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By W. H. Wilson, G. Bjarnholt, D. C. Holloway

This paper describes the results of field tests conducted on a 5x25m bench face to evaluate several new concepts for improving smooth wall blasting. These ideas involved the use of notched boreholes to direct the fractures and unconventional types and placements of explosives. The notches were made with a mechanical broaching tool, a water jet and with shaped charges. Borehole pressures were measured with an inexpensive and expendable pressure transducer developed for this program. The testing showed that at hole spacings 38% greater than normal practice, notched holes with half the normal loading density produced equivalent results.

Jan 1, 1986

By Robin Dill, Wei Song, Andrew Perham, Leo Martin

"AECOM completed a subsurface exploration program for an 18-foot diameter, approximately 4-mile long rock tunnel project in Hartford, CT. The tunnel crosses the regional geologic trend through the middle of a sedimentary basin, passing through four separate rock formations, including two volcanic flows. Faulting is prevalent, with at least 15 faults encountered during the site investigation. Severe artesian flows were also experienced in several of the borings, requiring highly specialized drilling equipment. Sixty-two borings and associated testing were performed to characterize the structural geology, hydrogeology, rock properties and ground conditions. A phased program was carefully planned and implemented with participation of geotechnical and geologic specialists, with thoughtful evaluation along the way to react to the ground conditions encountered, and make appropriate refinements. This paper will discuss the subsurface program details; the integrated efforts used to effectively characterize the ground; and will summarize the final characterization presented in the GBR. PROJECT DESCRIPTION The South Tunnel Conveyance and Storage Tunnel project is part of the Metropolitan District (MDC’s) Long Term Control Plan to address discharges to various water bodies throughout the greater Hartford area. This program includes a main conveyance and storage tunnel; a tunnel boring machine (TBM) launch shaft and retrieval shaft; adit tunnels with associated drop shafts and connection structures; consolidation conduits and manholes; and a tunnel pump station (TPS) at the downstream end of the main conveyance tunnel. AECOM is serving as prime designer for the MDC on this project. The main conveyance tunnel (hereinafter referred to as the “South Tunnel”) will be a deep rock tunnel with an invert depth of approximately 180 feet at its downstream end and approximately 218 feet at the TBM retrieval shaft. The tunnel will have a finished diameter of 18 feet. The tunnel will rise in grade at a slope of 0.1 percent to its upstream end. Total invert depth along the deep tunnel alignment ranges from about 180 ft. to as high as 320 feet. The total length of the tunnel will be 21,754 feet. The South Tunnel will be constructed using a shielded TBM and fully lined with pre-cast concrete segments."

Jan 1, 2016

By R. Stanevich

Studies by the National Institute for Occupational Safety and Health (NIOSH) and the Bureau of Mines (BOM) have demonstrated that: (1) 90% of diesel particulate is less than one (1) micrometer in aerodynamic diameter; (2) more than 90% of the respirable mass in electric coal mines was greater than one (1) micrometer in aerodynamic diameter; and (3) in coal mines using diesel equipment, average diesel concentrations can represent up to 53% of the MRE respirable coal dust standard. For mines approaching the 2 mg/m3 standard, compliance may be easier to attain by controls directed at diesel exhaust particulate as opposed to coal dust control. First, however, a Simple instrument is needed to measure diesel exhaust particulate concentrations. This paper describes such a sampler, its construction, and use. While results from this sampler are primarily utilized to identify control areas, NIOSH is also using the sampler for estimating worker exposures in epidemiologic studies.

Jan 1, 1988

By Edmund W. Murphy

At the first workshop on Surface Subsidence Due to Underground Mining (1 981 ) , Yarbrough delivered a paper (1 ) describing the mine subsidence insurance program in Illinois operated as the Illinois Mine Subsidence Insurance Fund (IMSIF) . In this paper we will present claims data and describe changes that have occurred in the Illinois Insurance Code during the first six years the IMSIF has been in existence. The change in the original Illinois Public Act 80-1 41 3 that most affected the public was an amendment increasing the maximum amount of mine subsidence insurance avail- able on a structure from $50,000 to $100,000. Seventy-two counties in Illinois have been undermined for minerals, primarily bituminous coal. In Franklin County (.in the southern part of the state) 36 percent of the land area has been undermined for coal extraction. In St. Clair and Madison Counties , two counties that include the St. Louis Metropolitan area along the western border of the state, 10 percent and 7 percent of the land surface have been undermined. Most of the undermined area in the two counties is beneath urbanized land. In 34 counties, approximately 1 percent or more of the land surface has been undermined (2). The first shaft mine in Illinois was sunk in the city of Belleville (east of St. Louis) in 1842, and the mines of early vintage have initiated the majority of the mine subsidence claims. Data compiled from the first six years reveals that most Illinois mine subsidence events that affect structures occur over abandoned mines where early methods of high- extraction mining practices were used. Maps of these mines operated in the 1800s reveal irregular development of room and pillar areas and poorly defined production areas (panels). Rooms in the mines varied from 15 to 40 feet in width and the pillars varied in width from 5 to 40 feet. The pillars were often long and narrow. Recovery rates in mines that operated prior to 1900 generally averaged 50 per- cent, and extraction rates as high as 80 percent have been documented (2). Some pillar robbing occurred.

Jan 1, 1986

By M. J. Laurila

The concentration of low grade iron ores to pellets or sinter fines suitable for blast furnace feed or direct reduction is highly energy intensive and costly in terns of both size reduction and separatory processes. Large equipment with high throughputs are required to make the fine grinds and multi-stage separations economical. Control of these circuits is essential to producing a high grade product. As a result, iron ore concentrators possess some of the more sophisticated instrumentation for process control in the mineral dressing industry. These instruments must characterize flowstreams of up to 8,000 gpm where 80% of the particles are less than 50 µm. The past decade has been one where enormous strides have been made in automated control of iron ore plants. This paper reviews state-of- the-art control systems for crushing, grinding, magnetic separation, froth flotation and sizing/classifying circuits. Trends toward computerization and plant automation in response to a dismal outlook for North American steel production are also examined.

Jan 1, 1984

By P. James

"INTRODUCTION Dissolved salts build up in leach solutions over time as leach solutions mature. Iron can be particularly problematic for hydrometallurgical copper production as it can compete with copper for separation during solvent extraction (SX), leading to iron bleed through and reduced electrowinning (EW) efficiency. In its ferric form (Fe+3) it precipitates as a hydroxide at fairly low pH and can promote clogging and disrupted flow within leach pads as a result of localized pH deviations – such as might result from weather events. The solids formed can cause relatively irreversible degradation to pad productivity. The precipitation pH point for solids formation falls as the ferric (Fe+3) concentration increases, making the issue more prevalent as leach circuits for ore bodies containing appreciable iron get older. It can also be more of a concern when leaching lower grade ore like tailings where lower copper content may increase iron/copper ratios which may increase iron accumulation rates. The precipitation pH point for solids formation falls as the ferric (Fe+3) concentration increases, making the issue more prevalent as leach circuits for ore bodies containing appreciable iron get older. It can also be more of a concern when leaching lower grade ore like tailings where lower copper content may increase iron/copper ratios which may increase iron accumulation rates. Control of ferric (Fe+3) levels could be used to lower ferric (Fe+3) levels and keep the ferric hydroxide [Fe(OH)3] precipitation pH sufficiently high to avoid the associated solids formation and potential leach pad occlusion. Control of ferric (Fe+3) and ferrous (Fe+2) tenors and ratios may also provide benefit for improving sulfide ore leaching or bioleaching performance and effectiveness. Traditionally options for such control were rather limited with bleeding contaminants off or chemical dosing treatments being prominent methods. Now Blue Planet Strategies introduces a new option. Blue Planet Strategies (BPS) focuses on improved resource utilization targeting electrolytic treatment and waste conversion into resources and has now demonstrated that its proprietary DeMet™ (Dynamic Electrolytic Metal Effluent Treatment) platform is highly effective at efficient low-cost conversion and control of iron content in leach streams in addition to targeted dilute metal recovery.[1-6] This powerful new capability provides a new treatment opportunity to control leach solution ferric (Fe+3) and ferrous (Fe+2) tenors to enhance leach operation production."

Jan 1, 2015

By R. D. Hagni

Pyrometallurgical products from lime roasting of refractory gold ores, pyrite, and arsenopyrite have been examined for their mineralogy, porosity, permeability, retention of contaminant sulfur and arsenic, and potential effectiveness of gold extraction. Cathodoluminescence microscopy, supplemented by X-ray diffraction and scanning electron microscopy-energy dispersive analysis study, has shown that the products phases have characteristic cathodoluminescence properties as follows: anhydrite (CaS04), deep blue; unreacted portlandite (Ca(OH)2), deep purple; oldhamite (CaS), light yellow to rarely pink or orange; a sulfur-rich, arsenic-poor phase, violet; an arsenic-rich, sulfur-poor phase, non-CL. Within the temperature range examined (400-850°C), hematite exhibits greater porosity and permeability at the intermediate roast temperatures of 600-650°C.

Jan 1, 1993

By Luther W. Hyde

Ground water, one of Florida's most valuable resources, is also one of its most abundant. The State is fortunate to have several prolific ground- water reservoirs that yield tremendous quantities of water to wells and to some of the world's largest springs. These ground-water reservoirs, or aquifers are of two types artesian and nonarteaian. Ad artesian aquifer is one that contains water under sufficient pressure to rise above the top of the oontaining formation. A nonartesian aquifer is one that contains water which is not confined, and the upper water surface (water table) is free to rise and fall. The principal source of ground water in Florida is the artesian Floridan aquifer which supplies most of the water users in the State. Nonartesian sources include the Biscayne aquifer of southeastern Florida, the sand-and-gravel aquifer of extreme western Florida, and a shallow aquifer which lies at depths of less than 100 feet over much of the State. The accompanying map shows the approximate areas where these aquifers are the main source of supply; however, the boundaries do not represent the areal extents of the aquifers. For example, the Floridan aquifer extends beyond the southern boundary shown on the map, but it is not of great economic significance there because the water is highly mineralized. Aquifers other than the four described occur in the State, but they are, for the most part, of minor significance. One of these is an artesian aquifer which is formed by permeable zones that occur locally within the confining beds above the Floridan aquifer.

Jan 1, 1966

By Stanley J. LeFond

After two disappointing years where most industrial minerals posted decreasing production, 1981 saw an increase in production over 1980, or at least in status quo. Several commodities did, however, post significant gains. For example, bromine posted a 14% increase; fluorspar production dropped 21% in 1980, but gained 24% in 1981; and sulfur production increased. However, perlite was off 13% and manufactured sodium sulfate off 18% in 1981, while natural sodium sulfate production remained almost even with 1980. In fact, many industrial minerals production either remained constant, increased, or decreased only slightly over 1980. Increasing energy costs had their impact on industrial minerals in 1981. Hopefully, the recent downward trend in energy costs, at least gasoline, will financially help some of the more marginal plants. The 1979 shortage of rail cars that did not carry through 1980 also did not carry through in 1981. Rail rates increased 12.8% over-all last year, while average freight rates for industrial minerals increased 15.5%, according to Neil Johnson, consultant. Also, the trucking industry posted a 5-6% decrease in rates due to deregulation. All in all, we in the industrial minerals section seem to be holding our own at least. Stanley J. Lefond Contributing Editor

Jan 5, 1982

By Gordon M. Pugh, David G. Rasmussen

INTRODUCTION Square-set timbering of stopes developed from stull support methods because veins too wide for stulling were being mined. In general, the void caused by the stoping of the ore is filled with a system of interlocking timbers. Timber framework supports the wall rock, the mining operations, and provides access. A timbered stoping method can be used in all widths of veins at any dip. The timbering can be normal and parallel to the planes of the walls or at angles to these planes. Irregularly shaped ore bodies can be worked by timbered methods. Pinching or swelling of the vein will not adversely affect the method. Timbered stoping is especially advanta¬geous where weak ground is encountered in the vein or the wall rock. The ground is unsupported only until timber sets are stood. These stopes can be filled with sand or waste when desirable. The customary method of developing a square-set timbered stope with haulage drifts above and below and timbered raises on each endline is shown in Fig. 1. The drifts are timbered the full stoping length. The raises have two compartments with the manway and timber slide in one compartment and the chute in the other. The drifts and the raises cannot be widely spaced as handling timbers is an important consideration. Stoping can begin immediately above the drift timber and continue in both directions and upward from the raise. Breasting down each blast round for only one timber set laterally is the common practice. The blasted ore falls to a lower floor where it is slushed or trammed to the chute. Drilling can proceed on the upper floor while broken ore is being removed from the stope. As shown in Fig. 2, square-set timbering may be used in cut-and-fill stopes to support the back and walls until fill is added. There are various ways of developing an ore block for timbered stoping. Fig. 3 depicts the situation for the main haulage in the vein with an intermediate drift separated from it by a sill pillar. The ore is slushed into the chute connecting the two drifts. The situation in Fig. 4 is identical to that in Fig. 3 except that no sill pillar is used. In Fig. 5, the haulage drift development is done in the footwall with crosscuts driven to intersect the ore body. The intermediate drift development is above the crosscuts with a minimal sill pillar. The main bottleneck in timbered stoping is in the timbering process. Drilling in the stope is generally held up while timbering is done. Two or three men are re¬quired to do the timbering and an additional one or two men are needed to transport and raise or lower the tim¬ber. Good organization is essential to timbering. Skill must be exercised in standing the timber to support the ground and still maintain good production. COSTS AND PRODUCTIVITIES Development and stoping costs are presented in Tables 1 through 7 for the situations in Figs. 3 and 5,

Jan 1, 1982

By Richard A. Bideaux

During 1977 and 1978, a comprehensive study was made of the drill-hole data obtained by Urangesellschaft, USA, from their Juniper Ridge, WY, uranium deposits. This included uranium grade data acquisition, application of a variety of mineral deposit modeling techniques, and estimates of minable reserves, which pointed towards a possible production decision.

Jan 1, 1979

By Dennis A. Beiter, Kenneth G. Fields, Richard T. Stoltz

Section 56.5001 Title 30, [Code of Federal Regulations1], establishes requirements for respirable dust exposure in Metal and Nonmetal Mines based on the Threshold Limit Values (TLV's) adopted by the American Conference of Governmental Industrial Hygienists. Respirable dust samples collected by Mine Safety and Health Administration (MSHA) inspectors at a sandstone quarry showed certain workers were exposed to excessive concentrations during development of a "coyote tunnel" that was ventilated with a blowing system. Because exhausting face ventilation systems are known to control dust more effectively than blowing face systems, a change was proposed to improve worker respirable dust exposures. This paper describes the evaluation of the proposed exhausting ventilation system in the quarry's "coyote tunnel" and compares the respirable dust exposures with those measured by the MSHA inspectors with the blowing system. Results showed the proposed exhausting ventilation system reduced respirable dust exposures to less than 20 percent of those measured by MSHA inspectors when the blowing system was used.

Jan 1, 1993

By K. Kim, H. Gao

A site-characterization strategy formulated for the probabilistic assessment of rock mass properties is presented. The strategy, which takes into account the variability of rock properties and the stochastic nature of the rock-failure process, entails determining probability distributions of rock properties by utilizing extreme value statistics and Monte Carlo simulation. Laboratory and, field test data are analyzed and presented to illustrate the methodology. The results indicate that the distributive characteristics of rock mass can be estimated by using this proposed method. The results of the analysis can be incorporated into the design of underground structures. Then, the relative uncertainties of the failure probability of the structures can be assessed.

Jan 1, 1997

By Harry E. LeGrand

The purpose of this paper is to outline briefly some basic principles of ground-water hydrology that have application in mine hydrology and to suggest that an understanding of these principles can help to forestall problems that are internal as regards mining operations and external as regards damage or harm to surrounding areas. The development of a mine below the water table pleads for accurate values to answers of hydrologic problems. Such internal problems involve both an inflow of water without adequate drainage facilities and a deficiency of water for the required mining or milling use. Methods which give acceptable quantitative values for the solution of many ground-water problems depend on rather simple geologic structures, characterized chiefly by homogeneous rocks of considerable areal extent. Unfortunately, mining operations are likely to be centered where rocks are not homogeneous. Consequently, methods that attempt to show values of the storage and transmitting capacity of rocks in mining areas must be applied with caution. In spite of complex geology and apparent uniqueness of individual mines and quarries, adherence to some concepts of ground-water occurrence and movement should be helpful to mine operators. Mining operations that extend below the water table disturb the natural hydrologic regimen. Prior to development, the water table slopes away from the site of a mine in at least one direction, but as soon as development begins and water is removed from the mine, the mine becomes the apex of a depressed cone in the water table, causing a convergence in the flow of water toward the mine, The behavior of the cone of depression, as it tends to deepen and to increase in areal extent when the mine is deepened, is related to the rate of withdrawal of water necessary to keep the mine dry. The significance and nature of the cone of depression in the vicinity of pumped wells has been studied by ground-water hydrologists for years, but the cone caused by pumping water from mines has generally been less systematically appraised. The geologic and hydrologic settings of mines range greatly so that a simple standard procedure for dealing with problems

Jan 1, 1972

By M. Girardeau, J. Blondin, M. Nomine

It is well known that surface moisture and fine content have a strong influence on coal handle- ability. It is known as well that these two criteria are not sufficient to classify coals. It seems that certain types of coals are intrinsically difficult to handle, the reason being still not well understood. For these coals, moisture and fine content are aggravating factors and have to be strictly controlled in order to avoid handling problems (sticking, arching in hoppers.. .). CERCHAR has undertaken research aimed at identifying the role of intrinsic characteristics of coal with regard to handling properties. Two of them have been identified as particularly relevant : the clay content and the "colloidal particle" content. Two simple tests commonly used for soils characterisation, but never used so far for coal, have been applied to characterise a wide range of coals. The derived classification has been compared with an empirical classification based on dry screening tests. DRY SCREENING EXPERIMENTS Dry screening at a relatively low screen aperture (e.g. 6 mm) may be considered as a good tool for assessing the handleability of wet fine coals. Difficult to handle coals always lead to rapid plugging of the screen whilst easy to handle coals can be screened without difficulties. A wide range of coals has been selected with various ash, moisture and fine content. Their characteristics are given in table 1. The experimental set-up consists of a 2 t.p.h vibrating screen equipped with two automatic re- cording balances collecting respectively oversize and undersize material. Provided that the feeding rate is accurately controlled, good reproducibility in the percentage passing as a function of time is obtained. Figure 1 gives examples of different types of relations that were obtained. - LMC coal exhibits a behaviour that is characteristic of easy to handle coals : % passing remains constant in the course of the screening operation. - B coal behaves slightly differently. One observe first a rapid decrease in the flow passing through the screen and then a stabilization. Hand- ling properties can be considered as intermediately - The curve relative to LHB coal is characteristic of a very difficult to handle coal that blinds the screen almost immediately. It is necessary to point out the fact that the classification derived from screening experiments is a global one, including not only intrinsic factors but also external factors such as fines and moisture content. The role of surface moisture is in particular very important and variations in this parameter may upset the empirical classification

Jan 1, 1988

By W. Lin

When a large area of open gob collapses suddenly, a windblast is produced that can cause considerable damage throughout the infrastructure of a mine. In a few cases, the windblast has been accompanied by ignitions of methane and/or coal dust. This paper describes the conduct and results of laboratory tests involving the compression of mixtures of air, methane and coal dust under a falling weight and while allowing controlled escape of the mixture. The transient responses on pressure and temperature sensors were recorded. In addition to an analysis of those records, the paper highlights those conditions in which ignitions occurred.

Jan 1, 1997

By L. Sandbak

A geomechanical rock mass classification or RMR is currently being used for the evaluation of an LHD mechanized mining system versus the conventional block caving method. The successes and failures of a Panel lS-19 LHD test block on the 2315 level can be attributed to the geologic and geomechanical zonation present. Trench blasting, caving procedures, and sequencing are other factors that were also considered. Lessons learned from the P.lS-19 test block have been used to alter the design for future LHD production blocks on the 2615 level. These modifications are designed to maximize the pillar and insure an adequate draw connection.

Jan 1, 1988

By P. M. Jones

Introduction The success of in situ leaching depends upon: • maintaining adequate injection and recovery flow capacities in wells, and • distributing leach solution effectively to contact and leach ore minerals within the target zone. Solution balance is an important indicator of the capability of a particular in situ hydrologic design to efficiently and effectively control and distribute leach solution within the target ore zone. As part of ongoing hydrological investigations into in situ leaching, the US Bureau of Mines (USBM) is studying factors affecting solution balance in an unsaturated copper ore deposit at the Cyprus Casa Grande Mine. The purpose of this study is to assess the effect of hydrogeologic features and in situ operating conditions on changes, during leaching, in: • solution storage within the deposit, and • solution balance. At the Casa Grande Mine, In-Situ-Leaching Test Facility, solution balance estimates are obtained for 10 selected intervals during 17 months of leaching under a variety of well configurations, pressures and flow conditions. Operational and geological description The Casa Grande Mine is located 32 miles south of Casa Grande, AZ. The mining area is within the Casa Grande Deposit, a porphyry copper stock consisting primarily of an intensely-fractured, granodiorite cap overlying a copper sulfide base. As a result of mine dewatering activity, unsaturated conditions existed in the deposit prior to leaching (Schmidt et al., 1990; Friedel and Schmidt, 1991). No groundwater was present in any of the in-situ-leaching, test-facility wells after drilling and prior to leach solution injection. Also, no ground water seepage occurs in the test-facility drift. The Casa Grande, In-Situ-Leaching Test Facility consists of 58 injection and recovery wells collared in an underground drift. The wells were drilled at various angles into the granodiorite deposit in a series of 15 divergent fans (Fig. 1). The fans are 12.5 ft apart and lie parallel to one another between the 900- and 1100-level drifts. Each fan contains three to five wells drilled at angles ranging from 6° to 89° from the horizontal. The wells can operate in an injection or a recovery mode. Currently, 36 flow sensors and 28 pressure transducers (part of a hydrologic, data acquisition system) are used to continuously monitor injection-well flow rates and pressures.

Jan 1, 1993

By Rong Guo, Qi He, Jinsuo Zhang

"Coal mine ranks No.1 among the 6 major high-risk industries, so it is no doubt that the senior managers in the coal mine bear more stress than other industry executives. The massive working stress results in serious influences on the physical and mental health of the senior managers of coal mine, which might also induce the safety risk in the coal mine. Therefore, to evaluate the senior managers’ working stress and put forward strategies of mitigating the stress is of great theoretical and practical significance in improving the physical and mental health of the senior managers and reducing the safety risk in the coal mine. The research on the working stress has been made in the western countries for almost 50 years, China began the research in the mid and late 1990s, the research is spread across multiple industries including education, medical treatment, computer and aviation. However, there is little research on the senior managers’ working stress of the coal mine, so the available literatures are few. In the paper, we, based on the working stress model of Robbins and through questionnaire survey, analyze the element composition of the senior managers’ working stress of the coal mine; then, we construct the evaluation indicator system of the senior managers’ working stress of the coal mine based on the individual factor, organization factor and environment factor; after that, we determine weights of each evaluation indicators by using the improved triangular fuzzy number weight method, and set up the evaluation model of the senior managers’ working stress of the coal mine on the basis of the fuzzy comprehensive assessment method. At last, in accordance with the model, we evaluate and analyze the working stress for the senior managers of some typical coal mines, and diagnoses the major sources and intensities of the senior managers’ working stress, and then propose corresponding strategies of mitigating the stress."

Jan 1, 2017

By B. F. Buie

In South Carolina and Georgia an area about 20 miles wide situated near the inner margin of the Coastal Plain and extending from the vicinity of Columbia, S.C. southwestward about 180 miles to the vicinity of Macon, Georgia is the principal kaolin-producing area of the world. It is especially important as the world's major source of clay used for paper coating. Essentially all of the land known or likely to be productive of quality kaolin in this area is now owned or controlled by producing companies. The least-fully developed portion is that in the vicinity of Wrens, Georgia which has been explored and developed since 1961. For the two decades following the end of world war II, demand for kaolin increased at approximately twice the rate for other industrial minerals. From 1945 to 1965 production increased four-fold. Since 1965 the rate of increase has been less, but the outlook now is that production and demand will double at least once again before the year 2000. A substantial part of this increased demand has come, and will continue to come, from industrial development, especially in the paper industry, outside the United States. The general effect, in addition to a continuing increase in production in the U.S., will be to cause: 1. Intensive research for development of more effective and more economical methods of beneficiation; 2. Concerted research effort to find an economical way of modifying high-viscosity kaolin so that it can be used for paper coating; 3. More active exploration in the U.S. and overseas, especially for deposits to supply the foreign markets; 4. More intensive study of the geological factors responsible for the accumulation of commercial deposits of kaolin; 5. Some modest rise in price, relative to the cost of other industrial minerals. The effect on kaolin producing region of Georgia and South Carolina will be to cause: 1. Working of progressively lower-grade deposits; 2. An increase in the acceptable thickness of overburden; 3. Mining of deposits farther down-dip, the use of which is now inhibited by large volumes of groundwater; 4. Increased threat of invasion of the kaolin in industry's market by substitute materials; 5. A substantial increase in concern over environmental factors; 6. More attention to be paid to the large opportunity offered for mining of silica sand and other co-products.

Jan 1, 1972