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By A. McKenzie

During the late 1990s Anglo American PLC was evaluating the Gamsberg zinc deposit in the Northern Cape Province. As the primary South African metallurgical technology provider, Mintek became involved in the evaluation of a pyrometallurgical process for the recovery of zinc from the Gamsberg deposit. eloping DC arc furnace technology for the metallurgical industry. This includes the patented Enviroplas® process for the treatment of zinc bearing waste materials. As a consequence Mintek has developed extensive expertise in the pyrometallurgical processing of zinc bearing materials. However, Mintek?s technologies stopped with the production of Prime Western Grade (PWG) zinc. Early on in the evaluation of the Gamsberg project, it became evident that the refining of PWG zinc would play an important part in the economic viability of the project. At the time the only pyrometallurgical technology available for the refining of PWG zinc to Special High Grade (SHG) zinc was the New Jersey process. This process represents 50-year-old technology and has not undergone significant refinement since the technology was developed. In order to provide a complete pyrometallurgical flow sheet for possible implementation at Gamsberg, Mintek assessed the New Jersey process with a view to either improving the technology or developing a new technology appropriate to the 21 century. In 1998 Mintek convened a consortium consisting of Mintek, the University of Cape Town, Eskom Enterprises, Bateman Metals and Anglo American PLC. The consortium successfully applied to the Innovation Fund for funding to develop a new process for the pyrometallurgical refining of PWG zinc to SHG zinc. The main aims of the project were to develop a new technology for pyrometallurgical zinc refining that conformed to the following: ?Decrease the production cost of SHG zinc metal to stimulate the development of the Gamsberg deposit. ?Be compatible with the current Enviroplas® process, preferably as an add-on to that process. ?Should use electrical energy to benefit from local price tariffs. The project commenced at the beginning of 2000 and consisted of three phases: ?an initial technology search and assessment of the fundamental aspects of zinc distillation. ?the design of a pilot plant for zinc refining. ?the manufacture, commissioning and operation of the pilot plant. A new technology based on a DC arc boiler feeding a packed distillation column was developed and during 2003 the project culminated in the successful operation of a 200 kg/h pilot plant. Ultimately, all of the aims of the project were met and the main outcome is that an advanced process for the pyrometallurgical refining of zinc (the Zincref process) has been developed. The main innovation of the technology was the switch from an energy transfer limited process to a mass transfer limited process (for which a patent has been registered).

Jan 1, 2004

This report provides concepts for two automated longwall mining systems; one for operation in a 42 to 57-inch thick section of the Lower Kittanning coal seam, the other in a 57 to 72-inch thick section of the Pittsburgh seam. The Lower Kittanning concept uses a single-drum bidirectional shearer and 500-ton capacity 4-leg chocks and the Pittsburgh concept uses a double-drum shearer with 500-ton shields. Each concept is designed to produce in excess of 2,500 tons of clean coal per shift with a 6-man face crew. The automated longwalls consist of off-the-shelf control system hardware attached to standard longwall equipment. Control hardware includes a control and display console; prime and backup mini-computers; multiplex data transmission, power, and control lines; several remote terminals which serve as interface junctions; and various sensors and actuators located on the longwall equipment. The system can be operated either fully automatically, remotely or manually.

Jan 1, 1976

By X. M. Zhou, T. Wu, He X. N.

"In order to study the support effect of concrete in the surrounding rock of a metal mine, the concrete and surrounding rock were comparatively studied. Firstly, the general situation of the ultra-kilometer deep shaft in Xincheng, surrounding rock and geological conditions are introduced. Secondly, the experimental scheme of material mechanical properties of surrounding rock and shaft lining concrete based on same standard dimension and shape are examined, and the mechanical tests of rock and concrete based on different shapes and sizes are carried out. The study shows that the compressive strength value and elastic modulus of the rock cube are 1.28 times and 1.78 times that of the cylinder, and the Poisson's ratio is 0.77 times. The dimensional effect law of the concrete cube and cylinder test data is consistent with the rock law. However, the test results of the concrete are less discrete than the rock; the test also shows that the strength and elastic modulus of the concrete lining of the main shaft of the Xincheng are much smaller than the surrounding rock that is also used as the concrete aggregate, which indicates that the support of concrete lining in the surrounding rock shaft behaved weakly. Based on the development of the modern NATM rock support theory, it is suggested that the mechanical test standard of artificial support materials should be unified into the mechanics test standard of natural rock, which will help to improve the accuracy and practicality of scientific calculation of geotechnical engineering.INTRODUCTIONConcrete and rock mass are usually attributed to a class of materials called frictional materials or geotechnical materials, of these two kinds of materials, the first is man-maded and the second is natural. The attentive scholar can find that there are many differences in the test criteria for the mechanical properties of the two kinds of materials among different industries or departments, reflecting the differences in their sources, tradition, and the application thoery of material strength among them.There is a situation wordwide, that there are many testing standards of the same material mechanical parameters such as elastic modulus, compressive strength, etc. The cube with a side length of 150 mm is adopted as a common sample in the architecture department in China, while a cylinder with a diameter of 50 mm and a height of 100 mm is as a standard specimen in geotechnical department.Knowing that mechanical parameter values are strongly affected by the the size, shape of specimen and loading rate etc, that is to say the mechanical parameter values of the same material are completely different when obtained from different test with different standards. However, such a problem has been long neglected in geotechnical engineering."

Jan 1, 2019

By Florian Häckel, Ulrich Schwarz-Schampera, Benjamin Bühring, Katja Schmidt, Dieter Garbe-Schönberg, KoschinskyM Andrea

"The Vanuatu island arc and its adjacent back-arc basins form part of the southern New Hebrides subduction system in the southwestern Pacific Ocean within a triangle defined by the Solomon Islands, New Caledonia and Fiji (Meffre & Crawford 2001). The back-arc region east of the volcanic front is characterized by three structural depressions namely Vate, Erromango, and Futuna (from N to S), also known as the Coriolis Troughs. Earlier studies of lava compositions reveal the occurrence of highly evolved alkali-rich dacites as well as ankaramite basalts within the individual troughs (McConachy et al. 2005).Here we present the first mineralogical and geochemical analyses of hydrothermal fluids and precipitates (sulfides, sulfates, Fe-oxyhydroxides), sediments, and volcanic rocks, sampled during the "Coriolis Volcanism and Vents" (COVOLVE) cruise SO229 in July 2013 aboard the German R/V Sonne. The samples were retrieved during several ROV Kiel 6000 dives directly from the newly discovered active vent sites at the neovolcanic Nifonea Ridge, and from multiple sites at water depths ranging between 200 to 2800 m within the Vate and Futuna Troughs as well as from the shallow water Epi Caldera NNE of the Coriolis Troughs (Fig. 1)."

Sep 1, 2014

By Edgar Stanfield

Since April, 1923, the Scientific & Industrial Research Council of Alberta has had an analyst, Mr. W. P. Campbell, steadily employed analyzing coal samples taken by the provincial Mine Inspectors. Mr. Campbell has also analyzed a number of coal samples taken by one of the Council's Geological Survey parties. We have thus, at the time of writing, over four hundred analyses of Alberta coal, made by the same analyst, and strictly comparable. These samples come from widely distributed points, some 88 townships being represented. The stream of samples is still flowing in, and it is hoped by the end of this year that the number of townships represented will be increased at least fifty percent. The time has arrived to publish some of the data collected; but it must be clearly understood that this is merely a progress report, subject to revision as further information is obtained. Three hundred and fifty-three coal mines in the province were operated during 1923. The production per mine varied from 5 up to 500,000 tons. With this number of mines to examine (and fresh mines are being opened every year) it is a large task to make even a proximate analysis of representative samples from every mine. Any chemist will appreciate the fact that to make ultimate analyses representative of even the larger mines will be a long and tedious task. It has seemed better to make a comprehensive survey by proximate analysis and determination of calorific value before beginning the more thorough survey by ultimate analysis. This paper, therefore, deals only with proximate analyses and calorific values; and, owing to limitations of time and space, does not deal with other chemical and physical characteristics of coal, such as storage quality and fusibility of ash, important though these are.

Jan 1, 1925

By Wittenberg J. C, Reid A. F

A method is described for the rapid and effective production of polished section mounts of ore and mineral particulates to the standards required for automated image analysis. This requires mounting without segregation of particles by mass or density, and it is also preferable to avoid having particles in contact so that they can be recognised as individuals. To achieve this, the particle samples are first split to an appropriate degree in a rotary riffle, and the sub-sample mechanically diluted and thoroughly mixed with an inert filler of the same size range as the particles. Just sufficient epoxy resin is added to make the mixture fluid. During setting, the ore particles are mechanically supported by the inert filler particles, thus maintaining the particles in a random three-dimensional distribution. A rapid two-stage grinding process, based on the successive use of 600 mesh silicon carbide and 1200 mesh (10 um) corundum on cast iron plates, is followed by three stages of mechanical diamond polishing on cloth laps. Hand polishing is entirely avoided, as is the use of papers.

Jan 1, 1984

By A. Stawsky

Effective use of diesel vehicles in underground mining requires attention to improved control of diesel exhaust pollutants such as particulate (soot), oxides of nitrogen and sulfur and carbon monoxide. An experimental survey Is made of diesel emission aftertreatment control techniques including water scrubbers, catalytic converters, exhaust gas recirculation (EGR) and soot filters. Use of two or more techniques in combinations are tested for optimum control of all exhaust pollutants. Dynamometer data is developed for steady state and computer controlled simulated mines duty cycles operation of Deutz F6L-714 and F8L-413 engines. Exhaust Quality indices (EQIs) are calculated from dynamometer data for comparisons among the control combinations examined. The Corning diesel particulate filter (DPF) is shown to be the most promising and practical device. The Corning DPF in combination with EGR provided the best (lowest) EQI from the test data. Use of the DPF requires additional regeneration mechanisms such as a catalytic filter coating or fuel additives. A metal mine field test is made of DPFs on an operational scoop tram to test practical applicability. Further development is proposed for application to various mine conditions and for improvement of DPF regeneration techniques including coating with newly available catalysts.

Jan 1, 1987

By J. W. Thompson

That the governor is hereby authorized to appoint a commission of seven persons, to be known as the industrial accidents commission-two of whom shall be employers of labor, two of whom shall be employees in either mines or industrial establishments of this Commonwealth, or duly accredited representatives thereof, two of whom shall be learned in the law, and one of whom shall be a person skilled and experienced in making investigations-to inquire into the causes and results of industrial accidents in the mines, mills, factories, stores, and upon the railroads, street railways, ships, wharves, and in all industrial establishments, and in all other places where men, women, and children are employed in manual labor in this Commonwealth; to study the most advanced methods for safe-guarding against these accidents; to inquire into the subject of fair compensation for those who are injured in these accidents, and for the families of those who shall be killed as a result thereof; and to study the operation and effect of the Workmen's Compensation Law of 1913. SEC. 2. The chairman of said co~mission shall be designated by the governor, and the person named on said commission as a skilled and experienced investigator shall be the secretary of the commission, The commission shall have power to employ such legal counsel and other officers and employees as it may deem necessary to properly perform its duties. SEC. 3. The secretary of said commission shall receive an annual salary of $2,400 and his actual necessary expenses; and the other members of the commission shall receive no compensation for their services, but shall be allowed their actual traveling and other necessary expenses. The salaries of any other persons employed by the commission shall be fixed by it.

Jan 1, 1920

By Ihsan Berk Tulu

In the Analysis of Retreat Mining Pillar Stability (ARMPS) program, the magnitude of the abutment loading adjacent to a gob area is calculated using an ?abutment angle? concept. The extent of the abutment loading is determined solely as a function of depth from an empirically derived equation. In the recommended calibration method for LaModel, it was believed that the empirical equations for calculating the magnitude and extent of the abutment load, as used in ARMPS, were the best available methods for determining these critical overburden loading values. Therefore, similar calculations were implemented in the LaModel calibration method. However, the latest in situ stress measurements of abutment loading performed in the United States and Australia have shown that there can be significant deviations in the measured abutment magnitude and extent as compared to the predicted values from the empirical formulas used in ARMPS and LaModel. It seems reasonable that the overburden geology, depth, extraction panel width, and mining height should have a significant effect on the extent and magnitude of the abutment load. However, the current empirical formulas do not incorporate many of these significant parameters into determining the abutment load on the pillars. In this paper, stress measurements from US and Australian mines were back-analyzed by using analytical and numerical methods to investigate the measured abutment extent and loading. Ultimately, it was determined that the original empirical abutment extent formula in conjunction with the original ALPS square-decay stress distribution function was supported by the case histories in this paper. Also, for depths less than 900 ft, the average 21° abutment angle was supported by the case histories; however, at depths greater than 900 ft, the abutment angle was found to be significantly less than 21° and should be calculated with a new proposed equation.

Jan 1, 2012

By William R. Yernberg

The 2002 Arizona Conference was held Dec. 8 and 9, 2002, at the Doubletree Hotel at Reid Park in Tucson. The annual conference is organized by the Arizona Conference Board of Directors, which represents SME sections in Arizona and adjacent areas. The conference provides an opportunity for individuals and corporations associated with mining in the region to get together once each year to discuss the status of the industry. The meeting featured a keynote address, a distinguished lecture and about 30 technical papers. As was the case at the 2001 meeting, the economy, the price of copper and foreign competition continue to be the hot topics of discussion among the attendees. The domestic copper industry still faces challenges due to low copper prices dictated by the world market and because of a weakened domestic economy. There continues to be concern about the Arizona region’s copper industry in light of the mergers, downsizing and shutdowns that have occurred during the past several years.

Jan 1, 2003

By E. O. Binyon

"INTRODUCTION The Gibellini manganese—zinc-nickel deposits, are in southeastern Eureka County, Nev, T. 15 N., R. 53 E. M.D.B.M. 27.5 miles south of Eureka, the county seat.The partly explored deposits, locally known as the Black Iron or Gibellini property, were examined in May 1944 by an engineer 3/ of the Bureau of Mines, who recommended that the deposits be explored further by means of core drilling. The proposed program called for drilling six diamond—drill holes, or eight if necessary, and for trenching at two places — 200 feet northeast and 280 feet west of the shaft on the main deposit.In 1946 the project was set up, and In July, of that year diamond drilling as started. Six holes were drilled for a total length of 1,198.5 feet. No trenching or other exploratory work as done. Work on the project was stooped. October 30, 1946.OWNERSHIPAccording to county records, the property comprises a group of 13 unpatented lode claims, and ownership is divided among Louis P. Gibellini,Louis P. Gibellini and wife (both of Eureka, Nev.), and the International Smelting & Refining Co., of Salt Lake City, Utah. Five claims of the group - the Black Iron No. 1, Black Hill No. 2, Manganese No. 3, Flat No. 1, and Flat No. 2 - are owned by Gibellini. Three claims - the Black Hill No. 1, Black Iron No. 3, and Manganese No. 4 are owned jointly by Mr. and Mrs. Gibellini, and the International Smelting & Refining Co. owns the remaining five - the Black Iron No. 4 and No. 5, and Black Hill Nos. 4, 7, and. 9."

Jan 1, 1948

By S. M. Rupprecht

Transportation accounts for some 26% of all mine accidents in South Africa. Although this figure may not be as prolific as fall of grounds accidents or accidents caused by seismic events, the enormity of transportation accidents demands attention, especially as many of these accidents are avoidable. This paper investigates vertical (i.e. shaft), horizontal (i.e. haulages), and in-stope transportation systems and their function as it applies to safety. Recent transportation accidents are discussed looking at current trends within the three transportation areas. In the shaft area, discussions will focus on handling of material cars, loading and off-loading of the workforce and management of shaft orepass systems. The horizontal component will focus on personnel transportation, safety around trains and hoppers, and handling of material in the cross-cut. The discussion on in-stope transportation will focus on the general congestion of the stope environment and how it affects the overall movement of personnel, material, and rock. Of concern are the large numbers of material handling and scraper winch accidents that occur and which will be discussed in some detail. The paper will conclude with a section on proposed guidelines within the three above-mentioned systems for mine engineers to consider in mine design and life of mine planning and scheduling. Keywords Underground logistics, safety in material handling.

Jan 1, 2011

By Gao Dazhao

This paper presents some research results of pile foundation performance. Measurements on the bearing behaviour and driving effects of piles have been carried out during pile testing and pile driving. It has been found that the ratio of end resistance to the ultimate load depends on the pile length, and the ratio in general is less than 40% for single piles. However for pile group, the end resistance seems to be greater than skin friction. The characteristic curves of skin friction and end re­sistance appear as interaction between soils and pile. The contribution of skin friction to the pile bearing capacity not only depends on soil properties but also on the relative position of the soil layer. The pore pressure and the lateral pressure in ground have been observed during the stage of driving piles. Based on the available data, the radius of plastic region and the coefficient of pore pressure A can be found analytically. The pore pressure time process curves which may be used for practice have been predicted by means of numerical method.

Jan 1, 1991

By John C. Niedzielski, A. Frances Ackerman

The existing US-70 Bridge over the Gila River at Bylas, Arizona is planned to be replaced due to its current poor condition. The replacement bridge is planned to be a 15-span, 1,904-foot (580 meter) long bridge, the third-longest bridge in the state highway system. Within the Gila River floodplain, granular alluvial soil is underlain by cohesive lacustrine soil; bedrock was expected due to nearby terrain but not encountered within the geotechnical exploration depth of 166.5 feet (51 meters) or less. Based on the anticipated loading conditions, design scour depths and the soil conditions, the full structural loads are carried by large diameter drilled shafts; therefore accurately determining the axial capacity of the proposed drilled shafts has important practical and safety implications. The conventional methods for axial capacity determination may be overly conservative to use for design in the site’s layered granular and hard clay soils. An Osterberg cell load test was performed to obtain detailed information on the load transfer from the drilled shaft in side friction and base resistance to allow optimization of the bridge foundation design. The fullscale field test played a significant role in the design of the project’s deep foundations, notably, by incorporating the specific subsoil properties and consequently, an exact prediction of the load-settlement behavior under loading. This paper provides the results of the geotechnical site investigations, drilled shaft load testing procedures, and shaft load distribution from the load test results. Comparisons were developed between predicted axial capacities of drilled shafts using several design methodologies and the axial capacity measured from static load testing. The load test results showed that underestimation of the axial capacity is evident using conventional methodologies. The exact prediction of the load-settlement behavior under loading combined with a site-specific design approach, resulted in a substantial reduction of foundation costs due to reduced drilled shaft dimensions.

Jan 1, 2017

By Bill Linville

During fiscal year 1969, the Bureau of Mines was engaged in 66 projects to improve methods for producing and utilizing petroleum. Programs directed toward the continued conservation of this natural resource, including crude oil, natural gas, and oil shale, were carried out through research performed at four laboratories supervised and supported by administrative personnel of this Bureau of the Department of the Interior. Over 100 technical papers and reports were published by the Bureau or others based on this research, and numerous other reports were prepared for U.S. Government agencies, State agencies, and organizations of the petroleum and natural gas industry engaged in cooperative research programs with the Bureau.

Jan 1, 1971

Jan 1, 1997

By G. R. Dunning, M. R. Wilson

"Abstract - The early Paleozoic accretionary tectonic regime that was established along the Laurentian margin of the Newfoundland Appalachians provided a favorable setting for shear-hosted gold mineralization along a major terrane suture, the Baie Verte-Brompton Line. The Stog’er Tight prospect is one of several mesothermal-style gold occurrences hosted by shear zones within accreted ophiolites and oceanic arc terranes on the Baie Verte Peninsula. It is an epigenetic, stratabound deposit that is confined to shallow-level gabbro sills within the volcanic cover sequence of the allochthonous Point Rousse ophiolite complex. Gold mineralization at Stog’er Tight is associated with hydrothermal mineral assemblages represented by chlorite-calcite, sericite-ankerite, red albitepyrite (±Au) and chlorite-magnetite alteration zones. Gold occurs with pyrite within the intensely altered gabbro, along the margins of syn- to late-shear, quartz-rich, replacement veins.The Stog’er Tight gabbro served as a rigid body conducive to shear deformation and fluid penetration, whereas, its high Fe-Ti oxide content induced fluid oxidation and gold-pyrite precipitation. Hydrothermal alteration involved progressive CO2, S, Na, and LILE metasomatism, along with significant enrichments in the REE, HFSE, and Th in the high-grade ore zone. Temperatures of vein formation and alteration are constrained by oxygen isotope thermometry to be between 250°C and 480°C. Isotopic compositions of vein quartz from the Stog’er Tight prospect are uniform (??O = +12.5 ± 1‰) and fall within the range cited for many shear-hosted, auriferous quartz veins.A U-Pb zircon igneous age of 483 +3/-2 Ma for the Stog’er Tight gabbro is consistent with its stratigraphic correlation with other Ordovician ophiolitic and volcanic arc/back-arc assemblages in Newfoundland. The U-Pb age of a rare variety of hydrothermal zircon recovered from the high-grade ore zone directly constrains the timing of gold mineralization to 420 ±5 Ma, in accord with a major orogenic episode of Silurian age that produced many of the magmatic and metamorphic rock suites in north-central Newfoundland. Our results are consistent with a post-peak metamorphic, late-magmatic model for gold mineralization that occurred during the waning stages of Silurian orogenesis, driven by emplacement of I-type granitoid intrusions into the crust."

Jan 1, 2000

Three stages are involved in the complete design and evaluation of an open cut mine. These are:1. design of the optimu,m ultimate pit, and selectionof the optimum mill capacity,2. development of production schedules, with intermediate pit designs, and3. evaluation of the project, based on factors from stages 1 and 2, by means of discounted cash flow analysis.Stage 1 requires relatively complete information on metal distribution, metallurgical performance, and cost factors, which is used to design a series of pits, corresponding to a range of milling rates.For each milling rate considered there is a unique pit design which maximizes operating profitability. The computer outlines this design (in terms of size, shape, and location; tons and grade of ore; and tons of waste), by considering the effect on profitability of mining each block in the mineralized inventory in relation to every other block which would be affected by its removal.The computer selects only those blocks for mining which result in an overall increase in profitability, maintaining pre-determined wall slope conditions throughout.Both operating profitabilities and capital costs increase with throughput, and the two curves will combine to produce a flat peak in net profitabilities at a certain rate, with its associated pit design.Stage 2 can maximize head grades early in the life of the operation. This increases the projected present value, but there are good arguments for maintaining steady head grades throughout the life of the mine.A modification of the programme used in Stage 1 is used here.Stage 3 is a conventional discounted cash flow analysis, using factors obtained in stages 1 and 2. It can be computerised, but a manual treatment is probably just as fast overall.

Jan 1, 1970

By Martin B. Hudson, Daniel Brescia, Zhangwei Ning

Due to the complexity and sensitivity of surrounding environments, instrumentation and monitoring (I&M) is critical for many urban deep excavations. The objects to be monitored include shoring bulkheads, retained embankments, adjacent buildings and roads as well as underground utilities. However, there are also plenty of constraints on the implementation of I&M such as: congested site space, difficult access to instruments, coordination among multiple contractors, and installation permits for private or public properties. Because of these constraints, sometimes the implementation of the initially planned I&M is delayed, its scope is reduced, and/or the continuity of monitoring is interrupted. For these reasons, it is important to implement the I&M with robust and redundant capabilities while providing minimal interference to the project construction and schedule. This paper describes state-of-the-art I&M technologies including automatic motorized total station (AMTS) with camera and automatic leveling device, reflectorless AMTS and IoT- based wireless sensors based on a case study of an on-going deep excavation project. INTRODUCTION In urban environments, deep excavations requiring the construction of temporary shoring bulkheads are becoming increasingly common, with the associated need to provide protection of the surrounding built environment during construction. Protection of the surrounding environment requires planning, engineering, construction, monitoring, and excessive shoring movement mitigation. The I&M is a vital component of the deep shoring implementation. In public projects, geotechnical I&M is often placed within the general contractor’s scope, whereas the majority of deep excavations are privately undertaken. For those private developments, sometimes the developer contracts the geotechnical I&M directly with the project geotechnical engineer or by a geotechnical I&M specialist hired by the project geotechnical engineer. In either the public project or private development case, the effective deployment of the I&M system relies heavily on coordination and collaboration with the project contractor.

Jan 1, 2018

By R. F. Tenney

The performances of four anthracite-burning stokers have been observed under identical 74-hp. steam boilers. Fifty runs of 12 and 24 hours' duration were made in accordance with the test code of the American Society of Mechanical Engineers (ASME). The capacity of each stoker for burning anthracite has been determined, the heat efficiency at each fuel rate has been computed, and the various significant items of hear loss have been estimated. The operating data and results from each run are tabulated in this report in the form used in the test code. The heat efficiencies and most significant operating data for each run are also presented graphically for each stoker to establish the trends with increasing boiler outputs, The tests show that rice anthracite can be burned on stokers adapted to its use at heat-conversion efficiencies of 82 percent or more under continuous firing. These high efficiencies are maintained with fuel inputs 30 percent higher and lower than the peak efficiency rate. Moreover, a heat efficiency of 70 percent or more is obtained at 50 percent of the optimum fuel rate, even though the excess air into the furnace cannot be held below 150 percent.

Jan 1, 1960