.3 and 2.8 m/s2) and a skid steer loader (1.7 m/s2). Major ?ndings from health and work history data showed 96% of participants reported having to bend or twist their necks, although 24% reported neck symptoms. Sixty four percent of participating operators reported experiencing back symptoms (e.g., pain, aching, stiffness, etc.). Recommendations included: specifying a seat that ??better?? isolates operators from jars/jolts with new tractor purchases; maintaining the seat/seat suspension and replacing worn or damaged cushions with NIOSH tested viscoelastic foam padding; using larger diameter tires with radial ply instead of bias ply construction, particularly on small utility tractor mowers, to aid in attenuating ride ??roughness??; using a swivel seat to reduce the stress on the neck from bending or twisting; and improving efforts to educate owner/operators of the adverse effects of WBV exposures. Since the data presented in this paper were collected from two independent samples, the authors were unable to draw any correlations or etiological inferences from the study. However, results were compared and contrasted with other studies which included similar vibration measurements in agriculture.

Jan 1, 2008

By Bruce I. Thomas

Reconnaissance studies of beach sands along the eastern part of the Gulf of Alaska were conducted by the Bureau of Mines to determine if any of these coastal areas warranted investigations as possible sources of valuable minerals. Because of difficult access and limited transportation facilities only a minimum of sampling, surveying, and camping equipment is utilized to obtain spot samples from key areas as well as information indicating the location, size, and character of the deposits. Such data were obtained from beaches located along approximately 247 miles of the Gulf of Alaska coastline during parts of the 1957 and 1958 field seasons. A total of 201 3-inch-diameter auger holes, spaced roughly at 1 mile intervals, were bored by hand methods to depths ranging from 3 feet to 27 feet; also 33 shovel samples were collected from selected localities. Samples were reduced to a rough concentrate in the field by panning; the rough concentrates were shipped to the Bureau of Mines laboratory at Juneau for further concentration and for petrographic and chemical analyses. Particular attention was given to the heavy-mineral content of the concentrates, although the types and relative amounts of rock-forming minerals were also determined. The reconnaissance indicated that, generally, the valuable mineral content of the beach deposits was too low to be of further interest except in the vicinity of Yakutat and Lituya Bays where erratic but possibly significant concentrations of magnetite and/or ilmenite were found. At Yakutat, about 20 miles of beachline extending southeasterly from Ocean Cape, yielded samplas containing from 1 pound to over 300 pounds of iron and from a trace to over 100 pounds of titanium dioxide (TiO2) per cubic yard of beach material. However, the higher grade samples represented local concentrations of minor importance. The general tenor of the Yakutat beach, as indicated by the airthmetical average of 27 auger-hole samples, is about 35 pounds of iron and 20.5 pounds of titanium dioxide per cubic yard of beach material. Ilmenite is the predominant heavy mineral of the beach deposits in the vicinity of Lituya Bay, but it was found in concentrations only in limited areas near the

Jan 1, 1962

By W. J. Boone

The Bureau of Mines has programmed for publication a series of Reports of Investigations of studies it has undertaken on low-temperature phase equilibria of several helium-bearing natural gases. This report is the second of a series of papers of the same general title; it is anticipated that additional reports of this series will be published. The advent of a helium conservation program and the prospect of production of helium by private industry, make it especially desirable to publish these data. This report describes phase equilibria related to a natural gas that is processed for helium at the Bureau's Exell Helium Plant (located about 35 miles north of Amarillo, Tex.). Carbon dioxide and water in the gas were removed before making the study; hence the data obtained are representative of the gas stream immediately after similar preliminary treatment in the plant process. Phase equilibrium data were obtained for conditions from -50° to -275° F. and from 100 to 500 p.s.i.a. Analyses of samples were made as follows: For a given run at a selected temperature and pressure, samples of the vapor and liquid phases were analyzed by mass spectrometer, and an additional sample of the liquid phase was analyzed for helium by means of special apparatus (7). 4

Jan 1, 1962

By T. G. Carnahan

In-place leaching is being actively utilized to extract uranium, copper, and evaporites from low-grade and difficult-to-mine ores. The purpose of this report is to discuss in-place leaching technology, its advantages and disadvantages, and to give an overview of past, current, and envisioned in-place leaching operations in the U.S.A. Application of in-place leaching to uranium ores has been particularly successful because the conditions for leaching are nearly ideal. The ores are generally in highly permeable sandstone aquifers bounded above and below by impermeable shales. The uranium ore minerals are easily leached with oxidizing acid or base solution and the pregnant solutions are amenable to ion exchange concentration and purification prior to conventional solution neutralization for recovery of yellow cake. Copper is routinely recovered from sub-grade rock by dump leaching at most major copper mines. Acidified ferric sulfate is either sprayed on the surface of the dumps or is ponded on top of the dumps. The leaching solution migrates down through the rock and dissolves copper. The solution is collected from the base of the dumps and copper is recovered by solvent extraction-electrowinning or by cementation on detinned steel cans. True in-place copper leaching is being practiced in the caved stopes of an exhausted underground copper mine. Other innovative in-place copper leaching techniques have been tested commercially and laboratory- and pilot-scale experiments have been conducted to investigate proposed in-place leaching techniques for extracting copper from difficult-to-treat chalcopyrite ores. Solution mining of evaporites is being conducted commercially for the extraction of sodium chloride and potassium chloride. A large-scale experimental project for solution mining of trona has been undertaken.

Jan 1, 1982

By R. W. Welker

A technique was developed for sampling and analyzing airborne dusts in diesel-operated, underground, metal, and nonmetal mines. Samples were taken using personal respirable samplers, operated as area samplers, and an 11-stage cascade impactor. Real-time particle size distribution and concentration measurements were made with a quartz crystal micro-balance cascade impactor and a portable electrical aerosol analyzer. The materials characterized were minerals (by XRD), elements (by XRF), particle size distribution and concentration (gravimetrically), and organic constituents (by HPLC). The method was used to characterize dusts generated at the chute pulling operations on the main haulage level of an underground tungsten mine. Analysis showed that respirable dust concentration ranged from 0.8 to 2.3 mg/m3. It consisted of silica (3-7%); organic constituents from diesel exhaust (0.11-0.31 mg/m3 equivalent airborne concentration); and mineral dust (39-100%). The remainder was diesel exhaust particles. It was concluded that size selective sampling may be used to estimate the ratio of diesel exhaust to nondiesel dusts in the respirable fraction.

Jan 1, 1982

By E. A. Mihok

A pilot plant for treating mine drainage by neutralization with limestone was designed, fabricated, and operated by the Bureau of Mines. The process consists of (1) producing a very fine (minus 400-mesh) limestone slurry, (2) mixing the slurry with the mine water, (3) aerating the resulting mixture to remove carbon dioxide and precipitate iron, and (4) separating the solids from the liquid by sedimentation. A mine discharge of pH 2.8, containing 1,700 ppm total acidity, 36 ppm ferrous iron, and 360 ppm total iron, was treated at rates from 300 to 400 gpm. The treated water ultimately reached a pH of 7.4 and contained no detectable iron. The resulting sludge compacted to approximately one-third the volume of sludge from mine water neutralized with lime. The advantages of the limestone process, compared with the conventional lime process are as follows: Widespread distribution and lower cost of run-of-mine, indigenous limestone; simplicity of plant design; reduced hazards and plant operating and maintenance costs accruing from the use of a less reactive chemical agent; higher solids concentration of the precipitated sludge; and no deleterious effect caused by accidental overtreatment. While total process costs could not be determined, it is conservatively estimated that the cost of treating mine water by the limestone process is one-third to one-half the cost of the lime process, depending on the characteristics of the water to be treated and the availability and quality of limestone used.

Jan 1, 1968

By Susan B. Robertson, Gregory E. Hinshaw

Many injuries are caused each year by rock falls in coal mines. Most of these injuries are not caused by major roof collapses, but from falls of smaller rocks from the immediate top or roof skin. Various surface controls are used in mines to control this surface rock. One that has been found to be very effective is roof screening. Depending on the size of the screen, roof coverage approaching 100%can be achieved. Many mines are reluctant to use screen for primary skin control because of additional costs of time and materials, but others are having great success at both controlling costs and surface rock. Data is presented from two mines that show a dramatic reduction in roof skin injuries when screening is used. Much of this success is due to innovations in roof bolting machines. Four case studies of roof screen experience are presented along with associated costs of materials, impact on bolting advance rates, and potential ergonomic risks. The effects of roof screening on skin control and safety are also included. Finally, this paper will provide information about features of different roof bolting machines that affect production and safety.

By Edward A. Barrett, Roberta A. Calhoun

MORE THAN 30 MILLION EMPLOYEES each year are exposed to hazardous occupational noise, and approximately one-third of them develop permanent hearing loss (NSC, 2000). Possible explanations for the latter statistic include worker regard for noise as a mere nuisance rather than as an occupational safety and health hazard, and consequent neglect of established protective measures (e.g., hearing protection); lack of knowledge concerning noise; improper use of hearing protection; worker indifference; and inadequate training (Bauer & Babich, 2004). Many drill rigs generate high sound levels, which vary according to the type of machine, its location with respect to the physical surroundings, and the location of the sound measured around the rig. A NIOSH study measured noise output from several types of drill rigs during field investigations (Ingram & Matetic, 2003). Two types of data were recorded the actual noise generated by the drill rig at various positions around the rig and the noise to which the drill operator was exposed during the drilling cycle. For the latter, sound level measurements up to 120 dBA were recorded while hammering casing (metal on metal) in drilling a water well (Photo 1). Several types of administrative controls exist to address worker exposure to loud drill rig noise. Rotating duties to decrease exposure time and posting warning signs in high-noise work areas are two such controls. Another is the use of hearing protection. Worker use of hearing protection has been disappointing, however. One study by the University of Michigan found that construction workers used earplugs and earmuffs only between 36% and 61% of the time that they were necessary (Lusk & Kerr, 1998). In a 2001 study, OSHA reported a relatively low rate of 30.4% for use of earplugs among construction workers (Walker, 2001). Why are rates for hearing protection use so low? One explanation may be workers' lack of concern for noise in the workplace. They expect workplace noise to be a loud and in¬ evitable nuisance and, therefore, accept it as part of the job. As a result, they disregard proven measures that can protect their hearing. Other workers may use hearing protection, but do so improperly. They may misguidedly remain in high-noise areas for extended periods thinking they are protected when in fact they are not. Other reasons for low use and/or misuse of hearing protection include poor education, lack of awareness and inadequate training (Nash, 2003). To address the training issue, NIOSH developed two exercises from which drillers can learn about noise, hearing loss and hearing protection. The first is an invisible ink exercise, titled "Drill Rig Incident" (DRI) (Photo 2); the second is a 3-D slide reel training aid, "Wearing Hearing Protection Properly" (WHPP) (Photo 3). DRI is an instructor-

By W. R. Crank

[Rock bursts are phenomena associated with mining operations, particularly with deep mining, although they occur throughout a wide rump, of depth, from 600 feel downward. The intensity is usually in direct proportion to the depth, varying from a loosening and sloughing off to a violent rupture, giving rise to the term, "explosive rock." The more or less violent breaking of rock in mine working, tins been variously designated as " flaking," "pressure hoists," "strain bursts," "bumps," "quakes," and "air blasts," which indicate opinions as to cause and effect. The term "air blast" lots probably beat most widely used, and naturally so, as a more or less rapid movement of air often accompanying the more violent phenomena of rock failure. Furthermore, there is an implied if not definitely expressed degree of magnitude in the action designited by a given term and in the cause and character of the action. For example. flaking, pressure bursts, and quakes are terms ranged to indicate action of increasing magnitude; and pressure bursts, strain bursts. bumps, and quakes are ranged according to the cause and character of the action.]

Jan 1, 1929

By V. F. Parry

[Subbituminous coal or lignite when stored in random piles or in ordinary bins in which the temperature fluctuates and air circulates quickly undergoes objectionable physical and chemical changes, which decrease the utilization value of the fuel and increase its tendency to ignite spontaneously. Slacking takes place, and the heating value decreases. These changes are accelerated by fluctuations of temperature and by circulation of air through the storage space. When such coals are exposed to the weather they lose moisture rapidly. If evaporation from the surface of a lump proceeds at a faster rate than that at which it is replaced by moisture from the interior, the rate of shrinkage at the surface is greater than that at the interior; consequently, stresses are generated at the surface which cause the coal to disintegrate. Likewise, when moisture is absorbed by the air-dried surface the outside layers expand at a faster rate than the interior, which results in further break-down. As these cycles are repeated the lump may disintegrate completely. Therefore, successful storage of low-rank coals depends on control of the factors that influence change of moisture.]

Jan 1, 1941

By Terry R. Neumann

During 1984, at the request of the U. S. Bureau of Land Management, U.S. Bureau of Mines personnel conducted a mineral survey of the 28,600-acre South Pahroc Range Wilderness Study Area (NV-050-0132). The Wit derness Study Area is located in Lincoln County, NV, about 900 miles north of Las Vegas. Five mineral properties and two mineralized zones were examined during this study; of these, one mineralized zone, two small prospect pits, and part of two claims are within Wilderness Study Area boundaries. Perlite resources were identified in the study area; however, no significant metallic mineral values were noted at any of the sites. The Wilderness Study Area is dominated by a Tertiary volcanic sequence consisting of an interlayered series of tuffs containing thin beds of volcanic pebble gravel, sandstone, and perlite. At the base of the exposed volcanic section, perlite occurs along a 2.4-mile-long zone that parallel s the study area's northeastern boundary. The perlite is suitable for filtration, insulation, and soil conditioning purposes, Because of prohibitive operation and transportation costs, the estimated 6 million tons of perlite occurring inside the South Pahroc Range Wilderness Study Area are classified as an indicated subeconomic resource.

Jan 1, 1986

Objective To reduce dust generated from soiled floors within enclosed operator cabs. Background Enclosed operator cabs are widely used on mobile excavation equipment in the mining and construction industries. They protect the operator from bad weather, noise, and airborne dusts. The inside cab environment usually must be kept at very low dust concentrations because of quartz dust generated from the excavation of silica-bearing rock. For cabs to be effective in controlling airborne respirable dust, they must have an efficient air filtration system, while providing a positive pressurization of the cab interior. Ideally, the recirculated and exterior makeup air filtration should provide at least 99% capture efficiency for dust particles as small as 0.3-µmaerodynamic diameters. Many of these enclosed cabs use heating units mounted or directed along the floor (see figure 1), which can cause dust entrainment problems. Prior studies by the National Institute for Occupational Safety and Health (NIOSH) of enclosed cabs have shown that these floor heaters can generate notable amounts of dust within the cab enclosure (see Technology News 486). Multiple-shift dust levels measured inside a drill cab increased on average from0.04 mg/m3 during the summer months to 0.68 mg/m3 during the winter months. Additional dust level measurements, made with optical particle counters during a nondrilling time period, verified that dust levels inside the cab increased from 0.03 to 0.26 mg/m3when the floor heater/fan was turned on. Dust levels within the cab also became greater than the ambient air dust levels outside of the cab when the floor heater was turned on during the nondrilling time period. This shows that the floor heater was a notable dust source problem. Cab floors are commonly soiled from operators tracking dirt inside the cab upon entering from the mine or construction site. Interior cab dust levels can be increased by (1) airflow disturbance of the soiled floor or (2) operator disturbance of the soiled floor. Approach During prior NIOSH surveys of surface coal mine drills, several drills with very low cab dust levels (#0.1 mg/m3) were observed to have floor sweeping compound spread out over the cab floor during the working shift. This initial observation led to a more controlled A-B study of the effect of the sweeping compound on the interior cab dust levels. This study involved dust sampling with and without sweeping compound inside the cabs of a bulldozer and two drills. Sweeping compounds commonly use sawdust or cellulose as the main bulk material, with oil or wax added for dust adhesion purposes. Sand is sometimes added to increase bulk density. Sweeping compounds with sand are commonly used to sweep up concrete floors. Those without sand (usually referred to as "gritless") are used to sweep up smooth finished floors. Most of the oils and waxes used for the adhesive ingredient are petroleum-base and can have an irritating odor. People sensitized to petroleum distillates could have allergic reactions to these sweeping compounds. A few companies offer non petroleum-base sweeping compounds, using either a natural oil or chemical additive for dust adhesion. During this study, it was decided that only the gritless sweeping compound (without sand) would be used, eliminating the addition of another potential silica source inside the cab. Since the sweeping compound would be applied on the enclosed cab floor for the duration of the shift, it was decided to use a canola oil-base sweeping compound to eliminate irritating odor from the petroleum-base sweeping compounds. The natural canola oil-base sweeping compound had a slight woody scent. The three pieces of equipment used in this study had notably different cab layouts. The bulldozer had a small cab with vents discharging air-conditioned or heated air from adjustable louvers at operator waist level and floor level. The bulldozer also had a cloth fabric seat and a carpeted floor. Drill A had a large cab with the heat directed from adjustable louvers at operator knee level and the air-conditioned air directed from cabin ceiling louvers. This drill had a vinyl fabric seat and a steel plate floor. Drill B had a small cabin with the heat directed across the floor (see figure 1) and the air-conditioned air directed down from the ceiling. It had a vinyl fabric seat and a worn rubber mat on a steel floor.

May 1, 2001

By J. L. Konchesky

In gasifying pulverized coal with steam and oxygen, the particle size of the coal fed to a gasifier should be as small as economically possible. A potential way to achieve this advantage is to reduce the particle size of the coal by passing the coal-steam mixture at high velocity through a heated tube coil before the mixture enters the gasifier. To evaluate this method the Federal Bureau of Mines flashed coal-water slurries in externally heated metallic tube coils to determine the extent of size reduction. The first of two different laboratory-scale apparatus reduced the average particle diameter of subbituminous C coal from 190 to 110 microns at an outlet velocity of 330 feet per second (f.p.s.). At the same outlet velocity, a second apparatus reduced the average particle diameter from 250 to 140 microns. An increase in velocity produced more finely divided coal, but variation in temperature and pressure had no effect except as they affected velocity. Virtually the same size reduction was noted for five ranks of coal at equal velocities. The rate of increase in particle-size reduction with increase in velocity decreased with a decrease in particle size. The pulverizing mechanism was concluded to be abrasion of the particles with each other and with the tube wall.

Jan 1, 1961