Technology News

From the Bureau of Mines, United States Department of the Interior

Technology news describes tested developments from Bureau of Mines programs. It is published to encourage the

transfer of this information to the minerals industry and its application in commercial practice. Mention of company

or product names is for documentation only and does not imply endorsement of a specific firm or product.

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No. 436, May 1994

Stope Leaching Reduces Surface Environmental

Impacts From Underground Mining

Objective

Modify conventional mining practices to reduce significantly the amount of material brought to the surface from

underground mines, or to “re-mine” existing surface mine

dumps by backfilling empty underground stopes.

Background

Conventional underground mining practices include excavating the ore and transporting the ore to the surface for

processing. Underground mining systems handle tons of

solid material to produce kilograms, and sometimes only

grams, of final product. Processing of ore on the surface

results in most of the excavated material remaining on the

surface exposed to long-term degradation from wind and

water.

The underground stope leaching mining system (fig. 1)

involves applying leach solutions to mineralized rock that

has been fragmented by blasting ore in place, or by backfilling empty stopes. Only enough material is removed

from the underground mining area (stope) to allow for adequate expansion during the blasting. This reduces the

amount of material brought to the surface by at least twothirds. After blasting, leach solution containing chemicals

and/or bacteria is circulated through the fragmented ore

to dissolve the target mineral. The resulting solution containing the dissolved mineral (pregnant leach solution)

is pumped to the surface where the product is removed,

and the leach solution is regenerated for recycling

underground.

Approach

Successful implementation of stope leaching in an underground mine requires control of the leaching solution,

as well as economical leaching solutions that are environmentally compatible. Detailed characterization of the rock

mass prior to leaching is critical for ensuring solution containment. Preleaching rock-mass characterization locates

zones of high permeability, such as fractures that can be

sealed prior to introduction of leach solutions.

U.S. Bureau of Mines (USBM) researchers have tested

and compared rock-mass characterization procedures utilizing geologic mapping, various geophysical techniques,

and hydrologic modeling at an underground test facility at

the Edgar Mine (an experimental mine in Idaho Springs,

CO), which is owned and operated by the Colorado School

of Mines (fig. 2). The test facility was constructed by

mining a simulated leaching stope using a blasting technique designed to maximize ore fragmentation and to minimize damage to the wall rock. Boreholes were drilled at

intervals around the outside perimeter of the stope for

core sampling and characterization of the rock mass. The

holes then became monitoring wells when solution was

added to the stope.

Results

Information on geologic features in the Edgar Mine and

surrounding area were compiled and evaluated. Detailed

geologic mapping of the stope area and core logging

This document was prepared by the U.S. Bureau of Mines. Neither the United States Government nor any person acting on behalf of the United

States Government assumes any liabiity resulting from the use of the information contained in this document, or warrants that such use be free from

privately owned rights.

.
focused on rock fractures. Data evaluation included statistical analysis to determine that the fracture sampling was

adequate to represent the rock mass for computer

modeling purposes.

Various geophysical techniques were applied inside

boreholes surrounding the stope to compare rock fracture

data with geologic mapping and core logging. Geophysical

testing included electric resistivity, radiometric, single-hole

sonic, and cross-hole seismic tomographic logs. The tomographic plot of seismic velocity waves between boreholes

gave the best correlation with physical fracture data.

The permeability along borehole intervals was measured

by placing expandable packers above and below test intervals and applying air or water pressure. The decay of

initial pressure over time was monitored to determine the

hydraulic conductivity of each interval. Fracture characteristics and results of permeability tests were used in a computer model, PLUME,l to predict the direction and rate of

fluid flow into the surrounding rock mass after solution

was introduced into the stope. Preliminary results using a

salt tracer in solution indicate that the computer model

correctly predicted the direction of solution flow into the

rock mass.

IReference to specific products does not imply endorsement by the . . , .

U.S. Bureau of Mines.

Leach solution

Fragmented

ore body

l —

Pregnant solution

Figure 1.-Stope leaching in underground mines.

Research by the USBM on leaching solution chemistry

continues; with progress reported on using sodium thiosulfate for leaching gold from refractory carbonaceous ores,

leaching copper from chalcopyrite ores with ferric chloride

solution, and preleaching oxidation treatment of various

minerals with biological agents.

Cooperative Research Opportunity

The USBM is seeking cooperators to assist in further

research and development of this technology. Additional

information on cooperative research may be obtained by

contacting Cad H. Schmuck, U.S. Bureau of Mines, Denver Research Center, Bldg. 20, Denver Federal Center,

Denver, Colorado 80225, or telephone (303) 236-0701.

For More Information

A USBM Report of Investigations (RI 9457), Predicting

Flow Characteristics of a Lixiviant in a Fractured Crystalline Rock Mass, is available from the author, Nadia C.

Miller, U.S. Bureau of Mines, Denver Research Center,

Bldg. 20, Denver Federal Center, Denver, Colorado 80225,

or telephone (303) 236-0724.

Upper level

Iine

main

f t

Figure 2.–Three-dimensional view of the solution control

research site prior to blasting of the simulated stope.

“U.S. Government Printing Office: 1994— 509-012/80019