SURFACE

MINING

METHODS

In this unit, you will learn the various surface mining methods used to extract ore from near surface deposits.

After completing this unit, you should be able to:

• Understand the geological factors for surface mining

• Understand the engineering factors for surface mining

• Explain what a placer deposit is

•Understand the surface mining methods:

What is Surface Mining?

Surface mine -

A mine in which the ore lies near the surface and can be extracted by removing the covering layers of rock and soil.

Almost all surface mining operations are exposed to the elements and require no roof support.

History

The history of surface mining is essentially that of mining coal, copper, and iron ores, and the nonmetallic minerals - clays, gypsum, phosphate rock, sand, gravel, and stone.

Changing public policy is exerting strong pressure favouring a reduction or elimination of surface mining; and, since the economic differences between surface and underground mining for the remaining mineral resources is narrowing, this increasing force may become the deciding factor in determining the future trend in surface vs, underground mining.

Evaluation of Surface Deposits

The following outline lists the basic factors which must be taken into account for evaluation of a prospective surface mine :

o Geography

Legal status of land and mining rights

Historical, political, and socialogical factors

Geology

Mining conditions

Ore treatment requirements

Economic analysis

Geography

Topography, a function of location, affects cost of development and operation of a surface mine. Geographic location establishes

climate.

Location establishes the condition of remoteness from or proximity to civilization and its developed facilities such as transportation systems, power supply, labour pool, manufacturing and supply services, and specialty repair shops.

Legal Status of Land and Mineral Rights

Land and other necessary rights should be checked, such as water use rights and the ability to acquire auxiliary land for plant site, roads, tailings disposal ground etc.

Historical, Political and Sociological Factors

It is important to determine the extent and nature of national and local laws and regulations in regard to conservation, water use, water and air pollution, tailings disposal, reclaimation, handling of explosives, taxes, royalities, import duties, mining safety and health codes, wage and labour conditions, pension requirements, and unions.

Geological

Geological evaluation may include wide-spaced drilling, drill-sample logging, testing and processing, plotting of the data on maps and cross-sections, preparation of specialized interpretive maps, calculation of reserves by grades, calculation of stripping requirements, groundwater studies, and economic analysis.

Mining Conditions

The geometry of an ore body and the topography of the land surface beneath which the ore body exists will affect the kind and cost of a surface mine. The depth and character of overlying rock and the physical characteristics of the wall rock also affect the configuration and cost of a surface mine.

Ore Treatment Requirements

Almost every potential surface mine must consider some phase of product upgrading (benefication).

This may vary from a simple crushing and sizing operation to a complex operation including multiple stages of size reduction, concentration and agglomeration. In many cases, pilot-scale testing is deemed advisable.

Economic Analysis

In the broadest sense, economic analysis for a surface mine involves the determination of market value of the product and all the elements of cost of production.

By subtraction, a margin of profit (or loss) can be calculated.

Many new surface mines require very high capital investments. There are 3 commonly used yardsticks to value investment worth :

1) degree of necessity,

2) payback period, and

3) rate of return.

Methods of surface mining can be subdivided into various classes and subclasses (E. Bohnit, 1992):

Placer MiningPlacer deposits are concentrations of heavy minerals, usually within loose alluvium that can easily be excavated and washed. Placer minerals such as gold, tin, and tungsten minerals, are of relatively high value, but the value of the placer gravel itself may be very low, often less than a dollar per cubic yard.

For deposits of such low grade to be worked they must be near water, on or near the surface of the ground, and should be only loosely consolidated so that drilling and blasting are not necessary.

Placer mining affects large surface areas for the volume of material mined, is highly visible and has serious environmental problems with surface disturbance and stream pollution.

The bulk of placer mining falls into three groups-- panning and sluicing / hydraulicking, and dredging.

Panning

In recent years, gold panning has become a popular outdoor recreation. There is excitement and appeal in panning an occasional nugget or a few small specks of gold. The remote chance of discovering a rich pocket somehow missed by the old timers provides a strong incentive.

In general, far more money is made selling manuals, maps, equipment, and gas and oil to these hobbyists than is made from the gold itself.

Sluicing

In SLUICING the placer gravel is shoveled, along with a stream of water, into the head of an inclined elongated sluice box with RIFFLES positioned across the bottom. These trap the heavy minerals and the lighter minerals are washed over the top and out as relatively barren waste.

Sometimes fine gold is trapped as an amalgam when mercury is placed within the riffles or on a copper plate at the exit of the sluice box. The gold in the amalgam is

recovered by retorting off the mercury.

Sluicing is dependent upon a good water supply.

Hydraulicking

HYDRAULIC MINING involves directing a high-pressure stream of water, via a MONITOR or nozzle, against the base of the placer bank.

The water caves the bank, disintegrates the ground and washes the material to and through sluice boxes, and / or jigs, and / or tables situated down-slope.

Hydraulic mining totally disturbs large areas and puts much debris into the drainage system. Presently, hydraulicking is used primarily in Third World countries. It is closely controlled or prohibited in the U.S.

Hydraulicking

Dredging

Large alluvial deposits are mined by floating washing plants capable of excavating the gravel, processing it in the washing plant, and stacking the tailings away from the dredge pond.

A Dredge floats in water and digs the gravel by an endless string of buckets. Coarse material is screened out and dumped out the back. The fine material passes into a series of sluices where the gold in recovered.

Dredging

Several types of excavation methods are in use:

DRAGLINE and BACKHOE PLANTS. Dragline use in placer mining with washing plants is limited to shallow digging depths. Its bucket is less controllable on the bottom than the backhoe, and it is less able to dig into the bottom to clean up all the ore that may be there. However, it has the advantage of a longer reach.

The digging reach of the backhoe extends to as much as 70 feet below the surface. It has the advantage of relatively low first cost, excellent mobility, and an ability to excavate hard

material.

Dredging

BUCKET WHEEL HYDRAULIC DREDGES are becoming more popular for underwater excavation, except where a high content of soft clay exists or where excessive oversize material occurs. It is dependent upon flooded pump openings that convey the material mined to the washing plant, and therefore it cannot work above water level. Placement of the pump suction is

critical.

DredgingBUCKETLINE DREDGES are capable of continuous excavation and are very efficient. They mine, process, and discard tailings to waste in one continuous stream. However, no storage opportunities exist, and the stream moves through the system by the force of gravity. Buckets, supported by a LADDER, dig the mine face. Material moves up the ladder and dumps into a hopper that feeds the washing plant. They are capable of high excavation rates.

Various methods are used to position the dredge --anchored by wire ropes or piling (SPUDS) at the rear of the dredge. Boulders can cause serious problems.

Dredging

SUCTION CUTTER DREDGES are similar to the Bucket Wheet Dredge except the digging device consists of a series of cutting arms rotating in a basket about a suction intake. The rotating arms break up the bank material, slurrying it so it can be drawn into the dredge suction. It has proven to be successful in mining unconsolidated beach

sands and offshore placers.

PLACER MINING COSTS

Capital Cost of Bucketline Dredge (1990):

Operating Costs (1990):

Because large placer deposits can be thoroughly explored before floating a dredge, such operations can lend themselves to thorough planning, and it is possible to carry out reclamation as mining progresses at only a slight increase in operating costs.

Strip Mining When orebodies are flat-lying and close to surface, it is sometimes economical to remove the overlying rock to expose the orebody. The surface soil is stripped off and stockpiled for later land reclaimation.

A stripping dragline with a long-boom or long reach shovels are common.

Large-scale continuous bucket excavators are gaining popularity. These large scale machines are designed for high capacity output and are tremendous in size, highly productive, and very expensive.

1 Electric drills prepare the overlying strata for blasting.

2 Removal of broken ore.

3 Removal of broken rock.

4 Extraction of upper ore seam.

5 Removal of upper ore.

Strip Mining

Removing Overburden

Open Pit MiningAlthough the basic concept of an open pit is quite simple, the planning required to develop a large deposit for surface mining is a very complex and costly undertaking.

At one mine, it may be desirable to plan for blending variations in the ore so as to maintain, as nearly as possible, a uniform feed to the mill. At another operation it may be desirable to completely separate two kinds of ore, as for example, a low- grade deposit where one kind of "oxide" ore must be treated by acid leach, but a second kind of "sulfide" ore must be treated by different methods.

Open Pit Mining

The grade and tonnage of material available will determine how much waste rock can be stripped, and there is often an ultimate limit to the pit that is determined more by the economics of removing overburden than a sudden change in the ore deposit from mineral to non-mineral bearing material. The ultimate pit limit and the slope of the pit walls are therefore determined as much by economics and engineering as by geological structure. Material that is relatively high grade may be left unmined in some awkward spot extending back too deeply beneath waste.

Open Pit MiningThe typical large open pit mining operation that has been in production for 10 years and more is operating under conditions that could not possibly have been foreseen by the original planners of the mine. Metal prices, machinery, and milling methods are constantly changing so that the larger operations must be periodically reevaluated, and several have been completely redeveloped from time to time as entirely different kinds of mining and milling operations.

Open Pit Mining

Sometimes the preliminary stripping of the waste overburden is contracted to firms specializing in earthmoving. Mining is usually done by track-mounted electric shovels in the large operations, and by rubber-tired diesel front-end loaders in the smaller operations. Scrapers are sometimes used in special situations.

Large bucket-wheel excavators of the kind used in European coal mines have not been applied to metal mining, because this equipment is best adapted to softer bedded, relatively flat-lying strata..

Open Pit Mining

Haulage is usually by truck, although railroads, inclined rails, and conveyor belts have been used.

The conveyance unloads directly into a primary crusher and crushed material is stored in coarse ore bins prior to shipment to the mill.

Open Pit Mining

Bench level intervals are to a large measure determined by the type of shovel or loader used, and these are selected on the basis of the character of the ore and the manner in which it breaks upon blasting and supports itself on the working face.

Open Pit MiningBlastholes are usually drilled vertically by self-propelled, track-mounted pneumatic or rotary drills. Bulk explosives are loaded in the holes and large volumes of ore are broken in a single blast. Sometimes the drill holes are routinely sampled and assayed to help plan the position of the shovels in advance of mining.

Blasthole assay control is especially desirable when exploration data are incomplete or lacking as in the case in the older pits which have long been mined past the limits of "ore" used in original planning.

Quarrying

QUARRYING or Quarry Mining is usually restricted to mining dimension stone - prismatic blocks of marble, granite, limestone, sandstone, slate, etc. that are used for primary construction of buildings or decorative facing materials for exterior and interior portions of buildings.

Quarries generally have benches with vertical faces from a few feet to 200 feet in height. Blocks are drilled and wedged free in a highly selective manner using time consuming and expensive methods.

Planning of the excavation is based primarily on geological factors such as the direction and attitude of bedding and joint systems.

Glory Holing

GLORY HOLING involves a mine opening at the surface from which ore is removed by gravity through raises connected to adit haulageways beneath, and tramming the ore to the surface. It is suited to mining on a hillside, and irregular deposits can be mined without dilution by waste wall rock. Mining can be quite selective and little waste rock accumulates on the surface. However,

reclamation is difficult.

Auger Mining

AUGER MINING refers to a method of removing coal, clay, phosphate, oil-shale, etc. from thin seams exposed in deep trenches or high-walls in strip mines.

The auger consists of two principal pieces. The first is a cutting head, generally from 1.5 to 8 feet in diameter. It may be single or multiple. The second is a prime mover, usually a skid mounted carriage, providing a mounting for the engine, drive head, and controls. As coal arrives at the surface it is transported via a conveyor belt or a

front-end loader to a waiting truck.

Operations are usually low-cost and highly productive, but recovery ranges from 40 to 60%. It can be implemented with relatively low capital costs.

Heap leaching

Heap leaching is also used in recovering metals from their ores.

Bacterial leaching is first used to oxidize sulphide minerals. Cyanide solution is then used to leach the metals from the mineral

heap.

Solution Mining

Solution Mining

Basic concept

The theory and practice of leaching are well-developed because for many years leaching has been used to separate metals from their ores and to extract sugar from sugar beets. Environmental engineers have become concerned with leaching more recently because of the multitude of dumps and landfills that contain hazardous and toxic wastes. Sometimes the natural breakdown of a toxic chemical results in another chemical that is even more toxic. Rain that passes through these materials enters ground

water, lakes, streams, wells, ponds, and the like.

Solution Mining

Although many toxic materials have low solubility in water, the concentrations that are deemed hazardous are also very low. Furthermore, many toxic compounds are accumulated by living cells and can be more concentrated inside than outside a cell. This is why long-term exposure is a serious problem; encountering a low concentration of a toxic material a few times may not be dangerous, but having it in your drinking water day after day and year after year can be deadly.

The main theory of leaching neglects mechanisms for holding the material on the solid. Although adsorption and ion exchange can bind materials tightly to solids, we will simplify the analysis and consider only dissolving a soluble constituent away from an insoluble solid. An example is removing salt

from sand by extraction with water.

Solution Mining

Countercurrent stage wise processes are frequently used in industrial leaching because they can deliver the highest possible concentration in the extract and can minimize the amount of solvent needed. The solvent phase becomes concentrated as it contacts in a stage wise fashion the increasing solute-rich solid. The raffinate becomes less concentrated in soluble material as it moves toward the fresh solvent stage.

'Heap leaching' is a countercurrent process where the solid is in a stationary heap and the solvent percolates through the solid. An example is a dump or landfill. This leaching is essentially countercurrent. In industrial leaching, solvent and solid are mixed, allowed to approach equilibrium, and the two phases are separated. Liquid and solids move countercurrently to the adjacent stages. The solvent phase, called the extract, becomes more concentrated as it contacts in stagewise fashion the increasingly solute-rich solid. The raffinate becomes less concentrated in soluble material as it moves toward the fresh solvent phase.

Surface Mining

End of Unit 13