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By Lennart Bergstedt, Olov Fägremo

Boliden Metall Aktiebolag, the prime base metal mining company of Sweden, treats some 10-10.9 t/y (11-12 stpy) of sulphide ore in nine concentrators. During the years considerable effort has been devoted to the development of all phases of the technology of ore prospecting, mining, milling, smelting and refining. This has been necessary due to the complex and low grade ores. The autogenous grinding technique was first studied within the company as early as 1945. Basic test work commenced during the early fifties, whereby the merits of pebble grinding could be evaluated. In parallel with the converting of the rod mill/ball mill grinding systems to rod mill/pebble mill systems, the development work concerning the feasibility of autogenous grinding went on resulting in the installation of a 8 6.7 x 2.1 m (8 22 x 7 ft.) cascade mill at the Vassbo lead mine in 1959. Long term tests at the Vassbo plant together with a number of pilot plant test involving low grade silicious ores and pyritic comples base metal ores successive led to a better understanding of the process of autogenous grinding. Before a two stage autogenous grinding system could be chosen for the expansion of the Aitik copper mine, a carefully planned survey concerning grinding characteristics and variances in the ore and flowsheet was undertaken. The expanded plant went on stream in 1972 and in the beginning of 1973 and since then has performed excellently.

Jan 1, 1979

By H. C. Kenny

THE annealing or softening temperature of cold-worked copper is appreciably increased by almost unbelievably small amounts of silver. As indicated by some data in this paper, the softening temperature of high-conductivity, tough-pitch copper may be increased from 200° to 350° C. by the addition of only 15 oz. per ton (0.052 per cent) of that element. Because it has this higher annealing temperature as compared with the silver-free metal, silver-bearing copper finds considerable use in industry. For example, when hard-rolled copper is used in operations that involve soldering or tinning, silver-bearing copper insures against loss of strength, whereas silver-free copper becomes "dead soft." Yet-though several investigators, beginning with Caesar and Gerner1 in 1916, have shown that small amounts of silver do definitely raise the softening temperature of copper-data are lacking on the problem: What is the specific influence of different amounts of silver on this tem-perature? It was to help in filling this gap in the knowledge of what might be termed "alloy coppers" that the data given herein were obtained. The softening of cold-worked copper depends, of course, on a number of factors, of which the composition of the material is only one. Other highly important factors are the degree to which the material is cold-worked and the time it-is held at the annealing temperature. The method of cold working and the grain size of the metal produced are also signifi-cant. All these variables were taken into account in the investigation reported here, and the influence of silver on the electrical conductivity and on the working was also considered.

Jan 1, 1934

By L. G. Huntley

A description f the geology and conditions affecting the occurrence and mining of oil; also the prospects of obtaining oil in different parts of the country. THE Colombian highlands consist of three parallel mountain ranges (Fig. 1) called respectively the eastern, central, and western cordillera of the Andes. A segment of the range that forms the backbone of the Isthmus of Panama also extends into Colombia, along the Pacific coast, as far southward as the mouth of the San Juan River. These four mountain ranges furnish the key to the varying climatic conditions and vegetation of the country. The river valleys are hot and covered with dense tropical growth. This is also generally true of the coastal plain, although in the northern portion there are some fairly open stretches where cattle are raised and even small districts where semi-arid conditions prevail. The population of this heavily wooded wet lowland depends almost entirely on river transportation for all intercourse. The elevated mountainous plateaus of the three ranges are more open and more generally cultivated; all traffic is by means of horses, mules, and the burro. The canoe and river-steamer transportation systems of the valleys are connected with the highland system of trails and roads by short cross trails and the few short railroads of the country. As these cross trails must pass through the intermediate foot-hill region, where the heavy rainfall reaches its maximum, they are exceedingly bad and the life of pack animals is short. The oil development of the Magdalena River valley is in this intermediate region, between the river and the sierras, north of Bogotá.

Jan 9, 1922

By R. Taggart, R. H. Ericksen, D. H. Polonis

MarTENSITIC a has been observed to form spontaneously from the retained ß phase during the preparation of thin foil specimens of metastable Ti-Cr alloys containing from 6.9 to 20 wt pet Cr. Similar spontaneous martensite reactions have been reported in thin foils of 0 brass' and other alloys.2 4 The absence of retained austenite in iron-based alloys has also been attributed to spontaneous decomposition during thinning.5 The present work indicates that a unique situation occurs during which the spontaneous transformation takes place when the transitional ? phase is present in the as-quenched or partially aged ß phase. Alloys containing 6.9, 7.0, 8.0, 12.0, and 20.0 wt pet Cr were prepared by the levitation melting of "io-chrome" chromium and iodide titanium. The 0.5-in.-diam ingots were vacuum-annealed at 1000oC, and wafers 0.020 in. thick were then cut from the ingots. These wafers were wrapped in molybdenum foil, encapsulated in Vycor under a reduced pressure of purified helium, and then held for 30 min at 1000°C. The specimens were quenched in water to retain the metastable high-temperature ß phase. The quenched alloys were thinned by mechanical grinding to about 0.005 in. thick, then electropolished in a solution of 300 ml methyl alcohol, 175 ml n-butyl alcohol, and 30 ml perchloric acid at a temperature of -40°C. Needle specimens 0.005 to 0.010 in. diam were taken from each ingot and heat-treated with the foil specimens. X-ray diffraction patterns indicated the structure of the 6.9, 7.0, and 8 wt pet alloys to be a

Jan 1, 1968

By J. Chipman, J. B. Gero, T. B. Winkler

IN the basic open hearth process the reaction of manganese in the bath with iron oxide in the slag attains a condition very closely approximating true equilibrium, and the distribution of manganese between slag and metal during the refining period is determined by the factors which control this equilibrium condition. For this reason a number of investigators have studied this reaction both in the laboratory and under various conditions of furnace operation. The principle reaction and its equilibrium constant are: FeO (in slag) + Mn = MnO (in slag) + Fe (liq.) [1] (MnO) .= (FeO),[%Mn] The symbols in parentheses represent activities of the two oxides in the slag which, in the simple slags consisting almost wholly of the two oxides, may be assumed equal to the respective mol fractions. The symbol (FeO)r is used to denote the total iron oxide computed as FeO; or it is the sum of the mol fraction of FeO plus twice that of Fe2O3. Early attempts to obtain laboratory data on this reaction suffered from failure to appreciate the great speed of the reaction. When slag and metal are cooled slowly together, the equilibrium shifts with temperature and the final compositions correspond to equilibrium at the freezing point rather than at bath temperature. The early work of Oberhoffer and Schenck¹ and of Tammann and Oelsen² apparently suffered from this source of error. In most of the other published laboratory investigationsa. '.' this error was not present but other uncertainties have existed, notably dependence upon optical temperature measurements and exposure to atmospheric oxidation.

Jan 1, 1951

By B. Egeberg

BRITANNIA metal is a white alloy consisting primarily of tin and antimony, the tin greatly predominating. The alloy usually contains a small amount of copper and occasionally very small amounts of one or two, other metals, such as bismuth, zinc, and lead. It is capable of being cold-rolled, it takes a high polish, and it is fairly noncorrosive; properties which make it a popular metal for many household and traveling utensils. Britannia metal is similar in many respects to pewter and has largely replaced pewter in the arts, but it should not be confused with that alloy, because pewter is essentially a tin-lead alloy, while Britannia is a tin-antimony alloy. In view of the fact that so little has been published concerning Britannia metal, the authors feel justified in presenting the results of their investigation, particularly since the behavior of this alloy under cold rolling and heat treatment differs so greatly from that of other metals and alloys. The old craftsman, engaged in the making of ornaments or tableware, heated the cold-rolled sheet metal or his finished product to a straw color corresponding to about 375° F., claiming to produce thereby a harder and more rigid article. The fact that possibly led him to this heating was that a reheated article or sheet made from Britannia metal has a distinct metallic ring when struck a sharp blow, whereas an unheated article or sheet has a leady, dead note. No doubt he has associated the metallic ring with a greater rigidity and strength, and justly so, as will be seen later.

Jan 1, 1928

By H. S. Gieser

THE flotation of oxidized silver, ores offers an interesting problem to the operating metallurgist, who has to treat this material. The advances made in the art of selective flotation by the use of newer collectors with accelerating and inhibiting reagents suggests that there should be something to help to decrease tailing losses. For many years there was no working hypothesis, so the subject was approached more or less in the dark, but with the publication of a paper by Taggart, Taylor and Ince1 giving results of flotation tests with many organic compounds and presenting a tentative theory, a better foundation has been laid and henceforth progress should be more rapid. Over a period of one and one-half years there was much experimentation with flotation of oxidized silver ores at El Tigre, Sonora, Mexico. The material coming to the mill was straight sulfide ore in a silicified rhyolite gangue, the ore minerals being sphalerite, galena, pyrite and tetrahedrite. This came largely from the lower levels of the mine. With the approaching exhaustion of these orebodies, attention was turned to oxidized ores left behind during higher prices of silver. Of late oxidized ore has been at times 25 per cent. of mill tonnage. Aerofloat 25, in considerable quantity, together with ethyl xanthate, has been used in the mill. The former raises large quantities of "insoluble" or gangue, which is difficult to reject when the density of the tailing pulp approaches 33 per cent. solids, as it did during periods of water scarcity. The mill tailing at times contained as much as 5 oz. of silver on a 32-oz. mill head.

Jan 1, 1931

By James Boyd

REACTIVE metals are not only those sufficiently radioactive to be used as fuels, such as uranium and thorium, but all metals that will find application in power reactors. It is required of such metals that they be relatively inert or chemically unreactive, dimensionally stable at high temperatures and pressures, and either very absorbent of radioactivity or transparent to it. To gain the right perspective concerning these metals it is necessary at this point to explode some erroneous concepts propounded so often they have become accepted as fact. The first of these is that the great industrial nations, especially the U. S., have reached their pre-eminent position chiefly because they possess abundant mineral resources. Some U. S. mineral economists, echoed by the politicians, have taken this position so often that many of their theories and policies are based on the presence of developed mineral resources as a sine qua non to the country's greatness. Actually the reverse is true- under a social and political system granting free rein to the initiative and creativeness of its people, many hidden mineral resources that otherwise would have lain fallow have been developed into productive enterprises to the benefit of the national industrial economy. Again, once the proper incentive was furnished, other countries have also been able to establish valuable mineral enterprises in recent years; resources long dormant for lack of a proper economic environment suddenly have become valuable and productive. It is possible to name immediately several nations where such domestic economic developments have had favorable repercussions within the last decade. The absence of productive mines in any country is no indication that workable deposits do not exist within its territory.

Jan 11, 1957

By M. G. F. Sohnlein

IN a Bolivian tin concentrator an appliance was needed to furnish a suitable product for fine jigging from a pulp of the following composition: Mesh Per Cent. +20 8.0 +40 36.5 +60 9.0 +80 10.5 +100 5.5 +150 4.51 -150 25.0 99.0 Loss 1.0 IN the jigging process, particles of cassiterite as small as 0.1 mm. can be recovered, provided they are not present in excess, that is if the material treated on the jigs contains sufficient coarser sands to keep the interstices between the grains open. Removing the fine material from the pulp by screening, to prepare the jig feed, is impractical, because that means the use of an 80-mesh screen or finer, and is not as effective as hydraulic classification. A screen would eliminate mineral grains from the feed which can be recovered by jigging, and in addition would throw more gangue and low-grade middling on to the jig. Formerly a one-spigot Richards vortex classifier had been used to accomplish this separation, but the tangential openings through which the hydraulic water enters became clogged by fine vegetal matter with which the water was contaminated, and which it was impossible to remove from the water. Consequently the work of the classifier was imperfect and a good deal of slime was sent to the jig. When designing the plant referred to in this paper it was decided to use a hydraulic classifier as shown in Fig. 1. The essential features had been copied from the Anaconda classifier.1 This remodeled classifier consists of a truncated pyramidal wooden hooper A with an attachment of cast iron

Jan 4, 1916

By R. H. Richards, C. E. Locke

In "Sorting Before Sizing " (a paper first announced at the Pittsburgh meeting, February, 1896, but delayed in preparation and now presented at the present meeting) it is shown that if slime-tables are to do their best work on slimes below 0.5 mm. or 0.05 inch in diameter, they must be carefully sorted or classified, and fed to a series of tables suitably adjusted to the different slime-sorts. The reason is that the coarser and finer slime-sorts require respectively different adjustments of the tables. Hence, if the two are fed together upon a table which has, as nearly as may be, average adjustments, fine galena, for example, will go into the tails, or coarse quartz into the heads, or both—losses which could have been largely prevented if the slime had been carefully sorted and each sort sent to its own slime-tables. As indicated in the paper just mentioned, the present tendency in this country is generally to do away with slime-sorting or "classification," and simply to take the overflow from the hydraulic classifier and divide it among the tables by means of

Jan 1, 1898

By A. J. Brown

THE causes that have formed fissures in the earth's crust, and the agencies that have converted them into metallic beds, are amongst the most important and interesting subjects that can engage the attention of the mining engineer or the scientist. They lie at the very base of the whole system of vein geology, and until something like a correct theory can be arrived at with regard to them, the science as such is necessarily very incomplete. The data for such a theory, owing to a lack of careful and systematic observations in countries pre-eminently metalliferous, like Mexico and the States of Central and South America, are almost entirely wanting. Doubtless there has been abundant speculation on the subject, from the earliest times down to the present, in every country where mining has been one of the leading industries of the inhabitants. Perhaps the theory of Elie de Beaumont more nearly approaches a satisfactory explanation than that of any contemporary authority. The most recent investigations, both in Europe and on this continent, tend to produce the conviction that the phenomena of metallic fissure lodes are in some way connected with, and- dependent upon, plutonic agencies-not, however, as the direct product of volcanic activity, but rather as the result of chemical agencies called into play through its aid. Active volcanoes generally produce only the well-known types of volcanic rocks, though, perhaps, well-marked cases can be shown where these contain ores of copper, etc., evidently derived from the same deepseated source as their hypogene gangue. To prove that there is a connection between metallic lodes and plutonic rocks, it is not necessary that the ores should in all cases occupy fissures within the limits of masses of propylite, rhyolite, or other like rocks, or be

Jan 1, 1874

By J. W. Pugh, Sam Leber

Single crystals of tungsten were made and deformed in tension at 3000°C. The slip traces so formed on these crystals were analysed to determine the apparent slip system. Results indicate that deformation occurs in the<III> direction on planes of maximum shear stress. This determination agrees with the early work of Taylor and Elam on other body-centered cubic crystals at lout temperatures. SLIP in face-centered cubic and hexagonal close-packed metals has consistently manifested itself in straight traces which indicate that glide takes place on planes of highest atomic density and in directions of closest packing. The slip traces observed on body-centered cubic metals are, on the other hand, frequently branched and wavy, and indicate that while glide takes place in the direction of closest packing, it does not necessarily choose the plane of highest density. The early work of Taylor and Elam1 4 indicated that glide took place in the close-packed direction on planes close to those defined by the maximum shear stress. Subsequent investigations have tried to rationalize these observations in terms of an elementary process on the close-packed planes (110). For example, Elam5 and Grenin~er&apos; have suggested alternate glide on nonparallel (110) planes with the observed result that the summation of these glides results in a trace which defines some plane other than{110). Madden and chen7 have recently reviewed the phenomenological and theoretical aspects of noncrystallographic slip. More recently Koehler&apos; has presented a model of dislocation propagation which permits another rationalization. He indicated that dislocation loops are transferred from one glide plane to another by double cross-slip of screw segments. It is reasonable to assume that, on the average, such transfers may result in a trace which approaches the plane of maximum shear stress. The work described in this paper will show that

Jan 1, 1961

Charles R. Eeyes, Des Moines, Ia. (con~munication to the Secretary*):—It is not in a spirit of criticism that I offer a supplemental suggestion or two on the subjects covered by this valuable and highly instructive memoir. There are two points which, in my opinion, should have received greater emphasis in Mr. Emmons&apos;s excellent paper. One is the fundamental r6le played by the chlorides under certain conditions in ore-formation. The other is the possible establishment of geographic relationships among the four pheilomena of (1) excessive chlo-ridic content of mine-waters; (2) the abundance of chloridic compounds of the precious metals; (3) the presence of man-gancse oxides; and (4) the diminishing importance, in ore-genesis, of the metallic sulphates. Although Mr. Emmons&apos;s notes refer to gold alone, it may be pertinently asked whether the same principles do not hold good for silver and copper also, since these metals form, together with gold, a distinct and well-known chemical group. That the reactions involved apply equally well to the other two metals mentioned, is sbown by a number of recent observations and discussions. Chloridic ores of silver are, as I have lately endeavored to show, mainly worked in arid or desert regions only; and the great deposits of disseminated copper-ores are similarly characteristic of such regions, in which both classes doubtless owe their formation to the abundance of saline materials derived from desert dusts, and to the plentiful and almost universal presence of manganese oxide. Under these climatic conditions, silver is somewhat more abundantly deposited than copper and gold, because its chloride is so much less soluble. During volcanic emanations, the metallic chlorides perform at all times distinct functions; when these are

Jan 1, 1912

By C. E. Arnold

A BRIEF description of this installation was included in a recent paper by H. Kenyon Burch.1 The purpose of the present paper is to amplify Mr. Burch&apos;s description, as it is felt by the writer that the subject merits more than passing attention. In making a comparison between the common method of hoisting men in mine shafts, and elevator practice in high buildings, the former shows to great disadvantage both as regards safety measures and smoothness of operation. It is surprising that in view of the inducements offered, the step of applying the building equipment direct to the mine shaft has not been taken before, as in many respects the conditions of service are similar, and the objects to be attained are identical. Even without doing this, it is certain that safety to life and limb in the usual method of hoisting men underground can be greatly enhanced by the application to the mine hoist, of some of the features of the building elevator. In the case under consideration, the elevator cage runs in a vertical, concrete-lined shaft compartment 5 ft. 11 in. by 5 ft. 6 in. (1.8 by 1.68:m.) in section and 585 ft. (178 m.) deep, while the counterweight runs in the pipe and ladder compartment of a twin shaft 102 ft. (31 m.) distant. The section of the division of this compartment occupied by the counterweight measures 2 ft. 4 ¼ , in. by 3 ft. 10 ½ in. (0.72 by 1.18 m.), and the counterweight is made in the form of a small cage which. suffices to carry two men for the purpose of making inspections or repairs in the pipe and ladder way.

Jan 12, 1917

An "international gold conference" was held at Spokane, Wash., on Sept. 5, 1918, under the auspices of the Northwest Mining Association, which was attended by a large number of engineers, mine operators, and bankers, representing the northwestern states, as well as Alaska and British Columbia. The principal subject under consideration was the doleful condition of the gold-mining industry, gripped between the diminished purchasing value of its product, and the increased cost of its operation. Two resolutions were adopted by the meeting. The first approved the resolution adopted Aug. 12 by the American Gold Conference, a permanent organization formed at Reno, Nev., in which it is stated that relief by the United States to the gold producers of this country should he made to correspond with the increased cost of production, the extent of the relief to be fixed from time to time to Meet changing conditions. The second resolution, adopted after serious and long-continued discussion, was introduced by F. A. Ross, mining engineer, of Spokane, and a member of the Institute, and was as follows: WHEREAS, it is now clearly understood and agreed by the allied nations that national gold reserves must be augmented without delay if rapidly expanding credits are to be secured; and WHEREAS, at this time, the production of gold is daily decreasing to an alarming extent because of well known conditions that, render the mining of gold-bearing ores unprofitable; and WHEREAS there is no way by which the mining of gold-bearing ores may be made profitable except by the direct&apos; aid and influence of the government; and WHEREAS the government has appealed to the mining industry for practical information and practicable suggestions upon this subject; therefore, be it

Jan 10, 1918

By T. H. Aldrich

This paper is intended only to give a preliminary account of experiments made, and conclusions reached, concerning the treatment of certain refractory low-grade gold-ores, the profitable reduction of which may furnish a useful guide to the solution of similar mining and metallurgical problems, not only in the gold-bearing belt of the Southern United States, but perhaps in other regions also. It must be regarded as simply a " report of progress," preceding a more elaborate and conclusive report of regular practice. The mines of the Hillabee Gold Mining Co. are located upon what is generally known locally as Hog Mountain, on the northern edge of Tallapoosa county, Ala., about 15 miles from Goodwater and 14 miles from Alexander City, stations on the Central of Georgia railway. Gold has been known since 1839 to exist on this property, where it was discovered in that year by a man named Johnston. The property was worked for placer-gold by the old miners, and there are now the remains of four attempts to operate it by stamp-mill and plate-amalgamation. The present owners purchasid it in 1896, and since that time they have experimented at frequent intervals, in order to devise a process that would satisfactorily recover the gold. In 1903 regular operations were begun. Geologically, the mountain consists of an intrusion of what has been pronounced true granite in altered sedimentary slates. The cooling of the granite produced shrinkage-cracks, which have become veins. There is considerable question as to the period of the slate formation; but it is undoubtedly, in my opinion, of Carboniferous age. The veins, about 50 in number, strike N. 60° to 80° E.; dip approximately 65&apos; from the horizontal; vary from a few inches to 25 ft. (being in gen-

Jan 1, 1909

By Halbert Powers Gillette

FROM the known laws of physical chemistry I believe it can be shown that progressive mass movement of water solutions in channels has seldom been the means of ore-concentration in veins. It is my purpose in this paper to show that the force known as “osmosis” has been the principal factor in ore-formation. Convection-currents have doubtless supplemented osmosis, and the two working together have been the agencies that have gathered the scattered particles of rare minerals into the larger masses which are called ore. Osmosis is commonly thought of as a vague, feeble force that causes slow diffusion of dissolving matter through the solvent. That it is a measurable force, often of great intensity, few know, except those who are familiar with the laws of modern physical chemistry. Therefore, I may be pardoned for going briefly into the elementary mathematics of osmotic pressure and of convection-currents. The velocity of water moving through any channel is given by the Chézy formula: (1) v = c [/rs], in which r is the hydraulic mean radius; s, the slope of the channel in which gravity is the sole propulsive force; and c, the coefficient of roughness of the sides of the channel. From this formula it is evident that the smaller the channel, the smaller the velocity of water under a constant head. In rock-crevices, therefore, where the head is constant, water moves much more slowly than in large fissures, and in moving through sand or porous rock it is to an even greater degree retarded. Hence, if a mass of water-saturated sand be heated, the finer the sand-grains, the more slowly will the temperature rise at parts remote from the source of heat. Conversely, cooling is retarded the smaller the interstices or channels through which the convection-currents move. The importance of this

Jan 1, 1913

By D. Turnbull, A. M. Turkalo

It is well known that if copper that has been severely cold-worked by rolling (70-98 pct) is heated to high temperatures a small number of large grains are formed at the expense of the fine grained structure that appears upon primary recrystallization. Dahl and Pawlek,&apos; Cook and Mac-Quarie,2 Ward,3 and Boas and Bowles4 have studied this phenomenon in copper. It has been reported as taking place with measurable velocities in the temperature range 700-1080". Boas and Bowles have reported that the time necessary for the transformation is of the order of days at 700°C. According to Ward3 the number of secondary grains which appear in OFHC copper decrease with increasing perfection of cubical alignment after primary recrystallization; also the perfection of cubical alignment increases with decreasing grain size prior to the final reduction. In this investigation two heats, A and B, of especially high purity copper (99.999 pct) supplied by the American Smelting and Refining Co., were used. The behavior of samples prepared from the two heats was qualitatively very similar providing that the grain sizes established prior to the final reduction were about the same. We observed that in specimens prepared from bars which were relatively coarse grained prior to final reduction secondary recrystallization took place at a rapid rate at temperatures as low as 500°C. This is about 200-300°C lower than the temperature necessary to obtain fairly rapid transformation in OFHC copper. A 0.22 in. thick rectangular bar of lot B was cold-reduced 60 pct by straight rolling and given a penultimate annealing treatment at 250°C. Following this it was cold-reduced 90 pct and heated to various temperatures in the range 450-1040°C. The specimens were about 8 mils thick

Jan 1, 1950

By V. S. Rood, J. A. Norden

The general system of workings at the Utah-Apex is similar to that found in many of the western metalliferous mines. There is a vertical three-compartment shaft extcnding to the surface, from which the various levels have been driven to the orebodies. The 1000-ft. (305-m.) level, about 3/4 mile long, was driven as a haulage and drain adit. Geology The formation is a flat north-dipping series of alternating limestone and quartzite beds. Extensive north-south faulting and fissuring have taken place, resulting in the formation of large replacement orebodies in the crushed zones in the limestone. The orebodies are irregular in shape and size, but follow closely the trend of the mineralizing fissures. The ore consists of galena, pyrite, sphalerite and small values in silver and gold. Large masses of non-commercial pyrite are frequently found in or adjacent to the orebodies, and more or less pyrite is found for some distance in the walls of the orebodies. Lenticular bodies of a black pyritic shale also occur in the zones where extensive faulting has taken place. Mining Methods and Condition of Workings Geological conditions, mentioned before, have made the ground extremely heavy. In mining above the 1000-ft. level, square-set and filling methods were used, but not altogether successfully. The crushing of timbers extended even into the filled portions of the stopes. The filling material itself, consisting of limestone, shale, and pyrite, possessed swelling qualities which added to the difficulty of maintaining

Jan 1, 1920

By H. Rush Spedden

R. T. Hukki has extended his study of ball mill grinding at supercritical speeds with an excellent mathematical analysis of tumbling mill grinding. When he speaks of supercritical speeds he is referring to the original Fahrenwald critical-speed formula. He said that, as an example of the use of supercritical speeds, if a fine grinding mill of any type is wet-grinding ore of 3.5 sp gr with steel balls at a certain conventional subcritical speed, the same power input and capacity should be obtainable with autogenous medium of 3.5 sp gr at a speed 100 to 120 pct of the critical in a grate- type mill of the same effective size. In addition, he introduced what he calls the base mill concept which, he stated, makes it possible to compare performances of mills of different sizes grinding with a metallic medium or with an autogenous medium of any specific gravity and at any speed. He showed the use of this concept and explained how it makes possible the evaluation of mill sizes and motors for any grinding purpose at any accuracy well within the conventional standards of design in milling practice. He further stated that the maximum capacity in pilot plant mills with various media, either metallic or autogenous, has always been found to be within the supercritical speed range. In his discussion of this paper he substantiated these findings with statements showing the increased capacity obtained in a milling plant in Finland which followed his principles.

Jan 7, 1960