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Mining Technology in the Future

J. B. Mudd, Member AIME

Introduction It is difEcult to think of any activity on which mankind has been more

dependent than mining, and certainly there is much evidence in almost every part of the world of old workings that clearly demonstrate that man from earliest times has dug and toiled to take from the earth precious stones, metals and materials that from time to time have been essential or of value to him.

In step with social development and industrial and technological advances the tempo of mining activity, regulated only by the laws of supply and demand, has quickened and in this century has accelerated rapidly in keeping with its dramatic development. Indeed, the rate of change of events has been such that if it were to continue to accelerate as fast as it has over the past decade there is reason to wonder how the insatiable demands for metals and minerals are to be met in the years that lie ahead.

Shallow mines containing high-grade ore deposits either have been or are being rapidly mined out and their place can only be taken in general by ore lying at greater depth, or by ore of low grade, situated for the most part in remote and in underdeveloped areas. The cost of exploration and development is rising at an alarming rate, with the result that the expenditure now required to bring a mine to production far exceeds anything previously contemplated. It is estimated that the total capital required to maintain existing operations and develop new ones during the next 5 years will be of the order of $50,000 million and of this, new capital to bring new mines to production will be of the order of $3,000 million. Today no single mining finance company can, from its own resources, find all the money for a major mining development, and as a consequence there has been a strong trend toward joint ventures and to the formation of multinational con- sortium~.

International lending institutions already play an important part in financing many new mining projects and their participation provides the financial umbrella under which other lenders can invest with greater security. There is little doubt that to finance the costly ventures in the future, governments and international lending agencies, banks and finance

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houses will have to play an even greater part than they have hitherto, and the mining finance groups on whom both the administrative and the technical responsibility must fall must continue as before to take the leading and important role of coordinating the raising of funds, the expenditures, the appropriations and the distribution of profits. Their task will clearly grow increasingly more complicated and difficult, but technical competence in every discipline must remain the fundamental requirement. This technological expertise will need to be matched with equally competent management.

In view of these considerations, I shall deal here with the present state of technology in the mining industry, and the direction in which it is progressing, and shall outline some of the possibilities for advancement in the future. I have drawn on the experiences gained in the Anglo American Corp., Charter Consolidated and De Beers Groups, whose mining activities not only are on a worldwide scale, but also are concerned with almost every type of mining. In this group of companies and their associates, the Anglo American Corp. provides the central technical services to the many mining companies. It is responsible for mining at least one-third of the world's production of gold (excluding that of Communist countries) and is involved in producing cadmium, coal, cobalt, copper, dianmnds, iron, lead, potash. silver, tin, vanadium, wolfram, uranium and zinc. The Anglo American Corp.'s engineers have, as a result, accumulated much experience in underground mining, and particularly in mining at great depths, and also in large earth-moving open-cast operations and even in underwater mining. These operations have been or are presently being camed out on a number of continents and under a variety of climates that vary from the arctic through the tropics and include working in the deserts of the Sahara and of South West Africa. The experience gained has led to the establishment of technical services equipped to handle a wide range of mining problems and geared to meet the needs of the mining industry at a time when the finding of new mines and exploiting them grows more difficult. more complex and vastly more expensive.

Underground Mining Only about one-third of thc Free World ore production conlcs from

underground mining operations, but the importance of urlderground mining is much greater than this figure suggests. Because the average grade of ore mined underground is higher than that of open pit ore, it is probable, if we exclude limestone and road metal, that about half the world's mineral production is derived from underground mines. Certainly by far the major part of the gold is mined underground, as is most of the lead and zinc. leaving open-pit mining to lead the way in copper and iron ore production.

The initial development expenditure and cost of working underground deposits means that preference will be given to open-pit mining wherever possible, but the profitable exploitation of deeper deposits must remain a most important target for progress toward higher efficiency and lower

J. B. MUDD 121

costs. If underground rnmmg is to maintain its pos1tlOn in the supplyof minerals, substantial advancement will be necessary in shaft sinking,and development and mining methods, as well as in productivity.

Possibly the most important factor in underground mining is the efficient use of labor. It is becoming increasingly difficult to find men preparedto undertake heavy manual labor in hot and uncomfortable conditions. Itmay· become imperative to mechanize, not because it is cheaper butbecause it is the only way to get the work done at all.

Shaft Sinking

The circular shaft is widely accepted as being the most satisfactory typeof shaft and the merits of the twin-shaft systems, vertical and subverticaJ,as compared with the single shaft divided by a brattice wall, have beenwidely debated in recent years. However, at depths up to 7,500 ft, opinionin South Africa seems to favor a single large-diameter shaft, divided by a

Fig. l Vaal Reef South, South Africa. General view of the twin headgears.

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central brattice wall separating upcast and downcast compartments. Withgood ventilation arrangements, shafts may be spaced as much as 10,000ft apart.

The present maximum winding depth for a single lift shaft is consideredto be about 7,500 ft, but this may increase with the development of improved high-tensile winding ropes and cage arrestors to overcome ropestretch difficulties. Where increased capacity or greater depth is required,two-stage winding in vertical and subvertical shafts is necessary, but thishas recently been achieved in a single shaft by using two lifts with atransfer bin in the same shaft. The trend is toward larger skips, callingfor more powerful winders. Skips of 20-ton capacity and more are in use;they are hoisted by a gearlessmultirope winder powered by two 11,000 hpmotors. To increase shaft hoisting capacity, pumping of the minus~10-mesh

fraction of ore crushed underground is being planned. It is envisaged thatreciprocating high-lift pumps will raise this ore from 8,000 ft in two lifts,delivering it directly to the mill. A similar system was successfully used atBancroft Mines in Zambia some years ago, though from a much lesserdepth. Eventually, hydraulic lifting of coarse material, perhaps up to 2in., may become possible and may be used to augment existing hoistingfacilities. .

Shaft-sinking techniques still favor the use of grabs (Fig. 3), butthe increased size of kibbles and the resulting congestion at the shaft bottom have led to suggestions that the grab should be used to load a chainconveyor leading to a surge bin some way up the shaft, from which thekibbles would be loaded without the need for uncoupling.

Although shaft boring has been successful down to moderate depthsand to sink shafts of small diameter, there is clearly much development

Fig. 1 President Steyn, No.4 shaft, South Africa. Single shaft, 35 X 37 ft, withbrattice wall replacing twin shafts (under construction),

J. B. MUDD 123

to be done before sinking production shafts for large mines can be dortein this way. Where circumstances favor its use, it is undoubtedly worthconsidering.

Development

Modem mining layouts, with their highly mechanized operations, callfor large development footages to provide the facilities and the environ-

Fig. 3 Cactus grab at work during shaft-sinking operations.


ment needed for high efficiency. It is possible to break and deliver to theplant or dump a ton from an open pit (rock) for the same cost as toadvance a millimeter of an underground development. Hence, if an underground mine requires 10 mOl of development per ton of are, the cost ofpreparing are for mining is the same as it would be in an open pit with a10: 1 stripping ratio. This illustrates the advantage of open-pit over underground mining, and also shows the importance of efficiency in development and the need for thrift in mining layouts.

The introduction of raise-and-tunnel borers to hard-rock mining represents an important advance, and holds out promise of speeding up development and thus bringing mines into production more quickly and at lowercost. These machines eliminate much of the arduous work associated withconventional development methods. In 1969, more than 50 raise-andtunnel borers working in various mines throughout the world, madeadvances totalling about 100,000 ft. It is certain that both the total number

Fig. 4 Raise-borer hole, Nchanga mine, Zambia. Used as ore-transfer drift undera block caving stope, 6 ft in diameter, 405 ft long, 21 0 dip. Actual drillingand reaming time - 165 hours.

J. B. MUDD 125

of machines in use and the footage advanced per unit will rise steadily. Atpresent, the diameter of mine openings bored is between 6 and 8 ft, butlarger machines are being developed that will be able to bore 12-ft tunnelsin rocks with a compressive strength up to 30,000 psi. Speeds of 5 ft/hrhave been attained, and in the next few years we may expect to seedevelopment boring become cheaper than conventional methods if dueallowance is made for servicing of capital and for the time value of money.

Although the early raise-borers were designed for vertical or nearvertical operations, these machines can now be used on inclines as lowas 15 to 20°.

Mining Methods

The most economic mining method for a given are body depends onthe grade, shape, and dip of the are body, on the physical characteristicsof the host rock, on the surface topography, and on whether it may becaved or not. New equipment and developments in stoping technologyhave tended to reduce rather than extend the choice of method available.

The twin problems of minimizing dilution and maximizing recoveryare likely to lead to an extension of hydraulic sand filling methods, especially in high-grade, tabular, steeply dipping are bodies. Open stopingmethods (mostly variations of sublevel stoping) are cheap and satisfactoryin the early stages of mining, but as operations develop in depth and thecapital cost of installations increase, problems of rock pressure and dilutionbecome more severe and with this the cost of handling waste rock becomesmore and more an important factor. Waste fill requires more labor thansand fiJI does, and the only justification for the former - that it is easierto control when mining pillars alongside fill - has disappeared with theintroduction of low-grade sand-cement mixtures. Such mixtures, used toconsolidate the top of the fill, have enabled rubber-tired or tracked equipment to be used in the stope, and have made it possible to remove freshlybroken are without diluting it with the fill material. The use of double and

Fig. 5 Joy-Sylvite continuous miner designed to excavate potash at a rate of 700to 1,000 tons/ hr. .


triple boom jumbos has simplified the drilling of long holes. Reports frommany mines indicate remarkable improvements in production, with stopeoutput exceeding 50 tons per man-shift in large operations.

Where wall rock is strong and pressure is not a problem, shrinkagestoping from raises developed by climbers and raise-borers using longholering blasting has enabled continuous and uniform ore production to beachieved. The problems and delays associated with chute drawing havebeen obviated by the use of drawpoint loading.

A development that is likely to show important advances is the useof load-haul-dump (LHD) equipment. The versatility of rubber-tired dieseldriven LHD equipment lends itself to highly productive low-cost tracklessmining and can be adapted to a number of mining methods. In additionto being used at draw-points, it will be used extensively in stope development and is particularly appropriate to sublevel caving and room-andpillar methods.

In methods involving caving and the movement of large amounts ofbroken ore, the use of glass models to develop the theory of ground movement has contributed to better ore control, to some reduction in dilutionand to an increase in recovery of ore. The spacing and location of sublevels, the drilling pattern and the hole burden can be planned to accordwith ore-flow characteristics, controlled by such factors as the ellipsoidof extraction, the limiting ellipsoid of motion, its eccentricity and its drawcone angle. Although dilution cannot yet be wholly eliminated, over-allrecoveries are improving with less waste dilution. Such scientific methodsbased on analogous experiments and simulation will increasingly replacethe old rule of thumb.

The development of the continuous miner for coal mining and its

Fig. 6 Western Deep Levels. South Africa. Underground refrigeration plant.

J. B. MUDD 127

subsequent adaptation to potash mmmg is a remarkable success storyand a fascinating one. The conditions prevailing in the potash mines haveencouraged the manufacturers to design suitable equipment and to makemajor developments in this field. In Canada spectacular advances havebeen made towards realizing full mechanized mining with the use of continuous miners and extensible-type conveyors that together can consistentlysustain production over long periods. Machines weighing 35 tons, poweredby 200 hp motors, with an output capacity between 50 and 75 tons/hrhave been replaced by bigger and bigger machines until today machinesare available weighing 205 tons, of 1,500 hp, capable of cutting roomsup to 12 ft high at a rate of 400 to 500 tons/hI. A new machine developedby Sylvite of Canada Ltd. in combination with National Mines Ltd. isequipped with four cutters and weighs 248 tons. It is powered by 1,500hp motors, is capable of cutting twice the normal width in rooms 8 fthigh, and will have an output of 700 to 1,000 tons/hr. This machine, inconjunction with the extensible conveyors developed by Sylvite and theJoy Manufacturing Co., will be put into service in 1971.

The development of these machines is certain to have applicationsother than in coal and bedded evaporite deposits. The use of the Mariettaminer in iron ore may be the beginning for the use of this equipment in

Fig. 7 Westem Deep Levels, South Africa. Refrigeration plant - end of ventilation pipe.


hard-rock mining. The close cooperation between the user and the manufacturer both financially and technically holds out the best hope for major developments because the cost of development of such machines is so high that the manufacturer is reluctant to go it alone.

Long-wall mining equipment, protected by remote-controlled advanc- ing hydraulic roof jacks is expanding in use and there will be a movement toward still larger capacity and more powerful units. At the same time, the application of these methods to hard-rock mining is receiving much attention. The productivity of continuous mine systems in low stoping widths using extensible belts and flight conveyor bridges is competing successfully with that in wider seams.

Block caving, long recognized as the most productive method in metal mines, has shown little improvement in recent years, while other methods have made substantial advances. However, caving can make use of many of the techniques developed for other systems, such as using raise-andtunnel borers to facilitate the large amount of development required below the cave area and improving ore drawing and transport.

Ground Control

The unfractured, clean-cut circular section excavated by a borer will stand up better to the rock stresses encountered at depth than openings developed by conventional blasting, and a regular, predimensioned section will be relatively easy to support.

In large underground excavations, monolithic concrete support of the roof and walls is being largely replaced by rock bolting with bolts up to 20 ft long in combination with strip-metal or wire netting. The roof and walls are presplit to avoid overbreak and to prevent shattering. Rock bolts and anchors have been developed to provide tensile strength up to 30,000

I

Flg. 8 Flowing characteristics of ideal solid flow mass.

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psi. Specific rock conditions are met by various types of bolts, including wedges and simple or tandem shell expansion bolts or prestressed bolts with epoxy or cement grout. Rapidity of installation, instant support, and relative cheapness have made rock bolting, with or without shotcreting, the most ~ d e l y used method of roof support.

In flat stopes, the all-timber mat-pack support is being replaced by a combination of timber layers and concrete blocks ("sandwich pack"), or by hydraulic jacks or props. Rapid-yielding hydraulic props have been used to advantage in very deep mines (8,000 ft) where rock bursts are common. With advances in the understanding of rock mechanics and better interpretation of ground behavior, a wider use of this type of equipment may be expected.

Instruments to measure rock stresses and to record movement have been considerably refined and these, used in conjunction with the analog computer are providing more and more the basic information on which excavations are planned.

Haulage and Handling

The concept of a fully mechanized mining operation starting at the excavation and ending in the concentrator bin has made considerable progress in the last decade and it is certain that it soon will be developed further. Well designed conveyor systems working together with continuous miners have operated successfully in many fields. Conveyors have replaced skips in some inclined shafts. Improvements in the strength of belting and the introduction of devices to prevent runaways have removed some of the limitations to long, inclined conveyor systems and have extended the depth to which they can operate. A conveyor system with a totalclength exceeding 100 km has been built, using steel cored belts.

In Sweden and elsewhere much has been done to improve locomotive haulage systems and particularly in the direction of automation. In some instances, rubber-tired vehicles operating in zig-zag inclines or spiral ramps have replaced shafts for hauling ore or men and materials.

Ventilation

For many years, dust and blasting fumes have been recognized as dangerous to health and in most established mining countries regulations under the mining laws dictate the minimum ventilation requirements. In designing mine layouts, the quantity and quality of the air to be circulated to the working places have always been of prime importance. Workings have become deeper and farther from the downcast shafts, and virgin rock temperatures have accordingly become progressively higher. Whereas attention was once mainly centered on the removal or dilution of fine dust and fumes, and on the provision of sufficient oxygen, as rock temperatures increase this attention is directed more and more toward obtaining an environment that promotes efficient work. As the virgin rock temperature

rises above blood heat, increasing quantities of air are required to maintain good working conditions; and where fresh air alone will no longer serve, refrigeration and air reconditioning become essential additions.

It has been proposed that in sublevel stoping the blocks of ore could be precooled by passing refrigerated )air through the cutoff raises, which would be bored; this way there would be no need for men to enter the block until working conditions are acceptable.

The importance of proper positioning of air conditioning units has been recognized for some time, and because these units must be sited to suit the heat rejection system, it is necessary to find means of transferring the cooling effect over considerable distances. This is done by circulating cold water to heat-transfer units near and even in the working places, and distances as great as 8,000 ft have been achieved with relatively small loss in heat efficiency.

In the deepest South African gold mines, the virgin rock temperature is about 126OF at 11,000 ft or more below surface, and the average quantity of air circulated to the workings has increased from 4.6 cu ft/min per ton mined in 1962 to 5.7 in 1969. The high cost of bringing a cubic foot of fresh or cooled air to the face makes it essential to avoid losses by short-circuiting. The use of polyurethane foam has helped to establish high standards in ventilation sealing.

Ventilation planning has been assisted by the introduction of computers and analog devices. Mathematical models have been developed for use with digital computers, enabling heat flow to be calculated and leading to the prediction of environmental conditions in ventilation districts and hence facilitating the estimation of ventilation requirements.

Underground recooling of return air for further use, which reduces the downcast capacity required, is increasing rapidly. Thought is being given to the use of vortex-cooled "cold suits" to provide acceptable conditions where only a few men are working in hot places. On the medical side, the degree to which pneumoconiosis is caused by outside factors - by dusts other than silica and by cigarette smoking, for example - is now known to be significant.

W a t d e t Technology ~ Pulsed water jets developing pressures of more than 300,000 psi are

capable of disintegrating the hardest rock. At this high pressure, the jet has almost the effect of an explosive charge fed continuously to the rock face. This is a promising tool for rock breaking and has obvious advantages over high explosive for certain applications such as the following:

1. Continuous mining with water-jet breaking and hydraulic transport in the case of coal, potash, phosphate and other relatively soft deposits.

2. Hard-rock development in a similar manner, in competition with boring machines.

3. Secondary breaking at the drawpoints of sublevel, shrinkage and block caving stopes, or in open pits.

J. B. MUDD- 131

4. Excavation of narrow veins without breaking the wall rock, leading to resuing with optimum control.

Theory and design parameters for this technique have been worked out and a small test unit built to verify them. Although 150,000 psi is the practical limit that steel can repeatedly withstand, higher jet pressures have been achieved using the "cumulative" technique. A jet velocity of 6,000 ft/sec should produce a pressure capable of disintegrating granite and other hard rocks. The maximum potential drilling rate is reported to be up to five times greater than the fastest conventional drilling method.

At lower pressures the water-jet technique is already in use. A recent development is a surface rotary drill fitted with a rotating water jet in conjunction with pumping to remove the rock excavated by the jet. By moving the jet up and down, it is envisaged that a cylinder of rock will be removed at depth so that the advantages of solution mining could be obtained in nonsoluble rocks.

Nuclear Explosives in Underground Mining

Research and large-scale testing of underground nuclear blasts have reached a point where the application of nuclear explosives to mining can be imagined. The tests have made it possible to establish empirical rela- tionships among explosive energy, cavity dimensions, shape, fragmentation and the extent of permeability of ground around the axis. Safety can be assessed and the cost per ton of ore broken can be estimated. The size

ORIGINAL

GROUND SURFACI

C O T O N . .ppARENT I I C I A T l l or LIP.

OF ORIGINAL

GIOUNO SUWACE

TRUE CIATIR IIOUNOAR*

Fig. 9 Nuclear crater and nuclear chimney.


and cost of a suitable nuclear device can be calculated. Work has so far been concentrated on the effects of single explosions. Double or multiple explosions producing contiguous chimneys remain to be investigated, and offer what may be a fruitful field for research.

The most probable application of the underground nuclear blast is fragmentation of a selected area to .form a rubble-filled chimney with sufficient permeability to allow circulation of leach solutions, either chemical or bacterial. Advances in solvent extraction technology would obviate some of the problems of chemical leaching, particularly the clogging of interstices with iron salts, and the need for large amounts of suitable scrap iron. The increasing demand for copper can be met only from deposits of lower and lower grade; and where these are too deep for economic open-pit mining, it may be that the only answer is nuclear blasting followed by in-situ leaching. Such methods may well be proved feasible by the end of the decade.

Nuclear blasting may, in addition, lead to a revival of interest in the block caving method. By blasting selectively interconnected rubble-filled chimneys of predetermined size, the ore could be fragmented so that the shape of the fragmented zone could provide the most favorable conditions. There is a similarity in shape for ore flow between underground draw cones and surface craters, and between the extraction ellipsoid and the typical nuclear chimney, so that extraction of the broken ground could probably be controlled using existing methods of draw-point control.

Open-Pit Mining Recent developments in open-pit mining have been largely in three

fields - increasing the size of equipment, improving the methods of pit planning and ensuring maximum slopes consistent with safety. The last decade has seen truck capacity rise from 50 to 200 tons, truck engines increase from 400 hp with mechanical transmission to 1,650 hp with diesel electric units; at the same time the bucket size of shovels has increased from 6 to 25 cu yd. The computer has been widely used for pit planning, and with more sophisticated programming it is being used to an ever greater extent. Research into pit slope stability is being actively pur- sued everywhere.

Equipment

As with other types of equipment, drills have tended to increase in size, and there has been a movement toward larger-diameter holes, wider spacing, and heavier burdens, in an attempt to increase explosives efficiency. When carried to excess, this development has led to poor fragmentation and hence to lower loading and hauling efficiency, and to the installation of very large primary crushers as an alternative to the problems arising from secondary blasting. Consequently, holes with diameters from 9 to 12 in. are now more common, and efforts are being directed toward

J. 6. MUDD 133

improving efficiencies in this range. Faster drill penetration is an obvious way to lower costs per foot drilled and this may be obtained by automation to give optimum rotation speeds and pressures. Research into bit metallurgy may be expected to increase bit life. There appears to be little probability that such unconventional^ proposals as lasers, electric arcs, el- tron beams or electrical disintegration will become standard practice in the next decade, but it is possible that a high-pressure water-jet drill may be successfully developed.

The use of more powerful slurry-type explosives, mixed at the site, is gaining popularity and can be expected to be developed further.

The electric shovel as a loading unit is now highly developed. Recent trends have been toward solid state and a reduction of moving parts, coupled with an increase in size. More recently, large hydraulically operated face shovels with more efficient front-end-loader buckets have been tried and can be expected to replace the rope-operated shovel. Extremely large stripping shovels and draglines are now in operation in strip mines. It is difEcult to imagine a machine with a bucket larger than 220 cu yd, but if the necessity arises for a bigger machine, present technology is capable of producing it.

Other developments have been in the field of belt loaders, which simply provide a loading boot into which material is pushed by a bulldozer. It is possible to foresee increasing use of these loaders as bulldozers increase in size and efficiency. However, this technique must generally be limited to softer materials.

It is perhaps in the field of haulage that the greatest strides have been made over the last 10 years. In 1960, a 65-ton rear-dump truck with two axles was hardly visualized; today, 200-ton trucks are in production and 300-ton units are on the drawing board. Engines have tended to lag behind what the truck size requires, but with the advent of the 200-ton unit the locomotive-type diesel engines have been brought into use. Engine size and power no longer restrict truck size, and the limiting factor is now tire technology. Tire problems are being overcome by the manufacturers and, over the next few years, there appears to be no technical reason why units with a capacity up to 500 tons should not be constructed. Whether mining operations will increase in size to the extent that it will be feasible to employ units of, say, 500-ton capacity is a Merent matter. Such a unit, operating on a 20-minute cycle would be capable of producing up to 8,000 tons/shift or more than 7 million tons a year. The scale of operations required to justify even a small fleet of these vehicles would be enormous and there must be few, if any, ore deposits of sacient size to justify their use. Consequently the demand for such units would be very limited and their high development costs would almost certainly make them compare unfavorably in operating costs with fleets of smaller units. Moreover, loader utilization with a small fleet of large trucks would be relatively low. Therefore, it is probable that 200 tons is about the maximum economic size that can be presently foreseen for reardump trucks.


As the size of other equipment has been increasing, so has the size of scrapers and rippers. However, ripping is still limited to relatively soft rocks or material that has been shattered by blasting. The recent development of pushers approaching 2,500 hp could alter this and make it possible to load harder material. Increased flexibility has been added to this method by the elimination of pushers either by using load elevating systems or by combining two units for the loading cycle. The development of larger and larger units is limited only by the capital cost; eventually the initial outlay on a few units would become so great that it would be cheaper to install a continuous handling system that would also be cheaper to operate. Diamond mines on the South West Africa coast will shortly be using the 657/666 30.6 cu m bowl-scraper of 950 hp in conjunction with push-pull equipment.

Bucket-wheel excavators and allied systems have been increasing in size and efficiency along with other types of earthmoving equipment. With increasing use of hydraulics making possible the construction of more powerful, compact machines it has become possible to excavate harder material than was previously possible, but use is still limited to relatively soft or friable material. A continuing disadvantage is the idexibility of the system. However, as operations increase in size this becomes less so, and the continuous excavation has distinct economic advantages. Further advances in design can be foreseen, which would enable these machines to mine harder materials.

Pitpaandq

As the depth of the pit increases, so the cost of hauling material from the bottom increases. At the same time, the amount of waste that has to be removed to uncover a ton of ore tends to increase. It becomes increasingly necessary to plan in detail to ensure that only enough waste is removed to enable the last ton of ore to be mined profitably.

Truck haulage costs increase rapidly as cycle time increases with depth. Maintenance problems increase; some mines are already experiencing difficulties with electric-powered-wheel drives on 100-ton trucks hauling from 600-ft depth. \%eel motors hauling 7,000 ft on an &percent grade tend to overheat. Inclined skip systems have been installed in a few pits but these are so inflexible as to create severe delays in truck turn-around time. Rail haulage has been used for many years but its limitation to low gradients makes for long hauls and inflexibility. It is apparent that none of the present systems will provide the perfect answer to the deep, large- scale operations of the future. Some compromise is necessary and the best arrangement for any particular condition must be some combination of the presently known methods. Flexibility combined with low costs in operation must be aimed for, and the first steps in this direction have already been taken. For example in the initial stages of an open-pit operation, large bowl scrapers have been combined with a conveyor system to remove a large quantity of soft overburden quickly and relatively cheaply.

J. B. MUDD

Fig. 10 Nchanga open pit, Zambia. Present depth, 700 ft; proposed final depth.1,500 ft. Total excavations in 1970 - 52 million tons, increasing annually. Stripping ratio - 10:1, increasing to 20:1. Vehicles on the haulroads are 110-ton dump trucks with electric wheel drive.

..;

~ 60

""~

z2•'"•,.o 40u~

z,.<•'"•ooz<~ 20z•o

CONVEYOR @lOOm. L,.,I

COSTS INCLUDE THOlE FOR ANCILLIARY EQUIPMENTDRILLING BLASTING ETC.

100 200

OEPTH IN M<::TEAS

100 400

Fig. 11 Owning and operating costs for a 15 million ton/year operation.

136 Al ME CENTENNIAL VOLUME

In the next stage it is more economical to use a conventional shovel and truck, but with increasing depth the capital cost of a larger fleet of trucks will make it more economical to install a conveyor belt system to take the material out of the pit. This point may be reached at a depth of about 400 ft. A crusher will usually be necessary to size the feed to the belt; and the elaborate layout can only be justified if it will be used a reasonable length of time - until the pit becomes deep enough to justify another system, possibly involving the movement of the crusher and the addition of another conveyor system to feed onto the hst. It may be possible to take the conveyor out within the confines of the pit, but the shape of the latter may demand a conveyor tunnel.

Since ore has to be crushed at some point in the circuit, crushing it in the pit will add little if anything to the cost. Where there is a lot of waste, it would be better to eliminate the crusher; hence it would be important to develop more effective drilling and blasting methods to ensure good fragmentation to make the ore acceptable to a conveyor system.

Fig. 11 shows the owning and operating costs resulting from a theo- retical study of a 15 million ton/year operation. Over a 15-year period a system employing a conveyor would show a total operating cost, including servicing capital, of 11 percent less than the over-all cost of hauling with conventional trucks.

Pit Slope Stability

A great deal of research has been carried out on the stability of open- pit slopes. Here, again, experience and rule of .thumb were the only av@l- able guides until recently. This work has shown that the shape and angle of a pit wall slope, the nature of the rock, and water pressure are all related, and that these factors determine the chances of a collapse and the nature of it.

By using suitable instruments in boreholes drilled from the benches and behind the face, it is possible to obtain data that will enable the behavior of the rock to be assessed so that the steepest slope can be selected that will still avoid a collapse. Given the height of bench and width of berm suitable for the mining equipment to be employed, it is possible to design the profile of the pit in detail. The optimum slope is not necessarily the same from top to bottom. A few degrees of pit slope may have a significant effect on the stripping ratio and hence on the economics of the operation; consequently more and more attention will be directed toward this subject.

The importance of water behind the pit face has been recognized and instruments have been developed for measuring the pressure. In some pits boreholes have been sunk to relieve the pressure and adits have been used to drain the ground.

J. 8. MUDD

Waste Dump Leaching

Waste dump leaching techniques have been employed for many years even though the fundamentals of the process have not been fully understood. Only recently has detailed research into the process been carried out. The objects of the research prbgram have been to discover methods of speeding up the leach process and improving recoveries. To do this it has been necessary to study each mineral involved in the process to discover optimum leaching conditions and methods and to examine the dumps themselves to discover which dumping methods and codgura- tions are most amenable to even distribution of the leach solutions.

One of the major problems has been that of obtaining an even distribution of the leach solution throughout the dump. Several methods have been employed, such as ponding and spraying; drill holes have been sunk, and perforated injection pipes emplaced. Layered compaction and precipi- tation of iron salts are among the problems to be overcome. It is possible that with the use of conveyors to the waste dumps the compaction problems will be alleviated and better control of pH values, by means of on-line sampling, assaying, and the use of the computer, may eliminate the problems of choking the permeability of dumps by iron salts. .

A great deal of research still remains to be done before the processes are fully understood and controllable. Chemo-autotrophic bacteria have been found to be important biological catalysts in the breakdown of copper and other sulfide minerals. Biological research is progressing now to the stage where more efficient biological agents can be proliferated.

Newly developed solvent extraction methods will eventually eliminate the large quantities of scrap metal needed at present to precipitate copper. This will make it feasible to employ leaching in poorly populated areas where little scrap metal is available.

Dredging In recent years, dredging has been marked by increases in dredge

capacity and digging depth, the emergence of the sea dredge and a revival of interest in the suction-type dredge. Land dredges are now operating at a depth of 180 ft, and suction dredges are operating in sea areas up to 250 ft. It is possible that greater depth will be achieved in the future, but this will call for changes in basic design, particularly in the bucket-line dredge.

Bucket-line dredges are using up to 160 buckets on the ladder, the buckets being cast of manganese steel in sizes varying from 7 to a maximum of 24 cu ft. Such dredges have a throughput of 800,000 cu yd/month of tin-bearing ground, more when overburden is being removed. The largest dredges have a pontoon length of about 400 ft and a width up to 9 0 ft. They incorporate automatic trimming with liquid ballast, closed- circuit television aids for the operators, and push-button control of the complex, heavy machinery.


Fig. 12 Tronoh mines, Malaya. 400,000 cu yd/month bucket-line dredge.

Bucket dredges dig more efficiently than suction dredges in hard"packed and clayey ground. By skimming a few inches off the rock, theyare better able to clean soft bedrock and ensure complete recovery of ore.In hard limestone-pinnacled ground, however, this does not apply, as thebucket line must be kept above the pinnacles for fear of damage.

Suction dredges are generally cheaper to build than bucket dredges,but because of the power needed to pump a pulp containing a low proportion of solids they tend to be more costly to operate. The largestsuction dredges are built for harbor clearance work with capacities upto 4,000 cu yd/hr (some 2 million cu yd/month) using 32-in. suctionpipes. However, for tin mining at sea the largest suction dredge constructed has a capacity of about 500,000 cu yd/month.

Suction dredges are better able to work thin sediment (10 ft thick toa few inches) on the sea bed. They are better in rough seas than bucket

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J. 8. MUDD 139

dredges, as it is easier to install the necessary compensating gear to keepthe suction head in contact with the sediment, but recoveries are oftendisappointingly low.

As the richer alluvial deposits are worked out, it will be necessaryto mine lower-grade material, including the tailings left by previous operations, or to dig deeper in an effort to recover rich material, now coveredwith tailings, that could not be reached by the smaller dredges of thepast. It becomes necessmy to build larger and more efficient dredges, andto extend offshore operations where higher-grade alluvium may yet remainto be mined.

There is a limit to how much the capacity of a single bucket-linedredge can be increased, owing to the weight and inertia of the ladderand buckets. To reach a throughput of 1Yl million cu yd/month, 35 ~u ftbuckets, weighing about 5 tons each, would have to be used. Up to apoint, capacity can be increased by speeding up the bucket line, but thisintroduces new problems, not the least of which is the shock to the wholesystem when the bucket strikes a firm obstruction such as a limestonepinnacle. A band of 35 cu ft buckets would perhaps cost £ 250,000,about 21h times the cost of a band with standard 18 cu ft buckets.

To achieve a throughput of 1Yl million cu yd/month, a design hasbeen suggested in which digging is carried out by two bucket ladders side

Fig. 14 Consolidated Diamond mines, South West Africa. Exposing beach fordiamond recovery.


by side, one operating at an elevation somewhat higher than the other.The ladders could be of standard design, using well tried equipment inthe way of buckets, tumblers, drives, screens and concentrating plant.Equipment handling and repair problems would not be increased, as theywould with a larger bucket line. Availability would be higher than in asingle-bucket-line operation, as one ladder could be inspected or repairedwhile the other was still working. It has been estimated that the availability of such a dredge might be 93 percent as against 85 pe1cent forthe conventional unit - an important consideration when the high capitalcost of the unit is considered.

The most recent advances in dredge design are related to drive improvements, as many dredges have had their capacity increased by simplyspeeding up the drive. Hydraulic equipment is now being tested to replacethe existing Ward Leonard and AC drives. It has the advantage of providing a smoother operation with less impact damage to the gear trains,shafting, tumblers and the buckets themselves, when an obstruction is met.

For offshore dredging, a dredge with combined bucket and suctioncutter has been proposed. A conventional bucket dredge operating in anunprotected offshore area in Southeast Asia may be able to operate Joronly half the year because the ocean swell during the monsoon season

Fig. 15 Plastic-covered sand banks supporting a movable-block waH so thatdiamond mining operations can be carried out closer to the low-watermark on the beach. Consolidated Diamond mines. South West Africa.

J. B. MUDD 141

lifts the bucket ladder off the bottom and drops it again with a damagingforce. The weight and inertia of the ladder make it difficult to use astabilizing device. With a suction cutter, it is easier to accommodate theeffects of the swell, but operating costs are higher. The combination dredgewould be able to deal effectively with the considerable variations in thickness of alluvium that characterize some offshore deposits.

It is likely, however, that entirely new concepts will supplement orreplace the conventional dredge for offshore operations. Two such developments are now being tested.

1. A remote-control submerged bulldozer that can be used to gatherthin sediments into piles for collection by a suction dredge (Komatsu).

2. An undersea dredge moving on tracks on the sea bottom, fittedwith a somewhat conventional suction cutter head and pumping thealluvium by pipe line to a shore-based treatment plant or to a mothership. Fresh air is supplied from the shore. The hydraulic boom supportingthe suction head is 30 ft long, and a 750 hp motor supplies the necessarypower, with a 50 hp hydraulic motor driving the cutter heads.

Other types of submarine dredges are being tested or are under construction for experimental work throughout the world. Research to conquerthe problems of mining the ocean bed is likely to accelerate during thenext few years.

Other Mining Problems

Transport

The transport of a mine's output to the market was once a matterlargely outside the miner's scope. Today, he may have to pay for anddesign a transport system or negotiate with some government or international financing agency about the provision of such a facility. Slurry

Fig. 16 Undersea dredge designed to work on the sea bottom.


loading of ore ships, pipeline transport over considerable distances, and unit trains have all emerged as methods to be considered. The transport cost of such minerals as iron ore and fertilizers may make the difference between profit and loss. Developments in the field of transport are important to the mining industry and may transform an ore body of doubtful potential to one of considerable interest.

Pollution

The recent wave of indignation against pollution of the earth, air and sea is no transient phenomenon. Mines and treatment plants are going to be obliged to plan their operations to avoid pollution. To some extent at least, this may be profitable. Certainly, mine and plant layouts will have to take into account to a far greater extent than before the disposal of their waste products.

Water Supply

The rapidly expanding domestic and industrial demand for water is becoming a problem almost everywhere. Whether mines are found in deserts or in well watered lands, suitable water for their process plants and housing is likely to cause a dii5culty and may in many cases limit the size of the operations. Economy in the use of water and its recircula- tion and purification will be among the most important considerations in determining the feasibility and viability of exploiting a new ore body.

Financial and Operational P l a d q

Whereas, in the past, mines were planned empirically from year to year, carefully prepared 5-year and longer plans are now common. These lay down in detail the development or stripping program, production tonnages, ore grade to be milled, and concentrates to be smelted.

Capital expenditure programs are prepared. Financial outlines are drawn up, and from them, cash flows, estimated appropriations, and divi- dend payments are derived. In this way it is possible to insure that enough development or stripping is planned to maintain an ore reserve, that major shaft sinking or expansion programs are undertaken at the right time and that profits are reserved as may be needed for replacements. Estimating, planning and budgeting are, of course, essential processes of management. Techniques to improve, refine and streamline these have made much progress, and much effort will no doubt be made to improve them still further in the future.

Operations Research The application of operations research (OR) tb the problems of the

mining induktry has made much progress in the last few years. It has taken some time to realize that it is not a magic formula for the solution

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of all problems and that a great deal of basic information must be accumulated before it can be used successfully. It provides information from data processing that reveals many ,hidden solutions or alternatives and quantifies new concepts, ideas and imaginative thinking, contributing greatly to the improvement of mining operations. Finally it renders the decision-making more reliable.

Computer simulation is being applied in solving routine engineering problems both in open-pit and underground operations. The main purpose of this is to solve problems of breakdown in continuity and to increase the efficiency and productivity of equipment. Simulation is also used in designing new operations and in optimizing methods, equipment and investment. Problems related to maintenance and the mechanical availability of equipment are also being dealt with by OR. The objective is to minimize the combined maintenance and replacement costs of equipment per effective working hour, and to improve the efficiency of equipment by increasing its mechanical availability. Statistical approaches to investment planning are in use and will be used more extensively in the future.

Mining investment planning has been facilitated by the construction of discounted cash flow (DCF) yield models, representing the investment climate of various countries as a multivariable function of revenue, working cost, capital cost, and their components (Fig. 17). These models are used to determine exploration targets and to obtain a quick evaluation of inferred mineral reserves.

Mining economics is based on a wasting asset, and mining and economic optimization studies are very necessary before any development is undertaken. The time and the opportunity to optimize a mining plan are at the outset, and once the optimum plan is missed its benefits are often irrecov- erably lost. Fig. 18 is a recently developed optimization package flow chart. It consists of more than a dozen interdependent computer programs that may be classified in four main groups: (1) ore reserve calculation and classification, (2) mine design, (3) optimization by incremental DCF yield analysis, and (4) final feasibility.

The flow chart illustrates the computer routines for deriving the optimum mining plan for an open-pit proposition. Multiblock and moving- weighted-average methods are used in calculating the ore reserves; the bench plans are prepared by a plotter; and costs are introduced via a mining simulation program. The optimum plan is selected by incremental DCF analysis. Mining plans are produced for various cutoff grades. For each cutoff grade, the program prints out the ore reserve that can be mined at a profit, the overburden that must be stripped and the profitability index (for example, DCF yield and payback period).

Conclusion The challenge before the mining industry today is to be able to

continue to satisfy the world's rapidly growing demand for minerals and metals. To meet this challenge the most important requirement is the

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efficient use of technical, financial and human resources. However, much will depend, as it has in the past, upon the large mining finance houses to provide the high-risk money, the initiative, the leadership and the expertise to undertake the prospecting, investigation and research on which new processes, inventions, improved methods and better practices rely. These alone can offset the rising cost of raw materials on which the modem world depends and make possible the exploitation of low-grade ore bodies of marginal profitabiity.

Technical improvements, scientific advances that underlie material progress, will continue apace, and achievements more dramatic even than those made in the past decade will no doubt be realized. This will mean higher standards of living, new ways of life, more rapid means of com- munication and inevitably management problems more complex than before. The wide spectrum covered by mining, its associated plant, its infrastructure, specialist services, metal sales and finance, calls for the recruitment, training and development of people on a scale even more comprehensive than in the past.

Mining finance groups must therefore be prepared for si&cant changes as well as new and often unexpected developments both in the use of materials and in political attitudes. In fact, it is not an exaggeration to say that their ability to survive as commercial organizations will depend on formulating policies far-seeing and imaginative enough to anticipate the changes, and on an ability to adjust their attitudes accordingly.

Understandably, governments of developing and underdeveloped countries where the important mineral deposits are found want their countries to have the majority share in the exploitation of what they regard as their own mineral wealth. Bearing this in mind, it must also be accepted that if the financing of large investments in mining propositions of the order visualized is to be undertaken from international capital resources, a formula must be found that not only takes into account national aspira- tions, but also satisfies the investor, both corporate and individual. The investor must be assured that the operation provides a reasonable measure of security and must be confident that large, long-term investments in mining can be made with safety.

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