some considerations in the design of

8/12/2019 Some Considerations in the Design Of

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SOME CONSIDERATIONS IN THE DESIGN OFBLASTS FOR IN SITU COPPER LEACHINGby

Dennis V. D Andrea, William C. arson,Peter G. chamberlain, and James J. 01son3ABSTRACT

Blasts designed for in situ copper leaching mustproduce adequate fragmentation and permeability forsuccessful leaching. Copper recovery depends to alarge extent on the particle size distributioncreated by the blast, the permeability in the blast-fractured zone, and the ability to maintain solutionflow with a minimum amount of channeling. This paperpresents the results of Bureau of Mines experimentswhere core drilling was used before and afterblasting to establish the degree of fragmentation.Data are presented from tests at the Sierrita mine ofDuval Corp., and also from tests at the Emerald Islemine of El Paso Mining and Milling Co. Copper re-covery is predicted from fragment size distributiondata using a computer model. Data from severalblasts are used to show how predicted copper recoveryis related to powder factor.

INTRODUCTIONObtaining maximum copper recovery within areasonable time is the most important requirement forsuccessful in situ leaching. Copper recovery dependson the ore body mineralization, the degree of frag-mentation produced by blasting, the permeability ofthe broken ore, the chemical characteristics of theleach solution, and the leach solution circulationsystem. Although all of these factors have signifi-cant effects on copper recovery, the blast design isthe most important variable under the control of themine operator. Blasting must adequately fragment theore and produce a particle size distribution that issuitable for good leaching. The blasting must alsoincrease permeability such that the leach solution

can continue to contact the broken ore, with a mini-mum amount of channeling, for the life of theoperation.This paper describes a method of designingblasts for in situ leaching based on fragment sizedistribution measurements obtained from drill coretaken before and after a series of test blasts.Geophysicist.GeologistSupervisory mining engineer.All authors are with the Twin Cities MiningResearch Center, Bureau of Mines, U.S. Departmentof the Interior, Twin Cities, Minn.

Bureau of Mines fragmentation experiments at theSierrita mine of Duval Corp., near Tucson, Ariz.2, 5 and at the Emerald Isle mine of El Paso Miningand Milling Co. near Kingman, Ariz. 1) re summarized.Copper recovery is predicted using a computer programand the results of fragment size measurements andlaboratory leaching tests.

BLASTING TESTSThe Sierrita mine test was a nine-hole blastdetonated with blasthole spacings of 15, 20, and 25feet and powder factors of 1.13, 0.64, and 0.41 lblton,respectively. These blastholes were 9 inches indiameter and 110 feet deep. The Emerald Isle mineblasts were detonated in two test areas. The Phase Itest area involved ore exposed in the pit bottom.Seven 8.375-inch-diameter blastholes 47 feet deep,spaced 25 feet apart, were used with a powder factor.of 0.78 lb/ton. The ore fragmented in ;he Phase I1test at the Emerald Isle mine was under 205 feet ofoverburden and extended to 277 feet. Twqblasts were

detonated in the Phase I1 area, the first with seven9-inch-diameter blastholes spaced 20 feet apart witha powder factor of 0.95 lblton, and the second withthree 9-inch-diameter blastholes spaced 18 feet apartwith a powder factor of 1.47 lblton. The secondPhase I1 test blast was detonated in an effort toimprove the fragmentation in the area broken by thefirst blast.CORE DRILLING

Core holes were drilled before and afterblasting with an NQ double-barrel wireline systemusing drilling mud. The drill core produced wasabout 2 inches in diameter. Drilling into orebroken by blasting presented no major problems andwas accomplished with return circulation of drill mud.The core drilling process did, however, createadditional fracturing of the core, resulting in afiner fragment size distribution than was actuallypresent in the blasted ore.

RESULTS OF FRAGMENT SIZE MEASUREMENTSTable 1 lists the results of drill coremeasurements. The quartz monzonite porphyry ore atthe Sierrita mine was more highly fractured initiallythan the Gila Conglomerate ore at the Emerald Islemine. The Sierrita mine preshot core data are theaverage of three holes, and both the postshot25-foot and the 20-foot spacing data are theaverages of two holes. The Emerald Isle mine

preshot core data are the average of two holes, onein the Phase I area and the other in the Phase I1area. Preshot core from the Phase I and Phase I1areas did not differ significantly so these two coreholes were combined. Unfortunately a postshot corehole was not drilled in the Phase I test area at theEmerald Isle mine. However, a postleach core holewas drilled in this area after completion of anin-place leaching test.Figure 1 shows the fragment size distributioncyrves for the Sierrita ore, and Figure 2 is a simi-lar plot for the Emerald Isle ore. These curves wereobtained by least squares fitting of the Weibulldistribution function to the actual size data. Theaverage size data listed in Table 1, based on 50percent passing, were obtained using the curvesshown on Figures 1 and 2.


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202 ROCK MECH NICS PPLIC TIONS IN MINING

FIGURE 1. Fragment size distributioncurves for Sierrita ore.

owing to differences in pH and leaching characteris-

Phase ~,~~slleach \JI haseD,pastshot o 7o o

8 15-ftpaclng

FRAGMENT SIZE. n

tics of the ore.Figure 5 is a plot showing predicted and actualcopper recovery versus time for the Emerald IslePhase I test area, which was leached in-place for 117days. The actual copper recovery was much less thanthe predicted recovery, because 1) the computermodel has limitations that result in overestimatingthe copper recovery, and 2) the drilling processcreated additional breaks and fracturing of the drillcore resulting in a finer indicated size distributionthan the true in situ size distribution. However, theprocedure of coring and predicting copper recoveryused in this paper does have value in establishing an

0 100 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0TIME, days

0.1 0.5 1.0 5.0 10.0 50.0 upper limit on the predicted copper recovery.FRAGMENT SIZE, n

FIGURE 2. Fragment size distributioncurves for Emerald Isle ore. FIGURE 3. Predicted copper recoveryversus time for Sierrita ore.COMPUTER-PREDICTED COPPER RECOVERYCopper recovery was calculated from the sizedistribution data using a computer program developedby Madsen, Wadsworth, and Groves 2). This computerprogram, based on a mixed kinetics model, assumes thatbulk solution transport is not rate controlling. Cir-culation of leach solution around each particle isassumed to be sufficient to maintain the necessarychemical reactions. In actual practice, however, theleach solution does not contact all of the ore becauseof channeling and permeability variations within theore body so that this computer model will overestimatecopper recovery values. Laboratory leaching testswere run to determine the two constants required asinput to the computer program. These constants dependon the type of ore being leached and on the character-istics of the leach solution primarily the pH).Figure 3 shows the predicted copper recoveryversus time for the Sierrita or?. These curves arebased on leaching with sulphurie acid at a pH of 2.Figure 4 shows predicted copper recovery for theEmerald Isle ore based on leaching with sulphuric acidat a pH of 1.5. These pH values were selectedbecause laboratory leaching tests were run at theselevels.) Although the measured fragmentation wasbetter in the Sierrita ore, the predicted copper re-covery was not as high as for the Emerald Isle ore

Phase 1,postleachPhase Il,postshot No

-0 100 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0

T I ME, days

FIGURE 4. - Predicted copper recovery versustime for Emerald Isle ore.


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D E S I G N O F B L S T S FOR I N S I T U COPPER LE C H IN G

TABLE 1. r i l l c o r e d a t aEmera ld I s l e

P r e s h o t ~ o s t l e a c h . 1 P o s t s h o t 1 1 Pos t sho t 2 ,o re run , f t . .

Core l eng th recovery ,p c t .QD, pet.......

Average s i ze , i n . . . . . . . . .(50 pass ing )

L a r g e s t p i e c e , i n . . . . . . . .

C or e r u n , f t . . . . . . . . . . . .Core l eng th recovery ,

p c t . . .................................QD, p c tAverage s i ze , i n . . . . . . . .

(50 pass ing )L a r g e s t p i e c e , i n . . . . . . .

Phase I

68 11 40 316.4 0 .5 3 . 1 1 .5

TABLE 2. - Powder f ac to r and p red ic t e d copper recovery

33

Powder fa c t o r ( lb / t on ) = 1814

9

where

20 16S i e r r i t a

P r e d i c t e d c o p p er Lr e c o v e r y , p c t

2304956

38546883

Locat ionS i e r r i t a

P r e s h o tPos t sho t , 25 - f t spac ingP o s t s h o t , 2 0 - f t s p a c i n gP o s t s h o t , 1 5 - f t s p a c i n g

Emera ld I s l ePresho tPhase 11 p o s t s h o t 1Phase 11 p o s t s h o t 2Phase I p o s t l e a c h

Pe = s p e c i f i c g r a v i t y , e x p l os i v eP = s p e c i f i c g r a v i t y , r oc k~ = b l a s t h o l e d i a m et e r , f t

S = b l a s t h o le sp a ci n g, f t

Powder factor ,l b f t o n

0.00. 41.64

1.13

0.00.952.423.78

fo r o re i n powder column zone only

Pos t sho t ,1 5 - f t

Pred ic t ed recovery a f t e r 700 days o f l each ing .Powder fa ct o r of 2.42 i s t he sum of powder fa c t o r s fo r two b l as t s w i thpowder fac to rs of 0 .95 and 1 .47.

P o s t s h o t ,20-f tPresho t Pos t sho t ,25-f t


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some considerations in the design of

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