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By W. N. Roman

This paper will examine the challenges involved in ventilating and cooling South African mines, with particular emphasis on solutions to the issues of heat (generally associated with depth), mechanization and energy efficiency. Comparisons will be made with other international mining areas. Some of the deepest mines in the world are to be found in the South African goldfields. The Mponeng mine is approaching 4,000 m below surface, with virgin rock temperatures (VRT) of between 60°C and 70°C. Mining at these depths has only been possible with the use of large refrigeration plants supplying cold air, water and ice to the workings. In the Bushveld Igneous Complex, where most of South Africa?s platinum mines are located, similar VRTs are being experienced at significantly shallower depths. This is due to the different geothermal properties of the host rock between the two areas. Existing mines are being forced to go deeper and new underground mines are being planned and constructed in other areas with widely differing thermal conditions. South African metal mining has traditionally been labour intensive for historical reasons as well as for the narrow tabular nature of many ore bodies. Due to cost pressures and labour problems, some mines have been investigating the possibility of using mechanized equipment, both diesel and electric, to increase production and to improve efficiency. Studies have been carried out to determine the effect of the additional equipment on the overall ventilation system and some of the conclusions are discussed. The use of diesel equipment adds heat and pollutants to the ventilation air but, although electrical equipment has a lesser effect on the ventilation systems, it is generally less favoured due to claims of lower flexibility/manoeuvrability. Mechanization holds the promise of more concentrated mining activities but, with higher localized ventilation and cooling needs. Systems which will control the provision of air and cooling more effectively are required and are being implemented. In the 1990s and early 2000s, the South African economy experienced significant growth which was not matched by the growth in infrastructure. Consequently, at present there is a shortage of electricity generation capacity which affects large industrial consumers, including mines. Increasing levels of heat and mechanization will require more electrical energy for ventilation and cooling. There are many initiatives underway to save power or to shift consumption to periods of national low demand and these will be described.

Aug 1, 2013

By B. Nanthakumar

The impact of stockpile oxidation on chalcopyrite was studied through collectorless and collector-induced flotation conditions. In both cases, the Cu grade-recovery performance of stockpiled ore showed relatively poor concentrate grades with respect to that of a fresh sample. However, the oxidation had a minor effect on chalcopyrite recovery itself. The poor concentrate grades resulted from dilution effect primarily by excessive flotation of pyrrhotite. Size-by-size analyses indicated that for the collectorless flotation, the recoveries in the coarse and fine particle-size regions, were higher for fresh ore than for stockpile sample. A greater impact was seen in the coarse size regions for both types of sample, compared to the collector-induced case. The Cu recoveries showed a remarkable increase in the coarse size range for both samples when a collector was used. However, the case for stockpile sample was a marginally higher compared to the fresh sample. This difference has been offset by an opposite trend that was observed between these two samples in the fine size region. It is concluded that in contrast to the case with nickel-bearing sulphides, stockpiling conditions do not show a significant adverse effect on behaviour of chalcopyrite due to its strong floatability, regardless of oxidation.

Jan 1, 2007

By S. Jordanov

The nature of processes that take place on top of copper bearing grains during the suppression of copper acid drainage from mine tailings was studied on a grain model. Cyclic voltammetry (C.V.) was employed as an experimental technique suitable for indirect measuring of copper leaching. Experiments were done in sulfuric acid (pH=2) at electrodes that progressively approach the real situation during acid main drainage, starting with Cu-metal, then with CuO and finally with CuO covered with CaSO4. Oxidation was found to be the only process that takes place on metallic copper electrode anodically polarized up to 2 V vs. Cu/CuSO4 reference electrode. Dissolution is intensive and reaches up to 50 mA.cm-2. When the same was repeated, but on CuO electrode, anodic currents were some 4 times lower than on the bare metallic surface. This indicates that the electrode material is not completely oxidized to copper(II)oxide and the Cu(O) or Cu(I) residues are oxidized during the anodic polarization. This explains the diminished dissolution. Dissolution is further restricted at electrodes covered with CaSO4. It is some 20 times lower than on the bare oxide electrode. When the second layer of CaSO4 was applied, anodic currents were another 15 times lower. Total decrease of anodic currents when transition is made from Cu-metal to CuO covered with 2 layers of CaSO4 reaches up to 7.500 times. C.V. measurements produced a proof that mechanical blockage is the way how lime prevents leaching of copper bearing grains in immobilization experiments referred elsewhere.

Jan 1, 2007

By Richard Murphy

"The general geology of the Great Bear Lake area has been described by; D. F. Kidd in the Summary Report of the Geological Survey for 1932. A brief description of the Eldorado mine was prepared by C. S. Lord for Memoir 230; Mineral Industry of the Northwest Territories; published by the Geological Survey in 1941. Recently; a detailed geological examination of the mine area has been initiated by the Geological Survey and; under A. W. Jolliffe's leadershif; J. D. Bateman; H. B. Thurber; and M. Feniak have prepared a number of maps and reports; as yet unpublished. Detailed mapping of the mine has been carried out since August; 1943; by A. W. Esty.Kidd has shown that, in the largely granitic country along the east side of Great Bear lake, the Echo Bay district is one of several underlain by older rocks . These he has divided into the Echo Bay and Cameron Bay groups. The former are strongly altered sediments, .porphyries, and lavas; the latter and younger group is predominantly sedimentary and far less altered. Associated with the granite, which everywhere intrudes the sediments and volcanics, are earlier differentiates. Later than all these rocks are various basic intrusives, collectively termed diabase. All the rocks are of Precambrian age."

Jan 1, 1954

By J. Ouellet

The authors participated in a study for a new mine shaft in Saskatchewan?s Athabasca Basin where ground freezing is desired to reduce water inflows. Constraints were imposed on the design in terms of permissible water leakage (less than 10 m3 / hour) inside the shaft. Since the shaft goes through highly permeable formations, water control measures had to be incorporated into the design. Ground freezing was selected as the most reliable and proven method for the sinking and a hydrostatic liner design was implemented. While exploring various technological options available for providing a fully hydrostatic liner, the authors came up with a new design concept that offers a more cost effective solution while meeting all the design constraints. The new liner concept is a composite design using concrete and a sprayed-on membrane system. A proof of concept program comprising laboratory testing and field testing was successfully implemented. This paper presents the innovative liner design, as well as the adapted sinking methodology, developed for this type of project. The new concept offers a new cost effective high performance hydrostatic mine shaft liner comparing favorably with traditional methods. RESUME: Les auteurs ont réalisé une étude pour un nouveau puits de mine dans le Bassin de l?Athabasca en Saskatchewan. Des limites sévères sont imposées sur le design en terme de volume d?infiltration d?eau acceptable (moins de 10 m3 / heure). Comme ce puits de mine traverse des formations très perméables la congélation des terrains a été sélectionnée, une méthode éprouvée et fiable. Un design hydrostatique a été choisi pour le revêtement du puits. Dans le processus de design, les différentes technologies disponibles ont été étudiées. Les auteurs ont développé un nouveau concept de revêtement composite offrant un rapport performance/prix plus avantageux que les technologies conventionnelles. Le nouveau design proposé est composé de béton et d?une membrane projetée. Un programme expérimental a été réalisé pour démontrer la faisabilité du concept. Cet article présente le nouveau concept ainsi que la méthodologie de fonçage de puits adaptée. Ce concept innovateur offre un revêtement hydrostatique performant à un cout avantageux par rapport aux techniques de construction utilisées jusqu'à maintenant.

Jun 1, 2012

By McNeely DuBose

INTRODUCTORY PROSPECTING for aluminum has always been, and still is, a peculiar combination of the technical with the physical approach, and 1 want to explain my conception of the term 'prospecting' when applied to aluminum, which differs in some respects from the ordinarily accepted idea of prospecting. In the early days of gold mining, prospecting was a search in the hills and the river beds for the metal, not for its chemical combinations. And so it was in the early days of aluminum, with the difference that the search for the metal was carried on, not in the hills and dales, but in chemical laboratories. Aluminum is never found in the elemental state and at that time the metal bad never been seen, but scientists of that day bad forecast that such a metal must exist and in 1807 Sir Humphrey Davy went so far as to give this unseen metal a name, aluminum, from the Greek word alumen. This word applies to our alum, and it was from alum that Davy sought to produce the metal. It is strange to think that, in laboratories throughout England and on the continent, prospecting for aluminum was busily under way by men who had never seen the metal they were searching for. Their situation was something like that of the fisherman and the 'big one' that got away. They could describe the metal, give its exact weight, and even name it, but they could not produce visual evidence of its existence. Eventually, however, the efforts of the scientist 'prospectors' were rewarded and in 1826 the metal was actually produced, possibly first by Wohler, though Oerstedt, a Danish chemist, is usually credited with the achievement.

Jan 1, 1951

By K. J. Aglias, M. Jeffery, J. V. Simmons, J. A. Johnston, S. G. Fityus

"Shales and mudrocks undergo weathering in the near-surface geological environment, and degradation when directly exposed at the surface. Despite having superficially similar appearances, many shales have very different degradation behaviours. For example, some shales break down readily upon exposure whilst others remain more or less intact. Further, for those shales which break down, the style of degradation can vary greatly. A phenomenon is described where fresh shale blocks that are exposed at the surface undergo rapid breakdown to a fragmented texture which then persists for relatively long times, without further breakdown to finer soil particles. This is illustrated for some coal mine waste rocks. Furthermore, this metastable fragmented texture seems to be particular to each particular type of shale. The paper presents a conceptual model for this physical breakdown of shales which accounts for the differences in fragmental texture that are observed. The importance of several factors including strength, permeability and expansive clay content are identified for their contribution to the degradation behaviour of different shales.INTRODUCTIONShales and MudrocksThe term shale is applied widely to a class of mudrocks having a range of distinctive but gradational appearances and mechanical behaviours. Mudrocks are sedimentary rocks primarily composed of silt and clay sized particles (Tucker 1982). Mudrocks form in low to very low energy environments such as flood plains, lakes and deltaic environments, where the finest detrital particles are able to settle out of suspension (Reeves et al., 2006; Stow, 2005; Tucker, 2001).Unlike coarser siliciclastic sedimentary rocks, mudrocks do not present with a wide variety of sedimentary structures and textures. The most common texture is a preferred orientation of clays and micas parallel to the bedding plane. This textural component is related to the property of fissility or the ability for such textured mudrocks to split along smooth planes that are parallel to the bedding. Another common structure is lamination, formed by variations of grain sizes and types during deposition, Massive texture describes the situation where no sedimentary structures or textures are apparent (Boggs Jr., 2009; Tucker, 2001). The term shale varies between sources and contexts, with Reeves et al. (2006) stating that a shale is a fissile mudrock, where “fissility is a property that has developed at a certain stage of weathering”."

Jan 1, 2015

By M. M. Papic

"This paper describes the Sasol coal liquefaction process, as used in South Africa. It covers coal supply, coal gasification, gas purification, Fischer-Tropsch synthesis, the major utility systems involved and the economics of the process.IntroductionThe Sasol coal liquefaction process is based on Fischer-Tropsch synthesis of liquid fuels from carbon monoxide and hydrogen, which are in turn obtained by gasification of coal with steam and oxygen. The process is shown schematically in Figure I.The first step in the process is coal preparation, in which the coal is crushed and screened to a 3-50 mm size for gasification on one side and to -3 mm size for steam and power production on the other. Oxygen needed for coal gasificatoin is produced in the air separation plant.Coal is gasified under pressure by steam and oxygen in the Lurgi gasification plant, where raw, impure gas is produced . The raw gas is first washed in a scrubber with a circulating gas liquor, heat is then recovered from it in a waste heat recovery boiler and the gas is further cooled for purification. Water-insoluble materials (tar oils) in the gas liquor are separated from aqueous solution and sent to tar refining for the recovery of tar products. The aqueous portion of the gas liquor goes to a Phenosolvan solvent extraction process, where phenols and ammonia are recovered. Water then proceeds to the effluent treatment plant.The scrubbed and cooled raw gas goes to a Rectisol gas purification plant, where all components other than hydrogen, carbon monoxide and methane are removed as completely as possible by absorption in refrigerated methanol. The acid gases (CO2, H2S) stripped from the methanol in the Rectisol unit are processed in a Stretford plant for recovery of sulphur. Regenerated methanol is returned to the process. The purified synthesis gas is passed to the fluidized-bed Synthol reactors, where Fischer-Tropsch synthesis reactions take place.The synthesis products are recovered in a product recovery unit as separate streams of decant oil containing higher-molecular- weight hydrocarbons, light oil comprised of lower-molecular- weight species, the aqueous stream containing water solubles, a C3/C4 stream, a C2 stream , a methane stream and a hydrogen-rich stream. Most of the hydrogen is recycled to the Synthol reactors; a major portion of the methane is reformed to hydrogen and carbon monoxide for recycle; the C2 fraction goes to an ethylene plant; and liquid streams proceed to the product refinery.The product refinery is used to upgrade and purify the Synthol products to specification fuels and chemicals.Sasol l, the first coal liquefaction plant in South Africa , was built in 1955. It is still operating and has a product output of about half a million tonnes per year. The Sasol II plant is in a commissioning stage and will be in full operation in 1981. It is about five times the size of Sasoll. It uses over 12 million tonnes of coal and produces more than 2 million tonnes of saleable products annually (1,2,3). The Sasol llI plant, which is a mirror image of Sasol II, is under construction and scheduled for completion in 1983."

Jan 1, 1981

By J. J. Brummer

"A reconnaissance geochemical survey, using the stream sediment technique, is described. The survey covered an area of about 400 square miles, in the Seal Lake area of central Labrador, underlain by basaltic flows, diabase and metadiabase sills, and sedimentary rocks. Previous work had indicated numerous small copper occurrences within the Seal Lake group of rocks. Stream sediments were tested for the readily extractable copper content and total heavy metal content. The results of the copper survey did not yield any new data on the distribution of copper mineralization. Results of the total heavy metal survey attracted attention to a remote portion of the area (Ten Mile Lake), which had hitherto been regarded as having no economic interest. The anomalous values found, suggested that zinc mineralization should occur in this region. Geological mapping indicated that the rocks exposed belonged to the Letitia Lake Group, which is older than the Seal Lake Group exposed to the east, which contains the copper mineralization of the district.Investigation of the area lead to the discovery of: (1) uneconomic lead-zinc mineralization; (2) radioactivity extending over a considerable area. Evaluation of the radioactive rocks led to the discovery of columbium-thorium mineralization in amphibolite and in diopside-actinolite schists along a contact with intrusive alkali syenite.Thus a geochemical survey, which was planned to evaluate a copper province, resulted in the discovery of lead-zinc-columbium-thorium mineralization."

Jan 1, 1960

By Y. L. Mehta

The working experience of the Zinc Smelter, Debari, Udaipur with process modifications and iron control practices is described in detail in this paper. Most of the modifications were made at different times as part of capacity enhancement campaigns that led to distinct advantages, thereby increasing both zinc production and recoveries. This smelter came into existence in 1968 with a humble beginning of producing 18,000 metric tonnes (MT) of zinc per annum, using the conventional batch hydrometallurgical roast-leach-electrowinning technology. Zinc production capacity was increased stepwise to 45,000 MT, 49,000 MT and 59,000 MT, by making suitable process modifications and adopting new technologies, particularly in the leaching section. During these modifications, the process of iron separation as Moore cake was also changed to jarosite cake production. At present, jarosite is being disposed in a secured landfill at the Zinc Smelter, Debari (ZSD) and as Jarofix at the Chanderiya Lead Zinc Smelter (CLZS). As a result of subsequent in-house performance improvement and debottlenecking campaigns, this smelter could achieve further increases in production. Zinc production for the financial year 2005-06 was 81,000 MT.

Jan 1, 2006

By Cheng-Ting Chiang

Information integration and exchange for the entire building life cycle is one of the most important issues in the AEC industry. Although the Building Information Model (BIM) technology is widely utilized to address them, BIM is mainly concerned within the design and construction (D&C) phases. In the operation and maintenance (O&M) phases, however, the information integration and exchange issue becomes more complicated because software applications here require not only BIM but other information outside of the AEC domain, e.g., disaster mitigation. Nowadays, most BIM-related research focuses on the D&C phases and pays less concern on the O&M phases, hence there is a research need that can help seamlessly transfer the information between the D&C and O&M phases. In this research, a software framework using ontology was designed to deal with aforementioned problems. The BIM data from the D&C phases were used as a foundation to generate new models for different software applications for the O&M phases. The ontology technique was employed to assist the model transformation process between the original BIM and the models for applications. Finally, a disaster mitigation example was chosen to validate the framework, in order to make sure that the transformation process is comprehensive and useful from domain experts? perspective.

Aug 1, 2013

By R. B. Kaunda

One fundamental requirement for surface and underground rock engineering is the ability to characterise the rock mass. An accurate characterisation of the rock mass allows rock engineers to optimize designs safely and economically. A number of classification systems have been developed in the past 50 years to help characterise rock masses such as RMR, Q system, Rmi, and several others. The challenge in developing rock characterisation schemes is often practicality, i.e. creating systems with relatively simple inputs for the everyday practitioner. A second challenge is the degree of subjectivity often inherent in commonly used classification systems. These challenges are especially exacerbated in weak rock or densely jointed rock masses. This paper introduces a new approach to characterise densely jointed rock mass using artificial neural networks and principal component analysis, based on principles commonly used in the geotechnical engineering of rock fills and waste rock dumps. Thirteen input parameters are dimensionally compressed using principal component analysis. Using a global database with more than 100 case histories, an artificial neural network is developed to conduct the rock mass characterisation. The advantage of this approach is less subjectivity as commonly used engineering (quantitative) terms are put to use. A further advantage is that given that the method is site-data dependent, it allows for ‘fine tuning’ of the model for a specific site.

Jan 1, 2015

By P. Hr?enjak

Two groups of methods are usually used in design procedures of underground excavation of dimension stone: the analytical and numerical methods of analysis. In all of these methods the in situ rock mass strength represents the main problem. This is due to massive rocks in dimension stone deposits, consisting of large blocks of rock separated with particular discontinuities, for which the Hoek-Brown empirical criterion, developed for jointed rock masses, cannot be realistically used to estimate rock mass strength, especially for a particular block of rock mass or a particular pillar. On the basis of the regular underground excavation process of dimension stone by separating blocks with the flat jack application, we developed the in situ flexural strength testing method. We carried out numerical analyses and evaluation of uncertainty of measurement in order to validate the in situ flexural strength testing method. On the basis of very good validation results, as well as results of the in situ flexural strength testing of exploitable layers on the ?Kanfanar? quarry, we introduced a new coefficient of reduction, which can be used for correction of other strength parameters determined by laboratory testing for obtaining a realistic value of rock mass strength.

Aug 1, 2013

By B. Burke

Magnesium and its alloys are attractive materials for use in automotive and aerospace applications because of the low density and good mechanical properties. However, difficulty in forming magnesium and the limited number of available commercial alloys limit their use. The present paper reviews the efforts to improve the attractiveness of magnesium through non-traditional processing, and presents preliminary results of current work on producing magnesium alloys via powder metallurgy P/M. P/M can be used to alleviate the formability problem through near-net-shape processing, and also allows unique chemical compositions that can lead to new alloys with novel properties. The feasibility of producing magnesium powder metallurgy products utilizing the industrially dominant process of mixed powder blending, uniaxial die compaction and controlled atmosphere sintering was investigated. An alloy composition based on the commercial Mg alloy AZ31 (3% Al, 1% Zn) was used to facilitate comparison to similar wrought products. The optimal processing conditions (compaction pressure, sintering time and temperature, and quenching temperature) were found to maximize sintered density and mechanical properties.

Jan 1, 2007

By G. P. Gu

The in-service 90/10 cupro-nickel tubes in a water-chiller system suffered from severe corrosion damage during their relatively short service life causing unexpected down time. In addition to downtime, the environmental risk associated with losing refrigerant was also a concern that initiated this investigation, Microstructural investigation using SEM and electron microprobe indicated that pitting corrosion had resulted in local nickel depletion, During the corrosion process, copper appeared to have re-deposited around the corrosion pits accelerating the local corrosion.

Jan 1, 2005

By T. W. Wong, M. A. Whitney, A. Hadadzadeh

Uniaxial compression tests were performed on cylindrical specimens, 10mm in diameter and 15mm in height, machined from an extruded AZ31B magnesium billet. The tests were conducted under isothermal conditions to a true strain of 0.5. The study compares macroscopic flow anisotropy between the extruded (ED) and transverse (TD) directions in the temperature range of 300°C to 500°C, and strain rate range of 0.001s-1 to 1s-1. Compression of ED specimens at low strain rate (0.001s-1) exhibited axisymmetric flow, and no significant differences were observed between geometries of specimens deformed above 350°C under the same strain rate. Deformation of TD specimens at low temperature and/or low strain rate resulted in elliptical cross-sections, with the anisotropy of flow being less significant at higher temperatures. Other studies performed using rolled plates have proposed that pyramidal slip and cross-slip at higher temperature eliminate the effects of initial texture, while the dominance of prismatic slip at lower temperatures will result in anisotropic flow. It is believed that the initial fiber texture common among extruded HCP-structured materials plays a role in the anisotropic deformation behaviour observed in the extruded specimens

Jan 1, 2015

By Ian Thomson

OBJECTIVES OF EXPLORATION ? Prove/Disprove presence of potentially economic mineralization ? Keep on budget ? Minimize negative environmental impacts ? Gain a ?Social License to Operate? (SLO) WHAT IS A ?SLO? ? Granted by the local community ? Intangible, informal, non-permanent ? Has to be earned and then maintained ? Defined as ? Ongoing Approval (BSR and AccountAbility) ? Ongoing Approval, broad social Acceptance (Joyce and Thomson) ? Ongoing Acceptance (Nelson, and many others) ?Expression of the quality of a ?relationship?

May 1, 2010

By D. Neely

As Saskatchewan potash mines grow in age and size, mining reaches greater depths.  The stress-relief design principals which were empirically derived at shallower depths are beginning to show fault.  Nutrien Vanscoy Potash has began to encounter new ground support and panel design challenges which were previously had negligible effect.  While the initial reaction was to reduce extraction and implement distance buffers between operating machines, it eventually became clear that these would only provide short term relief and that in order to continue safely and efficiently mining broad areas, a fundamental change in the mine design would provide the long-term protection required.  Presented is a case study with an overview of the challenges encountered at Vanscoy Potash in areas of greater depth along with an account of the design changes considered, implemented and recommended for future success. 

May 4, 2025

By T. Gerritsen, C. Martinez, F. Stober, T Ma, J. R. Frias, M. Sedighy, J. Janzen

Electrode control systems regulate power by physically positioning electrodes. Due to the weight and inertia of the electrodes and hoist equipment, this is a relatively slow process where power fluctuations of less than a few seconds are not fully corrected. Nickel laterite AC furnaces operating in high voltage, shielded arc mode typically incur frequent power variations of approximately +/- 20% or more around the power set point. To provide enhanced power control, Hatch has recently supplied, at commercial scale, a thyristor-switched reactance system called an SPLC. The SPLC incorporates predictive control software to operate furnaces at the maximum transformer rating by reducing the large power swings experienced without the SPLC. The resulting higher average power enables higher production, without increasing transformer or power plant capacity. A 60 MW production version of the SPLC has been commissioned on Falconbridge Dominicana's laterite nickel furnace, yielding excellent results with a 3 to 1 reduction in power fluctuations.

Jan 1, 2005

By Rodney G. Orr, Phil Robertshaw, Roland Loewer, Klau Lehnert-Thiel

This paper documents the 1988 discovery and subsequent evaluation (to 1991) of diamond bearing kimberlites in the Fort à la Corne area, Saskatchewan, by Uranerz Exploration and Mining Limited. Cameco Corporation began participating as a joint venture partner (50%) in mid-1989. The first indications that kimberlite-type intrusions might exist in the area carne from Geological Survey of Canada aeromagnetic maps. Because of the overburden thickness (over 100 m), magnetometer surveys continue to be the main exploration tool in defining pipe locations and sizes. Magnetometer surveys in varying levels of detail have outlined one main cluster and two sub-clusters, the main group being some 10 km by 50 km in size. In excess of 70 individual targets have been interpreted from the magnetics as probably representing kimberlites. The size range of the individual magnetically-defined bodies is between 1 ha and 74 ha. Proof that the magnetic anomalies were kimberlite intrusions carne in 1989 when each of seven test holes, all drilled on different targets, intersected the potential diamond host rock. The majority of the targets tested were diamond-bearing, with the indicator mineral chemistries suggesting that both peridotitic and ecIogitic diamond parageneses could be expected. To date, 18 individual kimberlite bodies have been drill-tested. A total of 39 bulk samples, amounting to 186 t (representing 360 t of theoretical sample recoveries) were processed in 1990 and 1991. One hundred and sixty diamonds were recovered, averaging 0.04 carats in weight, the largest being 0.6 carats. Most of the bulk sample grades are less than 2 carats/loo t; only three bulk samples have higher grades, the best being approximately 10.5 carats/l00 1. The diamonds are predominantly gem-quality stones.

Jan 1, 1992