Search Documents

By Scott A. Walker, Frank L. Rollo

"The San Francisco Museum of Modern Art (SFMOMA) in downtown metropolitan San Francisco, California, is supported on a mat foundation. Although the mat foundation of the original museum was designed to accommodate a future expansion, the plans for expansion that developed in 2010 required a larger footprint to accommodate the new space requirements of the museum. The new, larger expansion was designed by lead architect Snohetta. It will be a tall 12-story, steel-framed building occupying an area that was once the eastern part of the existing museum as well as two adjacent and newly acquired parcels.As designed, the expansion includes a 30-foot-deep excavation which is surrounded by adjacent buildings and improvements. Because of variable subsurface conditions at the site, geotechnical challenges included mitigating anticipated differential settlement between the existing and new foundations, preventing overstressing of the existing mat foundation, controlling groundwater to prevent settlement of surrounding improvements, and providing temporary support of adjacent structures during excavation for the new basement.To address several of the geotechnical challenges, soil-cement mixed (SMX) ground improvement was recommended by Treadwell & Rollo. However, during contract negotiations and construction, the specialty contractor proposed using jet grouting as an alternative technique for ground improvement. The jet grouting approach was accepted in principal, but when implemented was only marginally successful. Accordingly, the contractor eventually installed SMX panels using a Cutter Soil Mixing (CSM) machine. Because the CSM machine was a late arrival during construction, the contractor was not able to take advantage of the ground-improvement technique to install many of the shoring elements that we had originally recommended (such as the cut-off shoring wall). In the end, SMX panels were installed as ground-improvement beneath the new mat foundation to (1) reduce the potential for liquefaction in clean sand below the groundwater by providing increased rigidity to the soil, (2) reduce the potential for disturbing a weak underlying marsh deposit during construction, and (3) transfer vertical building loads to a layer of underlying dense sand.This paper also discusses the quality control and quality assurance programs that were implemented to check that the ground-improvement techniques used were thorough and complete and provided the required strength to transfer the new foundation loads to the underlying strata."

Jan 1, 2014

ONE DAY IN the mid-1880s, in a suburb 0f Pittsburgh, Pennsylvania, there dismounted from a train the two foremost leaders of the day in the coal and steel industries-Andrew Carnegie and H. C. Frick. Their attention was drawn to a well-loaded car of coke, which their experienced eyes recognized as a superior product. "I'll bet you that coke is from the H. C. Frick Coke Company," Mr. Frick challenged. "I'll bet it was shipped by my company." In a spirit of fun Mr. Carnegie accepted the wager. Mr. Frick lost the bet: the coke had come from M0rewood, one of three sites being operated under the direction of a young entrepreneur, Erskine Ramsay. If Mr. Frick's confidence was somewhat dented by this occurrence, the responsibility was his own. He himself had opened the way for Erskine Ramsay to become the youngest of mine superintendents; and he had dispersed all dissension regarding his choice with the words : "I'm not interested in his years, I'm interested in his record. He makes more coke in an oven than any other superintendent. And I'll take 'em out 0f the cradle if they can make coke... Suppose he is only twenty years old? ... The d --- kid is a genius, I tell you." Erskine Ramsay was to follow in the tradition of Carnegie and Frick. Not only was he ambitious and a worker his background had prepared him for it. Coal mining was in the Ramsay blood. His father, Robert Ramsay, of whom the son once stated that he had no superior as coal mining and mechanical engineer in this country, once accounted in these words for the phenomenal accomplishments which had attached to the name "It does no harm to belong to a family of mining engineers who are just as good as any in America-if it is myself that says it." Anyone acquainted with Robert Ramsay's character realized that he was not given to overstatement. And the above-mentioned Mr. Carnegie encountered young Ramsay again, a few years later in his career, and wrote of him to young Erskine's father, that he was "going to be a credit `tae us a' " (a quotation from Robert Burns.) Nor have Mr. Ramsay's achievements been identified exclusively with the industrial field in which he early asserted himself. He has been an inventor whose techniques in mining engineering have received international recognition. As an investor of foresight, he has benefitted others immeasurably, as well as himself, being an influential factor in the upbuilding of several communities in his chosen and well-loved adopted home state of Alabama since 1887. He is a philanthropist whose benefactions have run into millions, the chief beneficiaries being educational and charitable institutions. Amidst his accomplishments the lighter side of life has not been neglected and he is known for his hospitality, his

Jan 1, 1953

By Clark P. Mangelsdorf

The report consists of two parts. In Part I is a state-of the-act study of roof truss technology as of January 1979. Included are a history of the truss concept, a review of analysis techniques, a record of hardware available and equipment used in its installation and numerous examples of user experience. Part II is concerned with a study of the role of friction in the behavior of a roof truss. The appendices include laboratory tests by the author.

Jan 1, 1979

By Aly M. Mohammad

State and federal agencies during rehabilitation of existing bridges may require the existing bridges to be evaluated under seismic conditions. If the bridge does not perform under the seismic event, the bridge may require seismic retrofitting. Bridges are classified as: Critical, Essential, and Others according to their importance. Geotechnical considerations play a major role in evaluating the bridge behavior under a seismic event. Subsurface conditions influence the magnitude of ground acceleration and hence the forces and moments on the superstructure. This paper will provide a detailed description of the geotechnical aspects associated with the seismic retrofit evaluation for the 12th Street and 14th Street Viaducts located in Jersey City, New Jersey. The paper will also present a description of the geotechnical investigation program including seismic cross-hole surveys to determine the dynamic soil parameters, including the results of site-specific response spectra for both the 475 and 2475-year events. Liquefaction assessment methodology utilizing an equivalent factor of safety will also be described and the design method used will be summarized. In general, the pile foundation is modeled as 6x6 foundation stiffness matrix with 36 stiffness coefficients at the center of the pile cap. The seismic loading on the foundation is evaluated in a pseudo-static analysis, where the dynamic earthquake-induced loads are presented by static forces and moments. The computer program FB Pier was used to evaluate the foundation stiffness. The moments, axial loads, and shear are input into the FB Pier model. The stiffness of the foundation is determined and the results are then input into the Structural model to determine new forces and displacements. Adjustments are made until the loads are compatible with the stiffness used. The understanding of the geotechnical conditions at the 12th Street and 14th Street Viaducts and the coordination between the Bridge and Geotechnical Engineers were able to provide significant cost saving to the Owner. The Design Engineers were able to retrofit the existing Viaduct foundations with reasonable number of additional piles. It is believed that the results and finding presented in this paper will benefit both the Bridge Engineers and Geotechnical Engineers by highlighting the importance of interaction between the two disciplines.

Jan 1, 2006

By D. S. Kieffer

Ground characterized by hard boulders embedded in soft matrix represents one of the most challenging conditions for TBMs, as the cutterhead must cope with the non-optimal conditions of cutting hard blocks while simultaneously excavating through soft, soil-like matrix material. Dowden and Robinson (2001) and Hunt and Mazhar (2004) summarize several case histories relating to the problems of advancing a TBM through boulder-laden ground and provide guidance with respect to appropriate site investigations and baselining strategies. Some rock types constituting boulder-laden ground include: ?Sedimentary Rocks?cemented alluvial channel deposits, colluvium/talus, landslide debris, and glacial till. ?Igneous Rocks?agglomerates, lahars, and pyroclastic deposits. ?Metamorphic Rocks?tectonic mélange and cataclastic fault zones. ?Decomposed Rocks?corestone bearing saprolites. Disc cutters are commonly utilized to cut the hardblocks; however, as compared to the understanding of disc cutter behavior in hard continuous rocks, little is known regarding fundamental cutting mechanisms in highly heterogeneous bouldery ground conditions. To advance this understanding, a suite of full scale tests have been performed using a linear cutting machine at the Colorado School of Mines (CSM). This paper summarizes the observed cutting mechanisms and their practical implications for TBM excavations through boulder-laden ground.

Jan 1, 2008

By Michael Valencia, Alonso Marchena, Fredy Arcos, Miguel Chumbe

Cobrepampa es un depósito de cobre ubicado en Acarí (Caravelí-Arequipa). La mineralización se emplaza en pórfidos dioríticos pertenecientes al intrusivo Cobrepampa (Superunidad Linga) y está constituida por bornita, calcosina, covelina y calcopirita y en la zona de oxidación cuprita, crisocola, malaquita y hematita. El magmatismo es calcoalcalino con alto K. El intrusivo Cobrepampa evoluciona por fraccionamiento y contaminación en una corteza más delgada que la superunidad Linga. Petromineralógicamente se tiene epidotización con diseminaciones de calcopirita, pirita y bornita, gangas de cuarzo y calcita con hematita acicular. Las inclusiones fluidas muestran temperaturas de homogenización de 165.5 - 266.43 °C y salinidad 3.5 - 16.8 % eq. peso NaCl de un mesotermal o epitermal, pero que por sus características geomineralógicas sería un mesotermal. Por tanto, Cobrepampa presenta más de un evento de mineralización: una con magnetita y actinolita, y otro mesotermal de vetas de cuarzo con sulfuros.

May 22, 2019

By F. T. Cawood

This article is a continuation of a series of publications by the authors on the international debate on resource nationalism, with specific reference to South Africa. Previous outcomes have shown that mine nationalization is not a solution for South Africa, and that maintaining stability of the current mineral and tax regimes is fundamental. It will not be wise to replace existing law and policy instruments until there is a better understanding of the distinction between fact and public perception about the impact of the current system. The new scheme of state custodianship linked to a mining royalty that effectively targets economic rents provides for effective resource use. The methodology in this article is to consider how South Africa's mineral tax regime can be tweaked to achieve optimal management and benefit from mineral extraction and associated rents by avoiding changes feared by investors. The major finding is that the current suite of instruments can be considered a suitable platform for optimal management of resource rents. However, it will be necessary to first investigate ways of improving the mineral beneficiation intent of the royalty structure, and secondly investigate whether the mineral rent portion of the royalty formula can be ring-fenced for the purpose of establishing a sovereign wealth fund.

Jan 1, 2013

By J. Allaire, P. N. Muteb

The Meadowbank mine is operated by Agnico-Eagle Mines Ltd. and is located in the Kivalliq District of Nunavut in northern Canada. The process plant operates presently at a throughput of 11,500 tpd with gold grade varying between 2.9 and 3.6 g/t. The ore supplied to the process plant come from two different open pits and it expected to come from a new open pit early 2014. The ore is mid-hard materials so the feed size ore have a bigger impact on the throughput. The average gold recovery is 94.0% with about 15-30% of gold recover from the gravity circuit. The Meadowbank mine commissioning started in February 2010 utilizing a flowsheet consisting of one stage of crushing followed by primary and secondary grinding. The commercial production began in March 2010. During the commissioning several difficulties were observed to reach the designed throughput of 8,500 tpd. It was evaluated to insert a second stage of crushing. The insertion of the secondary crushing have two objectives, first to reach easily the process plant design throughput as well as to evaluated the mill capacity increasing. The process plant team evaluated various expansion alternatives eventually deciding to install a secondary crusher, increased the SAG and Ball mills motors capacities and maintain the rest of the process unchanged. Early 2011, with the improvement in the gold markets, decision was made to install the secondary crusher between the primary crusher and the primary grinding. The process plant continued to operate on the initial design until early June 2011. This paper presents the SAG mill throughput increasing from the designed tonnage of 8,500 tpd to 2012 targeted tonnage of 11,500 tpd, secondary crusher commissioning and the difficulties to operate in Canada Artic (Nunavut).

Jan 1, 2013

By L. E. Mordberg, E. N. Nesterova

Geochemical studies of the bauxite deposits show that the primary composition is gibbsitic. Some trace elements were relatively mobile during bauxite formation with the content of Th, U and Zr increasing and that of Rb, Ni and Co decreasing. Y and Pb were relatively immobile during lateritisation; their contents and ratios were inherited from different types of underlying rocks. The supply of Ni and Co took place at the diagenetic or epigenetic stages; their ultimate source was the crystalline basement in the region. The studies support the view that the bauxite was likely to have originated as a result of in-situ lateritisation and not through sedimentary transport and redeposition

Jun 18, 1905

By F. L. Pirkle

A heavy-mineral deposit about 1.6 kilometers (I mile) in length and averaging about three meters (10 feet) in thickness is exposed approximately130 kilometers (80 miles) east of Denver, along Interstate 70, in Elbert County, Colorado near the town of Limon. The deposit is exposed in a geomorphic feature known locally as Titanium Ridge. The dominant heavy minerals, accounting for about 70% of the total heavy mineral suite by weight, are ilmenite, garnet, and zircon. The mineralized zone appears in a series of visually interpretable facies that are stacked vertically and represent subtidal, forebeach, backbeach, washover fan, and aeolian depositional environments. These paleoenvironments are analogous to depositional environments represented by a modern heavy-mineral bearing facies tract on St. Catherines Island, Georgia. Thus, Titanium Ridge provides evidence that heavy-mineral accumulation models for late Cenozoic deposits in the Georgia and Florida coastal plains are applicable to heavy-mineral deposits of the Western Interior subsurface and, by implication, the Western Interior Seaway, extending from Mexico to the Canadian Arctic.

Jan 1, 2011

By Lottrey A. L, Sowerby B. D, Cutmore N. G

On-line monitoring of low rank coal quality during input to a process has the potential to provide benefits through: ò anticipation of coal quality variations prior to utilisation; ò provision of a tool to help determine the influence of coal quality on process behaviour, and ò provide a direct measure of coal quality for pricing. Moisture and ash are the two most significant coal quality parameters for combustion. They determine the energy content which can be derived from the coal and the problems arising from the presence of ash in the process (eg ash fouling, corrosion, erosion, particulate emissions and ash disposal). Extensive pilot-scale testing and full-scale experience has shown that specific coal/ash constituents (eg Na2O/ash ratio, acid-soluble aluminium, silica, sulphur contents, etc) are also important for the behaviour of Latrobe Valley brown coal during combustion, ash formation, deposition and removal of ash in precipitators and furnace ash hoppers. This paper discusses the range of on-line analysis techniques which have been investigated and their application to low rank coal analysis. Preliminary results from a joint SECV/CSIRO project which aims to develop two separate advanced instrument systems are also presented. These systems are based on: ò microwave attenuation and phase shift - for direct determination of moisture content; and ò inelastic neutron scattering - to provide total hydrogen, carbon, chlorine and ash contents, with potential for calculation of moisture content, specific energy, sodium content and Na20/ash ratio using correlations.

Jan 1, 1992

By Stephen M. Jasinski

This report describes the materials flow of mercury in the United States. It examines the mining and production of mercury and the manufacturing, uses, and ultimate disposal of mercury. Materials flowcharts were prepared for 1989 and 1990. An effort was made to include estimates for all major anthropogenic sources including fossil fuel combustion, waste incineration, and nonferrous metal mining operations. Mining and mineral processing released about 100 mt/yr of mercury to the environment in both 1989 and 1990. The combustion o ii fu1s released 175 mt/yr in the same years. The manufacturing, use, and disposal of products containing mercury accounted fix the release of 1,037 mt of mercury in 1989 and 938 mt in 1990. The manufacture of chlorine and caustic soda was a major source of recycled mercury, but it was all reused within the industry. Recovery from all waste products accounted for 36% of reported consumption in 1989 and 42% in 1990. The losses from manufacturing and end use have been declining, whereas mercury emitted from combustion sources has not been strictly regulated and will likely become the largest source of mercury releases to the environment.

Jan 1, 1994

By Phillip M. Dight

A number of impending and actual pit wall failures have been observed and documented which have been interpreted to initiate on newly generated structural features. This infers failure through intact rock. Failure mechanisms have been postulated resulting from circumferential stress, extensional strain and brittle flexure. These will be discussed in the context of the failures and the associated monitoring. The implications of failure initiating on ?unknown? features has a significant impact for deep open pit mines where designs are based on the expected structural regime. Being able to anticipate such failures is important for design. Equally important is management of such changes as they arise. In this context, monitoring using high resolution microseismic techniques will be discussed as a means of providing early warning of such failures.

Jan 1, 2006

The polymerisation of silica is found to interfere with thickening of leach pulps at Risdon. Hydroxyl ion and fluoride ion are catalysts for this reaction. Stability may be obtained by the presence of aluminium or ferric iron.INTRODUCTIONAt Risdon it has been a matter for satisfaction that, although the concentration, of silica in leach solutions is appreciable, it has ,given relatively little cause for anxiety.However the margin of safety is small and some intermittent difficulty in thickening leach residues during the last few years has prompted a further investigation, certain parts of which are described here since they may be of general interest.Leaching and ClassificationThe method of leaching which is practised at Risdon has been described in, two previous articles, by W. C. Snow (1936) and S. W. Ross (1949) to whiCh the reader is referred for detail. Continuous leaching is practised, the circuit solution flowing through five air agitated pachucas in series, to tlie first of which calcine is added. The circuit solution, which also enters the first pachuca, consists of spent electrolyte from, the electrolytic cells and contains 90 -gm. of free sulphuric acid per litre. About 80 per cent of the solublezinc oxide is extracted from the calcine in the first pachuca, at a pH value of 1-1 (7 gm.H2S04/1 at 55 to 60°C. In the later pachucas the leaching rate decreases sharply. The leach pulp which leaves the 'last pachuca contains an average of 17 gm. of free acid per litre, corresponding to a pH of 1.7; but, owing to variations in extraction, the end point fluctuates between 2.5 and 0·3 gm. free acid per litre (pH:> 1·5 to 2.5). The average flow through the plant is 1,250,000 gallons per day, which corresponds to a production of 300...

Jan 1, 1958

By H. Arima, T. Fujita, W. Yen

The copper catalyzed gold oxidation and reduction mechanisms in ammonium thiosulfate solution were investigated by Eh-pH diagram based on the optimum reagent combination for 60 mass% -75 µm silicate ore at 16g/t Au with the maximum gold extraction of 92% and the minimum ammonium thiosulfate consumption of 23 kg/t-ore for 24 hour’s leaching at ambient temperature. The gold recovery by cementation was nearly 100% by Zn and Al powders at Metal/Au mass ratio of 30, and around 95% by Cu powder at Cu/Au+ mass ratio of 50. In order to oxidize the gold efficiently with the less thiosulfate consumption, the stable region of [Cu(NH3)4]2+ should be as wide as possible in the region, where [Cu(S2O3)2]3- was more stable than [Cu(NH3)2]+. In order to reduce gold from the pregnant solution with the less metal powder’s (Zn, Cu, Al) consumption, the stable region of [Cu(S2O3)2]3- should be wider than [Cu(NH3)2]+ on copper precipitation. Zinc powder may recover gold more efficiently than copper powder with the similar trend on the effect of ammonia, copper and thiosulfate. However, the large amount of copper will be co-precipitated along with gold since the stability of zinc ammine and thiosulfate complexes are higher than that of copper’s. Aluminium may reduce gold efficiently at higher thiosulfate concentration and pH. These results were confirmed and compared with the optimum reagent combination for finer grind sample at 100 mass% -75 µm, which is the optimum ore size for the standard cyanidation at 1.0 g/ l NaCN to obtain about 95% gold extraction. The optimum reagent combination was concluded as 0.5 mol/l NH4OH, 0.0001 mol/l CuSO4*5H2O and 0.05 mol/l (NH4)2S2O3 with the result of 94% gold extraction and 3kg/t-ore of ammonium thiosulfate consumption. Gold and copper stabilities were well agreed with the result of coarser grind ore thermodynamically. The lower reagent combination will reduce gold oxidation power, but significantly minimize the reagent consumption. From this pregnant solution, nearly 100% of gold was successfully recovered by zinc precipitation at Zn/Au+ mass ratio of 30. Aluminium and copper precipitations resulted in less than 40% of gold recovery at Metal/Gold mass ratio of 30. The reagent consumption cost on this lower reagent combination followed by zinc precipitation was $1.6/t-ore, while the cyanidation was $3.1/t-ore. Thiosulfate oxidation mechanism indicated the similarity to Belousov-Zhabotinsky reaction.

Jan 1, 2002

By James Goldberg, Mark Withey

The Material Service Thornton quarry is a multi-million annual production tons complex. Surrounded by busy surface highways, and divided by a railroad on the north/south axis and further divided by a major interstate highway on the east/west axis, blasting is a daily challenge. This paper deals with the problems of production blasting in an area adjacent to that interstate highway, and paralleling a busy four-lane highway. Discussion includes primary blasting issues such as borehole size, drill patterns and flyrock controls. Discussion also includes blast designs, timing schematics, smooth-wall blasting solutions, traffic and vibration concerns and many other factors influencing the success of this project.

Jan 1, 2006

By A Rantala, K Aberkrom, O Haavisto, P Blanz

More complex ore bodies with lowering grades increase the challenges of maintaining high throughput, energy efficiency and required particle size for optimal downstream processing in grinding circuits. Continuous and often rapidly changing characteristics of the mill feed requires constant focus to operate the circuit at optimal level. Modern process instruments, on-line analyzers, monitoring and process control systems provide attractive and managed options to increase the circuits’ capability to respond to the challenge, often in phases and in rather short time period. Furthermore, well instrumented and performing base level automation in the grinding circuit enables advanced process control (APC) strategies that utilize measurements from the whole circuit to stabilize and optimize production. In this paper alternatives for incorporating process measurements and controls to typical grinding circuits are proposed. The expected benefits to throughput, availability, energy efficiency and particle size stability are described in recent case examples.CITATION:Rantala, A, Blanz, P, Aberkrom, K and Haavisto, O, 2015. Increasing grinding circuit robustness with advanced process control, in Proceedings MetPlant 2015, pp 129–141 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Sep 7, 2015

By Galen Bauer

"Up until the mid 1960's, the only available methods for making continuous weight measurements were through the use of mechanical and electro-mechanical weigh scales. It was during this time period that a new concept in weighing was introduced. It was known and is still known as the nuclear belt weigh scale.During the early days of its appearance in the market place, many things were learned about its advantages and disadvantages for weighing various materials. As with many new devices, it had to go through a ""wringing out stage"" before it became accepted by the weighing industry. Even today, even though it is a proven device, there are skeptics who believe the nuclear belt weigh scale will not survive. Let me assure you, it is here to stay.. 'PRINCIPLE OF MEASUREMENTThe nuclear belt weigh scale consists of a ""C"" Frame assembly which houses the radioactive source (Cesiwn 137) and the radiation detector (Ionization chamber). The ""C"" Frame assembly is put in place around the conveyor belt as shown in Figure 1. The radiation source emits a fan shaped beam of radiation which covers the full width of the material being conveyed on the belt. Radiation which has passed through the material on the belt is detected by the ionization chamber which is located in the lower arm of the ""C"" Frame assembly. The ionization chamber changes the radiation energy into an electrical signal which is related to pounds per foot belt loading. This signal is transmitted to a highly stable amplifier which combines the pounds per foot signal with a belt travel signal from a digital tachometer which allowed the scale user to be able to monitor total mass being conveyed. Many so-called conventional type scales use belt speed rather than travel to obtain mass per unit time, and then integrate the signal to obtain their weight signal. This unnecessary step tends to erode the overall accuracy of the weighing system."

Jan 1, 1975

By Michele Ashby

The mining industry is facing new risks from peak commodities, the energy and economic crisis and stock market pressures. The Green Mining Industry Initiative is a risk management strategy for the future of the mining industry. Defining the Risks The bad news is peak commodities, energy and economic crisis and stock markets collapsing are the issues that are already hitting everyone?s bottom line and draining the treasuries of every mining company around the world. Higher energy costs are affecting the bottom line more than ever before, which is creating more risk for companies. Mining companies have major land positions around the world and they have major energy needs. Uncontrollable increases in energy costs, increased regulations for diesel fuel emissions and energy intermittencies are costing mining companies more time, money, and man-power, which are cutting into profits and hitting the balance sheet. The good news is, there are practical energy solutions which are off the grid, tried and true methods which can be implemented in a cost-effective manner and will increase profitability and save shareholder values as well as energy security for these companies and their projects. To date, most of the mining resource industry has not yet considered or used other forms of power like clean, sustainable, or alternative energy for their own energy needs. The reasons have primarily been pushback from the mining industry due to beliefs of older technologies in alternative energy, a lack of understanding of the cost advantages long term, and perceptions of environmental relativity.

Jan 1, 2008

By Arthur Mackwell

The role of the precious metals is changing rapidly. They are becoming primarily materials of modern industry, and their decorative and monetary functions are diminishing in relative importance. Certainly gold is still the premier metal for jewelry, as silver is for silverware. But these uses are not growing nearly so fast as are the uses of these metals in a variety of industries. This is even more true of the platinum metals. Moreover, this is true despite the fact that the precious metals are highly expensive and liable to substitution by the base metals in cases where this is feasible. For the fact is that the physical and chemical properties of the precious metals make them relatively invulnerable to substitution; where they can be replaced by the base metals, they have been, and the opportunity for further substitution is, therefore, limited. SILVER In November 1969 the United States Treasury stopped minting the only remaining coin with any silver content. That was the silver-clad Kennedy half dollar, which had a silver content of approximately 40%. The step was taken out of necessity, because of the rising shortfall between world mine production and consumption. It was important to divert the 35 to 40 million oz of silver per year used for the minting of this coin into the open market, where it would be available for industrial consumption. This decision was important, not only for its practical effect but also for what it symbolized. Only four years earlier, in 1965, the Treasury had used 320 million oz of silver" for the full range of coinage. Now not an ounce of silver goes into the United States coinage. The reason is simple and yet complicated. For many years the U.S. Treasury had purchased silver at prices which provided a subsidy to domestic producers. As a result, silver producers were eager to sell to the Treasury, and that agency built up a huge supply of the metal -almost 2 billion oz by- 1961. But, as the free-market price neared $1.29, the basic melt value of the coinage, the Treasury became a seller of silver, not a buyer, and its stock was depleted rapidly. The Treasury lost more :than a billion ounces of silver in the years 1962 through 1965, and it continued to lose silver, both through coinage and through sales to consumers, through 1969. The Government continued to sell silver to consumers at the rate of 1.5 million oz a week, even after terminating the 40% silver Kennedy half dollar. But it was apparent that the Government stocks could last no more

Jan 1, 1976