Search Documents

By W. R. Kube, W. H. Oppelt

Five low-rank coals, including two lignites, a steam-dried lignite, and two subbituminous coals, were carbonized at 940°F, in a bench-scale carbon-ize~ with a nitrogen and hydrogen atmosphere, or both, using pressures from atmospheric to 1000 psig. Coking characteristics were observed, and yields, composition, and distribution of products from thermal degradation were determined. The influence of pressure on yields from coal carbonization, as well as on modification of coke structure, has long been of interest. One of the first attempts at pressure carbonization was that of Capps and Hulett1 who in 1917 investigated the effects of pressure ranging from 1 to 20 atm on yields at 600° C for bituminous and subbituminous coals. High-pressure carbonization has not been used commercially for coke production. More recently, carbonization at relatively high pressure in the presence of catalysts and hydrogen has been proposed for the production of metallurgical-grade coke from low-rank fuels.' Another approach involves carbonization at high pressure in a hydrogen atmosphere to produce heating gas and a solid residue that is gasified with oxygen and steam to produce the hydrogen required for hydro-genation 3-4 or, alternatively, is burned to raise steam. Recently it has been proposed to carbonize coal under pressure, burn the gas in a gas turbine to produce power, and use the solid residue as fuel in a conventional steam-power plant.5 Our primary interest in pressure carbonization stems from an attempt to determine the influence of pressure on the coking characteristics of lignite to design a high-pressure slagging gasifier using oxygen and steam. At atmospheric pressure, North Dakota lignite is generally considered to be non-coking. However, lignite and subbituminous coals have been reported to show increased agglomeration properties in steam, hydrogen, or carbon-dioxide atmospheres at elevated pressures, although no strong coke was formed at pressures up to 3000 psig.6 If lignite would agglomerate seriously under gasification conditions, proper flow of charge through the gasifier could not be maintained without installing a mechanical stirring device or destroying the caking properties of the coal before gasification. Data on the caking properties of lignite or other gasification fuel, as well as on yields of products under conditions simulating the carbonization process in the pregasification zone, were required in the preliminary design stage of the gasifier. No information was available concerning the influence of pressure and type of atmosphere on North Dakota lignite. To provide this basic data, the U. S. Bureau of Mines investigated the effect of pressure and a hydrogen or nitrogen-rich atmosphere on the yield of products and on coke formation in a bench-scale pressure carbon-izer. SCOPE OF INVESTIGATION Five coals were tested at 940°F in nitrogen and hydrogen atmospheres, at pressures from atmospheric to 1000 psig. Yields and composition of solid, liquid, and gaseous products were determined as a function of the process pressure, and attention was given to the physical appearance of the solid residue. Percentage of carbon converted, sulfur retained in char, and thermal balances were calculated. In this paper, primary consideration is given to carbonization in a nitrogen atmosphere and to data concerning the solid phase. The effect of pressure in hydrogen is presented in detail for only one lignite. More complete information on the response of the other coals in a hydrogen atmosphere will be published in a USBM Report of Investigations. DESCRIPTION OF EQUIPMENT Details of the bench-scale pressure carbonizer are given in Fig. 1. The body of the carbonizer was

Jan 1, 1961

By C. R. Fettke, Bela Hubbard

During the summer of 1916, while on a visit to the United States Agricultural Experiment Station at Mayaguez, Porto Rico, the writers were told by D. W. May, the director, that an occurrence of manganese ore had been reported from the mesa southeast of Mayaguez. A visit to the locality showed that the mesa is covered by a mantle of bright red soil resting upon serpentine. Here and there over its surface concretionary masses of limonite have been mashed out of this soil. These were apparently mistaken for manganese ore. On account of the similarity of this occurrence to the extensive iron deposits of northeastern Cuba, it was thought that a brief description based upon a two days'reconnaissance survey of the area mould be of scientific interest. Mayaguez is on the west coast of Porto Rico, at Mayaguez Bay, which is the third in importance of the harbors of the island. The city itself is about 3/4 mile inland on the low-lying plain at the mouth of the

Jan 1, 1920

F. W. Gordon, Philadelphia, Pa.: There can be no objection to the use of cross-compound blowing-engines; and when they are condensing they are susceptible of great range of duty with slight loss in efficiency. By those who have use for the full heat of the waste gases, this type of engine is advantageous; but many furnaces are not so located. The pig-iron is their finished product, and the gases are to be employed only for furnace-use. In this case simple engines are preferable, being less expensive and having less parts to take care of. These reasons determined the introduction of the engines, a sketch of which is distributed at this session. There are three of these engines to furnish blast for two furnaces, for a make of abont 250 tons per day, using 75 per cent. anthracite and 85 per cent. magnetic ores; and they are expected to blow up to twenty pounds pressure per square inch. For blast-furnaces I prefer the single to the coupled engine, except

Jan 1, 1894

By G. G. Griswold, B. S. Morrow

The copper-bearing ores concentrated at the Anaconda plant of the Anaconda Copper Mining Go. are principally a mixture of copper and iron sulfides associated in a gangue consisting of quartz, lightly altered granite and feldspars, with sphalerite and galena and more rarely barite and hubnerite as accessory minerals. The predominant sulfide is pyrite. The copper minerals present are: chalcocite, bornite (erubescite), enargite, chalcopyrite, covellite, tetrahedrite and some tennantite. Normal concentrator practice removes a slime prior to ball-mill grinding. The deslimed ore is ground to approximately 10 per cent plus 65 mesh for flotation. Two selective flotation circuits are used, one for deslimed ore and one for slime. Microscopic studies of concentrator products disclose such an interlocking of copper minerals with pyrite that any material increase in concentrate grade entails additional grinding unless recovery is sacrificed to a considerable extent. During the latter part of 1931 and the early part of 1932 the problem of producing a high-grade concentrate (40 per cent Cu) with at least no

Jan 1, 1935

By Joseph W. Roe

MaNY questions arising in the work of the mining engineer may be solved quickly and with suffcient accuracy by the methods of descriptive geometry; but, unfortunately, this subject is more often considered from a mathematical view-point than as an effective tool for solving practical problems. Some of the principles involved are merged into the ordinary operations of mechanical drawing, and are used unconsciously by the engineer in his every-day work. The rest are stowed away on the book-shelf with his forgotten or rarely-used college text-books. One reason for this condition of " innocuous desuctude " may lie in the uifortunate and often cumbersome notation which it has been found necessary to introduce. If some of these principles can be recalled to the engineer's attenti011 in terms of' every-day use they may prove of practical hearing. Recently, in talking with several mining engineers, I used some of the drawing-room methods of a machine-designer, to the evident surprise of those around me. These men were perfectly familiar with the methods, but their application to mining-work had not occurred to them. At their suggestion the present paper is offered, not in ally way as a treatise, but merely to suggest a certain point of view, in hope that it may lead others far more familiar with mining-problems to pursue the subject further. I. To Find the Strike. As the simplest and most fundamental problem, let it be required to find the strike of a vein when the location and elevation of three points on the outcrop are known. It mill be assumed for the present that the rein is a plane.

Jan 1, 1911

By R. B. Russell

The expansion coefficients of hexagonal (a) zirconium have been calculated from the lattice expansionparameters of both coefficientsofhexagonallow and high hafnium alloys in the range 0° to 600°C. It is found parametersofboththat the coefficients are straight-line functions of the temperature and that the effect of hafnium is to depress the mean coefficients by about 1 pct at room thattemperature and 9 pct at 600°C. The cubic (ß) form was stabilized so that its coefficient could be determined from 400' to 900°C. COEFFICIENTS of thermal expansion for hexagonal-close-packed (a) zirconium were determined up to 600°C (1112°F), without any interference from the effects of preferred orientation, by measurement of the lattice parameters obtained from a symmetrical back-reflection focusing X-ray camera. This paper combines the results for a 1.2 atomic pct hafnium alloy from an earlier report' with those for hafnium-free zirconium. The problem confronting the investigator is essentially that of getting sharp, high-angle diffraction lines which represent the material at a known constanl. temperature and chemical composition. In order to do this, it was necessary to use the proper X-ray emission wave length (Co K was chosen) and to have a fine-grained metal whose atomic spacings would be free from effects of cold work (the specimen was annealed) and to prevent a changing composition owing to gas absorption during the heating of the metal to temperature (the sample was heated in a vacuum). A summary of the effect of temperature on properties is listed in Table I which gives data for ordinary zirconium, as well as new values for the low hafnium metal. The coefficients of the linear thermal expansion that have been reported are given in Table 11. The discrepancies shown may be explained perhaps by the effect of preferred orientation, and McGeary and Lustman2 have done this successfully for sheet metal. The differences in texture cannot explain all the differences in expansion coefficient shown in Table II. liesults by the author agree most closely with those of Kroll.3 Chemical analysis of the high hafnium metal pre- viously reported1 showed that it was 98.22 atomic pct (97.40 wt pct) pure. The low hafnium metal was dehydrogenated Westinghouse crystal bar, designated SMZ-137A. Its analysis was as given in Table 111 (chemical analyses 5377, 5431, and 5432).

Jan 1, 1955

By D. W. Fuerstenau

Locked-cycle grinding experiments on mineral mixtures provide a means to study circuit performance in simulated continuous grinding systems. Such experiments with calcite/quartz mixtures revealed that approximately twenty-five 2-minute cycles are required before the composition of the circuit product and the recycle material attain steady state. The circulating load, after steadily increasing during the first 15 cycles, continued to fluctuate over a narrow range. Previous batch ball mill grinding of mineral mixtures has shown that breakage rate functions of minerals are time- independent but composition-dependent. In the closed-circuit grinding of ores with constituents having different grindabilities, the grinding rate parameters for the components also would therefore change during the transient period. With fluctuations in feed composition occurring randomly every half to one hour in processing plants, it is quite likely that the grinding circuit can be in unsteady state perpetually. The significance of these observations is discussed in this paper in the con- text of the models used in control schemes for the grinding circuits

Jan 1, 1984

By Walter L. Crow

MUCH has been published on highly technical phases of grinding. Very little has been written on how a mill man can improve his operations by determining the optimum size assortment of grinding balls that should be added as a makeup charge. This ball rationing is not to be confused with the ball ration used as an initial charge when the mill is started up. Ball rationing is considered for one or more of the following purposes: 1) to increase throughput of the mill, 2) to reduce the power required per ton of ore ground to the desired size, 3) to reduce steel consumption per ton of ore ground, 4) to improve product size, and 5) to lower retention time of ore in the mill.

Jan 7, 1957

By A. M. Johnson, M. M. Singh

The mechanism of failure under a drill bit is still improperly understood in spite of several investigations of the subject. Generally, the cratering process under static loading conditions is considered to be similar to that achieved dynamically by impact. This paper attempts to indicate that, although the sequence of fracturing in the two cases appear to be identical, at least some dissimilarities exist. For example, the width-to-depth ratios of the craters vary to some extent, and the amount of energy consumed per unit of volume of craters is unequal for the two different loading conditions. Prevalent rock penetration processes are dominated by methods utilizing mechanical attack on rock. It is, therefore, generally accepted that a better comprehension of the mechanism of rock failure under a wedge would prove beneficial towards improving present drilling techniques. Several attempts have been made in recent years to explain how craters are formed under a drill bit, but the mechanism of failure beneath a bit is still improperly understood. 1-11 Most investigators, to date, have inferred the sequence of events occurring during crater formation from analyses of force-time diagrams,1"6 from theoretical considerations,7 or from a study of the configurations of final craters.8-l0 These analyses have led to the presentation of widely divergent models for rock failure beneath a drill bit, ranging from brittle to viscoelastic. The cratering process under dynamic loading commonly is regarded as being similar to that obtained under gradually applied, or 'static', loads. But the effect of rate of loading on the action of a bit is still disputed. Some investigators11-12 maintain that there should be no such effects, whereas others have demonstrated experimentally that these exist.13-17' The purpose of the investigation reported in this paper was to examine petrographically the damage done to rock under the action of a chisel-shaped wedge, both with 'static' and dynamic loading, and to determine if rate-of-loading effects could be detected. Significant quantitative differences in crater volumes and depths were found to exist for a given consumption of energy. On the basis of this data, an attempt was made to indicate some of the rheological properties that a proposed model should possess. All the work reported herein was conducted at atmospheric pressures. EXPERIMENTAL APPARATUS AND PROCEDURE Two types of rocks were employed for most of the experiments reported in this paper, viz. Bedford (Indiana) limestone and Vermont marble. The mechanical properties of these rocks are given in Appendix A. Actually two types of Vermont marble were used, but since no marked difference could be discerned between the two varieties (as seen in Fig. 10) the data was used collectively for the analysis. Stronger rocks were not employed owing to difficulty in generation of observable craters without damage to the equipment. Six-in. diam cores were drilled from the rock samples and embedded in 8-in, diam steel pipe with 3/8-in. wall thickness, using hydrostone to fill the annulus between the core and the pipe. This procedure was adopted to confine the rock specimen so that fractures would not propagate to the edges of the cores. This goal was achieved satisfactorily for these tests because no cracks were observed to extend into the medium surrounding the rock, even when craters were formed only 1 in. from the rock core periphery. Three to four craters were formed on a core face, because the rock damage from any one crater generally did not appear to extend into the others. Whenever, interference between damaged areas around adjacent craters was suspected, the data was rejected for purposes of the analysis. The limestone and marble samples were tested with a 60-degree, wedge-shaped bit, 1 5/8-in. in length, made of tool steel. The bit shank had two SR-4 type electrical resistance strain gages, mounted axially, to record the force-time history during the loading operation. The static indentation tests were conducted using a 50-ton capacity press fitted with an adapter for drill bit attachment. See Fig. 1. The force exerted by the bit at any instant was measured with strain gages affixed to the bit shank. An aluminum cantilever, with two SR-4 strain gages mounted near its clamped end, was employed to measure bit displacement. Both sets of gages were included in Wheatstone bridge circuits,

Jan 1, 1968

By L. A. Wright, H. L. McKinley

PINE Creek mine of Union Carbide Nuclear Co. is some 23 miles northwest of Bishop, Calif., in the Sierra Nevada Mts. Office and mill are 7800 ft above sea level, the 1500 level portal is at 9300 ft, the A level portal at 10,800 ft, and the 3050 portal at 11,500 ft. Original claims were located in 1916, and the mine worked intermittently until 1936, when it was purchased by Union Carbide Corp. Since then the property has been operated continuously, except for two brief periods when operations were suspended because of manpower shortages. Mine production through 1956 amounted to 2,792,425 dry tons of ore.

Jan 10, 1957

By John R. Keith

Lead and zinc contents of elm, maple, and oak trees and of soils in which the trees grew were determined in samples from the Upper Mississippi Valley mining district, and from an area outside but contiguous to the district. The lead and zinc analyses of plant materials served to differentiate the mineralized and nonminer-alized areas. Stems contained about two times as much lead and about two to five times as much zinc as did leaves taken from the same stems. Lead and zinc analyses of all soil horizons could be used to differentiate between the two areas. Soils at most sample sites in the mineralized district seemed to be more reliable than plants for geochemical prospecting and are a more practical sampling medium. Soil samples from areas with thick loess cover, however, failed to indicate the presence of mineralized bodies whereas tree samples from the same sites contained anomalous amounts of lead and zinc. It is suggested that in areas of thick loess cover, plant samples could be used in prospecting for lead and zinc. The important part that plants play in the cycling of elements through the soil horizons is well-known. The concentration of elements in the upper soil horizons by the action of plants has been called the Goldschmidt enrichment principle.' The fact that plant species vary greatly in their ability to concentrate particular elements in their tissues is also well-known.' Agricultural literature has dealt extensively with relationships of the major nutritive elements in soils and plants3 The extent, however, to which plant actions cycle the minor nutritive and the nonmetabolic elements in the soil has not been adequately investigated. Lead is non-nutritive and toxic to plants, whereas zinc is a micronutrient in low, but toxic in high, concentrations.4 This report presents the results of a biogeochemical study concerned with the amounts of lead and zinc in some plants and soils that occurred both within and adjacent to the Upper Mississippi Valley lead-zinc district. Field work for this study was done in the summers of 1961 and 1962. SAMPLE TARGETS AND CHEMICAL ANALYSES Geologic and physiographic descriptions of' the district have been made by Heyl 5 and several other authors in a series of U.S. Geological Survey (USGS) reports and quadrangle maps.6-I Sedimentary rocks, mostly dolomite, limestone, shale, and sandstone, of Cambrian, Ordovician, and Silurian age predominate in the area. Mineral deposits are of the vein and cavity-fill type; the principal ore minerals are galena and sphalerite. Vegetation of the area was characterized by Curtis1 6 as southern hardwood forest interspersed with patches of oak savanna and prairie. Plant genera typical of the eastern United States Temperate Zone, including ash, cherry, dogwood, elm, hackberry, hickory, maple, oak, and other woody and herbaceous genera, are present. The natural vegetation has been greatly modified by agricultural, logging, and mining operations. The species of the original flora are present, but not in their former relative abundance. Soils of this district belong, for the most part, to the Brunizem and Gray-Brown Podzolic great soil groups. The Brunizems are the soils developed under prairie vegetation.'' The Gray-Brown Podzolics are the soils developed under deciduous forests,18 They are derived from both bedrock and from loess parent materials, and

Jan 1, 1970

By Alan B. Nicol, W. T. Parry

The outcome of these tests has been twofold. They have served to demonstrate that the tiltmeter is a useful and dependable instrument which could prove to be of significant value in reaching the goals of landslide prediction and control, and they have raised a good many more questions about the mechanism of slides than they have answered. It seems likely that many of the old concepts and assumptions will be modified or discarded as more knowledge of actual field phenomenon is gained. Safety and economy are paramount concerns in determining slope stability. These two considerations govern the feasibility and character of slope construction from start to finish.

Jan 1, 1970

By John B. Mitchell

Approximately 90% of all potash produced is employed as a fertilizer, either directly or with nitrogen and phosphorus. The remaining 10% is utilized in the manufacture of other chemical products. The original source of potassium was wood ash, however, today the term potash refers to all commercially important salts of potassium. The world's first potash mining operation commenced in Strassfurt, Germany, circa 1860, and since that time the industry has grown steadily to its present annual output of about 30 million tons of potassium chloride products or 18 million tons of Kg0 equivalent, potassium chloride or "muriate of potash" being equivalent to 63.1 % pure KPO.

Jan 5, 1972

By D. C. Livingston

The Pilares de Nacozari mine is located ill Sonora, 75 miles south of Douglas, Ariz. The town of Douglas is on the International Boundary and is the place at which the ores from the Bisbee mines are smelted. The Moctezuma Copper Co. owns the mines at Nacozari, and the copper-concentrates shipped to the Copper Queen smelter, at Douglas, make a good smelting-mixture with the Bisbee ores. The Moctezuma Copper Co. and the Copper Queen smelter at Douglas, as well as the railroad from Douglas to Nacozari, are owned by the Phelps, Dodge & Co. The ore-deposit occurs in the form of a large ellipse with a major axis of approximately 2,000 ft. and a minor axis of about 600 ft., the major axis bearing about 9" W. of N. The

Jan 1, 1913

By R. V. Wheeler

A matter of much concern at present to the coal-mining industry of Great Britain is the effect of the working of the Coal Mines Act, 1930. This Act, which received the Royal Assent on Aug. 1, 1930, contains five parts: (1) production, supply and sale of coal; (11) the reorganization of the coal-mining industry; (111) hours of work; (IV) the Coal Mines National Industrial Board; (V) general. The first three parts are of major interest and discussion can be confined to them. Part I.—Production, Supply and Sale of Coal Under Part I of the Coal Mines Act, 1930, provision is made for central and district schemes to regulate the production, supply and sale of coal by the fixation of standard tonnage, quota and minimum prices. By Nov. 1, 1930, all the district schemes came into force with the exception of that for Scotland, where the majority of the coal-owners did not see their way to accept the amendments to their draft scheme demanded by the Mines Department. A compulsory scheme for Scotland was thereupon prepared by the Mines Department and put into force on Nov. 24, 1930. It is administered by the coal-owners of the district. The essence of the schemes is the allocation of tonnage for each district by a Central Council, and the allocation of quota for each pit in a district by an Executive Board for that district. Up to Dec. 31, 1930, the various bodies of coal-owners engaged in administering the schemes were employed in work preliminary to their full operation. Standard tonnages had to be determined and coals had to be classified. In consequence, it was not found possible to put the regulation of output into force until Jan. 1, 1931. The determination of minimum prices proved to be an even longer process, and only in a few districts were minimum price schedules operative during the first quarter of this year. The working of the schemes during the March quarter, 1931, has been reviewed in an official publication by the Mines Department,' which can be summarized as follows:

Jan 1, 1932

By Claus G. Goetzel

SOME of the reactions and procedures upon which modern techniques in the production of metal powders are based were used for 2000 years by the ancients to reduce iron and other metals from their ores. Despite this, however, the art of powder metallurgy is relatively new and probably stems from the introduction of incandescent electric lamp filaments. Attempts to improve upon Edison's carbon filaments led to the production, in order. of metallic filaments of osmium. tantalum.. and finally of tungsten, which ha, replaced the others and is the commercial product of today. These metallic filaments were originally manufactured by the extrusion and sintering of a mixture of the metal or its oxide with a reducing agent which also served as a binder. Further progress in this direction was in the pressing of a small percentage of nickel with the tungsten. The sintering operation was carried out in an atmosphere of hydrogen, at a tem-

Jan 1, 1944

By S. Wiederhorn

The energy and force of interaction between a crack and a slip band have been calculated. When the distance between the crack and the slip band is greater than the dislocation spacing of- the slip band, the force on the cgrack is perpendicular to the slip band. The energy needed to propagate a crack against this force can be lawe enough to pevmit crack propagation parallel to noncleavage planes. SEVERAL recent experiments have demonstrated the occurrence of crack-slip band interaction in ionic crystals having a NaC1-type structure. Stokes, Johnston, and Lil have studied the nucleation and growth of cracks in MgO. From their observations, they conclude that slip bands have the ability to halt and stabilize cleavage crack nuclei. Gilman2 has shown that a crack, propagating along a (100) cleavage plane in a previously deformed LiF crystal, tends, at certain points within the crystal, to switch to (110) planes. He reasons that plastic deformation reduces the cohesive energy of the (110) planes below that of the (1130) planes, making possible (110) crack propagation. Keh3 has demonstrated that when indentations are made in MgO, crkcks form along (110) planes at the indentations. Gilman2 attributes the formation of these cracks to plastic cieformation. There is some doubt as to the formation of cracks on (110) planes. Kuznetsov4 in his discussion of crack propagation in rock salt, shows that splitting along (110) planes requires 1.76 times as much energy as splitting along a stepped (100) surface. Therefore, the observed (110) cracking in LiF and

Jan 1, 1962

By Albert E. Reed

THE development of the modern high-speed vibrating screen, together with the increasing availability of long-lasting stainless-steel screen cloth for relatively fine-mesh separations, means that more screen circuits will be used in the future. It seems pertinent therefore to examine a few characteristics of screen-circuit products, and especially the points of difference as compared with products of classifier circuits. The most noticeable change that takes place when substituting screening for classification in a grinding circuit is a substantial drop in the circulating load, owing to the greater ease with which the "fines" leave the circuit. Where circulating loads of 500 to 600 per cent are considered fairly normal in classifier grinding circuits, the grinding circuit equipped with screens for equivalent work will have a circulating load of only 20 to 75 per cent. As a result, the products of screen circuits will show a minimum of overgrinding or sliming as compared with a classifier circuit. The desirability of this characteristic will naturally vary according to the metallurgical requirement in each case. Because screen separations make a sharper division according to particle size, the products of screen circuits can be expected to show greater crowding of particles near the point of separation. These changes in product characteristics are so important that other factors, both in the grinding and concentration processes must be adjusted if the operator is to take full advantage of the increased tonnages and higher efficiencies available to him through the use of the screen circuit. One factor that too often is overlooked when changing to screen circuits is the importance of adjusting the ball-mill or rod-mill charge, to compensate for: (I) increased tonnage of new feed, (2) decreased circulating load, or (3) more granular character of feed. These same factors should also be taken into consideration for their effect on the various types of concentration or flotation equipment that may follow the screen circuit. Over a period of years, this equipment has generally been designed and adjusted to the product of the classifier, therefore it seems obvious that some research on the adjustment and adaptation of concentrating and flotation equipment to screen-circuit products will be most profitable to the operators. To illustrate the points mentioned above, the results from three recent installations follow, with data on the comparative characteristics of screen and classifier circuits. A range of separation from 10 to 48 mesh has been chosen, using two general types of screening equipment and two types of classifiers. The first of the installations of which the results are to be examined is that of an iron-ore installation where eastern magnetite was being concentrated prior to sintering. Fig. I shows the original grinding circuit, using classification, while Fig. 2 shows the same circuit equipped with a

Jan 1, 1945

By Albert E. Reed

The development of the modern highspeed vibrating screen, together with the increasing availability of long-lasting stainless-steel screen cloth for relatively fine-mesh separations, means that more screen circuits will be used in the future. It seems pertinent therefore to examine a few characteristics of screen-circuit products, and especially the points of difference as compared with products of classifier circuits. The most noticeable change that takes place when substituting screening for classification in a grinding circuit is a substantial drop in the circulating load, owing to the greater ease with which the "fines" leave the circuit. Where circulating loads of 500 to 600 per cent are considered fairly normal in classifier grinding circuits, the grinding circuit equipped with screens for equivalent work will have a circulating load of only 20 to 75 per cent. As a result, the products of screen circuits will show a minimum of overgrinding or sliming as compared with a classifier circuit. The desirability of this characteristic will naturally vary according to the metallurgical requirement in each case. Because screen separations make a sharper division according to particle size, the products of screen circuits can be expected to show greater crowding of particles near the point of separation. These changes in product characteristics are so important that other factors, both in the grinding and concentration processes must be adjusted if the operator is to take full advantage of the increased tonnages and higher efficiencies available to him through the use of the screen circuit. One factor that too often is overlooked when changing to screen circuits is the importance of adjusting the ball-mill or rod-mill charge, to compensate for: (I) increased tonnage of new feed, (2) decreased circulating load, or (3) more granular character of feed. These same factors should also be taken into consideration for their effect on the various types of concentration or flotation equipment that may follow the screen circuit. Over a period of years, this equipment has generally been designed and adjusted to the product of the classifier, therefore it seems obvious that some research on the adjustment and adaptation of concentrating and flotation equipment to screen-circuit products will be most profitable to the operators. To illustrate the points mentioned above, the results from three recent installations follow, with data on the comparative characteristics of screen and classifier circuits. A range of separation from 10 to 48 mesh has been chosen, using two general types of screening equipment and two types of classifiers. The first of the installations of which the results are to be examined is that of an iron-ore installation where eastern magnetite was being concentrated prior to sintering. Fig. I shows the original grinding circuit, using classification, while Fig. 2 shows the same circuit equipped with a

Jan 1, 1947

By Albert E. Reed

The development of the modern highspeed vibrating screen, together with the increasing availability of long-lasting stainless-steel screen cloth for relatively fine-mesh separations, means that more screen circuits will be used in the future. It seems pertinent therefore to examine a few characteristics of screen-circuit products, and especially the points of difference as compared with products of classifier circuits. The most noticeable change that takes place when substituting screening for classification in a grinding circuit is a substantial drop in the circulating load, owing to the greater ease with which the "fines" leave the circuit. Where circulating loads of 500 to 600 per cent are considered fairly normal in classifier grinding circuits, the grinding circuit equipped with screens for equivalent work will have a circulating load of only 20 to 75 per cent. As a result, the products of screen circuits will show a minimum of overgrinding or sliming as compared with a classifier circuit. The desirability of this characteristic will naturally vary according to the metallurgical requirement in each case. Because screen separations make a sharper division according to particle size, the products of screen circuits can be expected to show greater crowding of particles near the point of separation. These changes in product characteristics are so important that other factors, both in the grinding and concentration processes must be adjusted if the operator is to take full advantage of the increased tonnages and higher efficiencies available to him through the use of the screen circuit. One factor that too often is overlooked when changing to screen circuits is the importance of adjusting the ball-mill or rod-mill charge, to compensate for: (I) increased tonnage of new feed, (2) decreased circulating load, or (3) more granular character of feed. These same factors should also be taken into consideration for their effect on the various types of concentration or flotation equipment that may follow the screen circuit. Over a period of years, this equipment has generally been designed and adjusted to the product of the classifier, therefore it seems obvious that some research on the adjustment and adaptation of concentrating and flotation equipment to screen-circuit products will be most profitable to the operators. To illustrate the points mentioned above, the results from three recent installations follow, with data on the comparative characteristics of screen and classifier circuits. A range of separation from 10 to 48 mesh has been chosen, using two general types of screening equipment and two types of classifiers. The first of the installations of which the results are to be examined is that of an iron-ore installation where eastern magnetite was being concentrated prior to sintering. Fig. I shows the original grinding circuit, using classification, while Fig. 2 shows the same circuit equipped with a

Jan 1, 1947